Psychology Twin Studies
by
Nancy L. Segal
  • LAST REVIEWED: 05 December 2016
  • LAST MODIFIED: 19 March 2013
  • DOI: 10.1093/obo/9780199828340-0101

Introduction

Twin research is an informative approach for understanding the genetic and environmental influences affecting behavioral, physical, and medical traits. The simple yet elegant logic of the twin method derives from the differences in genetic relatedness between the two types of twins. Identical (monozygotic or MZ) twins share 100 percent of their genes, while fraternal (dizygotic or DZ) share 50 percent of their genes, on average. MZ twins result when a fertilized egg (ovum) divides during the first two weeks following conception, while DZ twins result when a woman simultaneously releases two eggs that are fertilized by two separate sperm. MZ twins are always same sex, whereas DZ twins may be same-sex or opposite-sex. However, rare events occasionally produce unusual MZ and DZ twin variations. Twin researchers compare the resemblance between MZ and DZ twins with reference to a particular trait, such as height or weight. Greater resemblance between MZ twins than DZ twins demonstrates that the trait under study is under partial genetic control. There are also various ways that twins and their families can be used in research to increase the potential yield of a study. Sophisticated biometrical techniques can estimate the extent of difference among people associated with their genes, shared environments and nonshared environments. Twin research has proliferated in recent years. This is largely because the power of the twin method for understanding the origin and development of human traits has become increasingly appreciated by investigators representing diverse fields. Twinning rates have also increased dramatically since 1980, especially the rate of fraternal twinning as a consequence of fertility treatments. There have been stunning advances in quantitative mathematical methodology that continue to increase the value of twin studies. Lastly, there have been enormous developments in the molecular genetics and genomics fields with respect to associating genes posing increased risks for specific behaviors and disease. Twins will continue to play a prominent role in these endeavors. The sources presented in this article represent a wide range of areas and topics within twin research. General overviews of the field, both historical and current, are provided, as well as a listing of special collections in twin research, that is, books and journals focusing on a particular topic or theme and web addresses. The largest section includes topics reflecting the widening range of psychological, biological, and medical traits that have been examined via twin research methods. The section on twin-based perspectives provides sources treating unusual twin-related topics.

Reference Works

Twin research has had a successful yet controversial past, a trend that has continued through the present. Despite the wealth of information that has been derived from twin studies, various methodological and interpretive aspects continue to be questioned. The historical roots of twin studies, its acceptance into the mainstream of psychological and medical research, and its challenges are documented in a number of books, articles, and essays. The resources in this section span a wide range of twin-related topics. The five books are appropriate for experienced investigators and new scientists, as well as general audiences searching for information about the many ways twins are used in scientific studies. Johnson, et al. 2009 and Boomsma, et al. 2002 go more deeply into current trends in twin research but will interest anyone concerned with what twin studies have (and can potentially) reveal about the origins of variation in human behavioral and physical traits. The selections here include general overviews of the biological and psychological aspects of twinship (Scheinfeld 1967), the nature-nurture debates (Wright 1997), overviews of unusual topics in the study of twins (Segal 2000), and cultural issues (Stewart 2003, Piontelli 2008). An older, but still informative account of the biology and psychology of twinning is also provided (Bryan 1983).

  • Boomsma, Dorret, Andreas Busjahn, and Leona Peltonen. 2002. Classical twin studies and beyond. Nature Reviews (Genetics) 3:872–882.

    DOI: 10.1038/nrg932Save Citation »Export Citation »E-mail Citation »

    Describes and documents the potential of large twin registries to study complex human traits. Discusses various twin research designs (e.g., classic twin study, co-twin control, genotyping of marker loci) and their application in scientific research. Includes lists of twin registers in and outside European countries.

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    • Bryan, Elizabeth. 1983. The nature and nurture of twins. London: Ballière Tindall.

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      A comprehensive examination of biological and psychological aspects of twinning by a British physician. Includes helpful information on twin types, twinning rates, and related topics. Also includes some specific topics not covered elsewhere, such as twin loss and twins with special needs.

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      • Johnson, Wendy, Eric Turkheimer, Irving I. Gottesman, and Thomas J. Bouchard Jr. 2009. Beyond heritability: Twin studies in behavioral research. Current Directions in Psychological Science 18:217–220.

        DOI: 10.1111/j.1467-8721.2009.01639.xSave Citation »Export Citation »E-mail Citation »

        Makes the argument that the heritability of most behavioral traits is now known, yet twin studies retain a vital place in psychological research. Twin research should direct greater attention to environmental influences on behavior in a quest to identify its underlying mechanisms.

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        • Piontelli, Alessandra. 2008. Twins in the world: The legends they inspire and the lives they lead. New York: Palgrave Macmillan.

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          Examines beliefs and practices regarding twinship from a cross-cultural perspective. The author’s background in neurology, psychiatry, psychoanalysis, and obstetrics substantially enriches the firsthand experiences she describes.

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          • Scheinfeld, Amram. 1967. Twins and supertwins. Philadelphia: J. B. Lippincott.

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            An older, but complete survey of the history, biology, and psychology of twins before this became mainstream science. Often includes information that is difficult to find elsewhere.

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            • Segal, Nancy L. 2000. Entwined lives: Twins and what they tell us about human behavior. New York: Plume.

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              A comprehensive overview of the background, methods, findings, and implications of twin research. Nine of the sixteen chapters address special topics such as athletic performance, legal circumstances, conjoined twinning, and noteworthy twin pairs. Written by a professor of psychology.

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              • Stewart, Ellen. 2003. Exploring twins: Towards a social analysis of twinship. London: Palgrave Macmillan.

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                Addresses the social, societal, and cultural aspects of twinship. Also considers various views of twins from the perspectives of the twins, their family members, and society at large. Draws on sources from multiple disciplines.

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                • Wright, Lawrence. 1997. Twins and what they tell us about who we are. New York: John Wiley.

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                  An account of research concerning genetic and environmental events making MZ twins both alike and different in behavior. The focus is largely, but not exclusively, on separately raised twins. A very good starting point for work in this area, although more recent publications from the Minnesota Study of Twins Reared Apart should be consulted. Written by a well-known journalist.

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                  Textbooks

                  Each of these textbooks offers comprehensive treatments of twin research methods and findings, set within the broader behavioral genetics discipline. They are excellent resources for students being introduced to the field for the first time, as well as researchers who wish to gain greater understanding of a particular topic of interest. All three books have been written by experts renowned for their contributions to twin research. Selections include a general overview of the field (Carey 2003), a summary of personality research (Loehlin 1992), and a detailed survey of research and methodology (Plomin, et al. 2012). Volumes that specifically target medical genetics (Speicher, et al. 2010) and disease risk are also included (Kendler and Prescott 2007). Also see the separate Oxford Bibliographies article by Lisabeth DiLalla on Behavior Genetics.

                  • Carey, Gregory. 2003. Human genetics for the social sciences. Newbury Park, CA: SAGE.

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                    An excellent comprehensive survey of behavioral genetics covering a wide range of topics in genetics and behavior. Includes sections to acquaint students with the biology needed to understand the material, as well as exercises, practice tests, and links for learning.

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                    • Kendler, Kenneth S., and Carol A. Prescott. 2007. Genes, environment, and psychopathology: Understanding the causes of psychiatric and substance use disorders. New York: Guilford.

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                      A complete look at the methods and findings from the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders. Includes overviews of twin research methods, genetic and environmental risk factors, and integrative models of disease risk.

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                      • Loehlin, John C. 1992. Genes and environment in personality development. Newbury Park, CA: SAGE.

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                        A concise primer on methods and findings from twin studies of personality. Recommended especially for individuals new to the field.

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                        • Plomin, Robert, John C. DeFries, Valerie S. Knopik, and Jenae M. Neiderhiser. 2012. Behavioral genetics. 6th ed. New York: Worth.

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                          An excellent introduction to the background, procedures, findings, results, and controversies in behavioral genetics and twin studies. Twin studies are included in every chapter on behavior, and an extensive reference list, glossary, and resource section are provided.

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                          • Speicher, Michael, Stylianos E. Antonarakis, and Arno G. Motulsky, eds. 2010. Vogel and Motulsky’s human genetics: Problems and approaches. New York: Springer.

                            DOI: 10.1007/978-3-540-37654-5Save Citation »Export Citation »E-mail Citation »

                            An updated and comprehensive compendium of research on human and medical genetics. Includes timely topics in the field, ranging from modes of inheritance, genome-wide association studies, and disease susceptibility.

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                            Special Collections

                            Twin research has now been incorporated into the research programs of individuals working in diverse areas of the behavioral and medical/neurobiological sciences. Stunning advances in molecular genetics have spawned new collaborations between psychologists and geneticists searching for specific genes underlying behavioral traits. The sources listed here reflect these changing trends and provide strong hints as to where twin research may be headed. Selections include special edited collections on varied twin topics (Plomin, et al. 1993; Bouchard and Propping 1993; Kim 2009), religion (Kirk and Martin 1999), and molecular genetics (Plomin, et al. 2002; McGuffin, et al. 2004).

                            • Bouchard, Thomas J., and Peter Propping, eds. 1993. Twins as a tool of behavioral genetics. Chichester, UK: John Wiley.

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                              A volume resulting from the 1992 Dahlem Workshop on mechanisms mediating genetic and environmental influences on behavior. Experts assess a wide variety of topics, including cognition, personality, and psychopathology, using findings from both child and adult twin samples. An evolutionary perspective on personality and recent findings from molecular genetics are provided.

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                              • Kim, Yong-Kyu, ed. 2009. Handbook of behavior genetics. New York: Springer.

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                                Provides a comprehensive overview of the behavioral genetics field, including chapters on history, methods, and findings. Given the detailed coverage, it is mostly recommended for graduate students and scientists, both new and familiar with the field.

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                                • Kirk, Katherine M., and Nicholas G. Martin, eds. 1999. Special issue: Twin Research 9:59–179.

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                                  A still timely compendium of original twin research papers on religiosity. Topics include associations between religious practices and health, religion and depression, and factors affecting religious observance. Twin samples in the various studies come from a variety of countries.

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                                  • McGuffin, Peter, Michael J. Owen, and Irving I. Gottesman, eds. 2004. Psychiatric genetics and genomics. Oxford: Oxford Medical.

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                                    An edited text covering the basic principles of molecular genetics, genetics of normal and abnormal development, genetics of abnormal adult behavior, and applications and implications of current findings related to the molecular genetics of psychiatric disorders.

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                                    • Plomin, Robert, John C. DeFries, Ian Craig, and Peter McGuffin, eds. 2002. Behavioral genetics in the postgenomic era. Washington, DC: American Psychological Association.

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                                      Provides an excellent overview of the growing involvement of behavioral geneticists in the molecular genetics field. Chapters cover a range of topics written by experts in intelligence, personality, psychopathology, and aging who describe attempts to link specific behaviors with specific genes.

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                                      • Plomin, Robert, and Gerald E. McClearn, eds. 1993. Nature, nurture and psychology. Washington, DC: American Psychological Association.

                                        DOI: 10.1037/10131-000Save Citation »Export Citation »E-mail Citation »

                                        A comprehensive series of chapters addressing past and present trends in behavioral genetic research, as well as specific study areas. Contributions come from twin and adoption researchers, as well as from experts outside the mainstream of behavioral genetic research. In addition to substantive study areas, such as cognition and personality, the broader concepts of nature and nurture are given considerable deserved attention.

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                                        Journals

                                        Twin studies can now be found in virtually every psychological and medical journal. That was less common in the 1950s, 1960s, and 1970s when genetic views of behavior were largely rejected in favor of environmentalist and psychoanalytical paradigms. Scientists frustrated with the environmentalist climate of the 1950s and 1960s founded Behavior Genetics in 1970, the flagship journal of the Behavior Genetics Association. Twin Research and Human Genetics, which developed out of the more specialized Italian-edited journal (Acta Geneticae Medicae et Gemellologiae—Twin Research), has grown in response to the proliferation of twin research in psychological and medical fields. Forums for molecular genetic research on psychiatric disorders are also available. Neuropsychiatric Genetics includes genetic studies of psychiatric and nervous disorders. Molecular Psychiatry includes articles on preclinical and clinical research, emphasizing cellular and molecular aspects, as well as epidemiological considerations and treatment outcomes.

                                        Methodology

                                        Twin studies often use the classic comparison of resemblance between MZ and DZ twin pairs. However, twin research has benefitted greatly by the availability of many genetically and environmentally informative kinships that variously include twins, their spouses, their children, their parents, and/or their nontwin siblings. The identification of twins reared apart, as well as twins reared together, has provided fresh insights into the significance of genetic effects and impact of the environment on behavior. Topics covered in this section include recruitment bias in twin studies (Lykken, et al. 1987), methodology (Neale and Cardon 2011; Falconer and Mackay 1996; Turner, et al. 1995; DeFries and Fulker 1985). Martin, et al. 1999 discusses twin studies as an informative approach to understanding behavior, as well as the important topic of why identical twins differ.

                                        • DeFries, C., and D. W. Fulker. 1985. Multiple regression analysis of twin data. Behavior Genetics 15:467–473.

                                          DOI: 10.1007/BF01066239Save Citation »Export Citation »E-mail Citation »

                                          Describes a technique for analyzing twin data in which one twin’s score is predicted from his cotwin’s score and the coefficient of genetic relatedness (1 for MZ twins, 0.5 for DZ twins). This approach is especially well suited to twin samples in which one member of each pair has been identified for having a deviant score on a given measure.

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                                          • Falconer, Douglas S., and Trudy F. C. Mackay. 1996. Introduction to quantitative genetics. 4th ed. Harlow, UK: Longman.

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                                            The classic book in quantitative genetic analysis, covering genetic concepts, resemblance between relatives, heritability, genetic and environmental correlations, and a great deal more. Twin research concepts account for a relatively small portion of this book, yet the discussion provided is important and illuminating.

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                                            • Lykken, D. T., M. McGue, and A. Tellegen. 1987. Recruitment bias in twin research: The rule of two-thirds reconsidered. Behavior Genetics 17:343–362.

                                              DOI: 10.1007/BF01068136Save Citation »Export Citation »E-mail Citation »

                                              Reviews existing studies to confirm the bias present in volunteer twin samples, namely the overrepresentation of twins who are MZ and female. Discusses the consequences of such features for heritability estimation, and suggests ways of insuring that twin samples provide valid results.

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                                              • Martin, Nicholas, Dorett Boomsma, and Geoffrey Machin. 1999. A two-pronged attack on complex traits. Nature Genetics 17:387–392.

                                                DOI: 10.1038/ng1297-387Save Citation »Export Citation »E-mail Citation »

                                                Outlines the utility of twin studies in tackling complex traits and shows the value of multivariate genetic methods for increasing the power of linkage and association studies. It foreshadowed the use of discordant MZ twins for understanding epigenetic mechanisms and other causes of sporadic disease.

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                                                • Neale, Michael C., and Lon R. Cardon. 2011. Methodology for genetic studies of twins and families. Dordrecht, The Netherlands: Springer.

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                                                  The first extensive overview of quantitative methods designed specifically to analyze data from twin studies. The book developed out of a series of international workshops designed to acquaint researchers with this material. Documentation and downloads can be found online.

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                                                  • Turner, J. Rick, Lon R. Cardon, and John K. Hewitt, eds. 1995. Behavior genetic approaches in behavioral medicine. New York: Plenum.

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                                                    A volume resulting from the increased participation of behavioral geneticists in behavioral medicine, namely the fields of psychology, medicine, nursing, epidemiology, and public health. Experts examine methods and findings from twin research in alcoholism, blood pressure, cardiovascular response, smoking, and other medical conditions.

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                                                    Historical Perspectives

                                                    Twin research has had a rich, yet controversial past. Interestingly, Galton 1875 and its description of the classic twin method occurred prior to our understanding of the biological origins of the two types of twins, MZ and DZ. Galton’s analysis relied on life history information provided to him in correspondence from identical and fraternal twins and people who knew them. In later years, Galton’s role in the discovery of the twin method was challenged by some researchers but defended by others in the field. Presently, much more is known about factors responsible for DZ twinning than for MZ twinning, and the classic twin method remains only one of several research designs. An early examination of the biology of twinning is offered by Arey 1922. Subsequent papers both challenge (Rende, et al. 1990) and support Galton as founder of the classic twin method (Bouchard and Propping 1993).

                                                    • Arey, Leslie B. 1922. Direct proof of the monozygotic origin of human identical twins. The Anatomical Record 23:245–251.

                                                      DOI: 10.1002/ar.1090230403Save Citation »Export Citation »E-mail Citation »

                                                      Provides the first scientific proof that MZ twins originate from a single yolk sac and, therefore, from a single fertilized ovum. Even though it was generally believed that human MZ twins had such a common biological origin, the evidence rested on experimental studies and on the observation that the nine-banded armadillo gave birth only to MZ quadruplets. Particular human twin specimens provided the direct scientific proof discussed in this paper.

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                                                      • Bouchard, Thomas J., Jr., and Peter Propping. 1993. Twins: Nature’s twice-told tale. In Twins as a tool of behavioral genetics. Edited by Thomas J. Bouchard Jr. and Peter Propping, 1–15. Chichester, UK: John Wiley.

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                                                        Describes key events in the historical development of twin research, defending Galton as father of the twin method. Briefly surveys important study areas in twin research as a prelude to the full volume.

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                                                        • Galton, Sir Francis. 1875. The history of twins as a criterion of the relative powers of nature and nurture. Journal of the Anthropological Institute 5:391–406.

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                                                          The landmark paper that defined the classic twin method, in which Galton famously explained that their history “affords means of distinguishing between the effects of tendencies received at birth, and of those that were imposed by the circumstances of their after lives” (p. 3910). The biological aspects of twinning had not been worked out at that time, but Galton surmised correctly that “look-alike” twins shared 100 percent of their heredity, while twins that did not look alike shared relatively less.

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                                                          • Rende, Richard A., Robert Plomin, and Stephen G. Vandenberg. 1990. Who discovered the twin method? Behavior Genetics 20:277–285.

                                                            DOI: 10.1007/BF01067795Save Citation »Export Citation »E-mail Citation »

                                                            A statement to the effect that Curtis Merriman, not Sir Frances Galton, deserved credit for discovering the classic twin design. This conclusion, which has been refuted by several investigators, was based on the fact that Merriman analyzed his twin data quantitatively as is done today, whereas Galton brought a qualitative approach to his material. Two years later in 1924, Siemens introduced the similarity method for diagnosing twins as “one-egg” or “two-egg” in origin.

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                                                            Research Methods and Design

                                                            Twin research offers a unique array of methods for participant recruitment, study design, and data analysis. The methodological features of twin studies, once described mostly in specialized sources, have become much more widely available in recent years. Journal articles, special issues, and online resources have familiarized many individuals with the terms and procedures that must be understood and applied in twin research. The classic twin design (Martin, et al. 1978) and the necessary adjustment of twin data for age and sex effects (McGue and Bouchard 1984) are key to the field. More specific issues of twin methodology include interactive modules for analysis (as provided in Plomin, et al. 2012 under Textbooks), the use and interpretation of intraclass correlations (Jensen 1971) and heritability estimation (Lykken 2007). Techniques by McGue 1992 for estimating genetic effects on schizophrenia and by Visscher, et al. 2008 for other human traits in the current molecular genetic era are seminal works.

                                                            • Behavioral Genetic Interactive Modules.

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                                                              Free program and guide to quantitative concepts and analysis in behavioral genetics. Designed to accompany the text Behavioral Genetics (Plomin, et al. 2012, cited under Textbooks).

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                                                              • Busjahn, Andreas, and Yoon-Mi Hur, eds. 2006. Special issue: Twin registers as a global resource for genetic research. Twin Research and Human Genetics 9:705–1037.

                                                                DOI: 10.1375/twin.9.6.705Save Citation »Export Citation »E-mail Citation »

                                                                A special issue on twin registers around the world. Information for the different registries variously includes the number of participating twin pairs, methods used for distinguishing MZ and DZ twins, past and ongoing projects, future directions, and who to contact for use of the registry.

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                                                                • Jensen, Arthur R. 1971. Note on why genetic correlations are not squared. Psychological Bulletin 75:223–224.

                                                                  DOI: 10.1037/h0030417Save Citation »Export Citation »E-mail Citation »

                                                                  Explains why intraclass correlations (ri) are not squared as are ordinary (Pearson’s r) correlations to obtain the proportion of variance in one variable that is explained by the other variable. Intraclass correlations derive from analysis of variance, and express the ratio of the between group variance to the total variance; they already express the variance shared between cotwins.

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                                                                  • Lykken, David T. 2007. A more accurate estimate of heritability. Twin Research and Human Genetics 10:168–173.

                                                                    DOI: 10.1375/twin.10.1.168Save Citation »Export Citation »E-mail Citation »

                                                                    Provides evidence that psychological and physiological traits vary around stable set points, due to environmental influences. Heritability estimates based on single measures may underestimate heritability. The argument that heritability is higher and more reliably assessed using multiple measures is demonstrated by various longitudinal data gathered on young and middle-aged twin pairs.

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                                                                    • Martin, Nicholas G., Lindon J. Eaves, M. J. Kearsey, and P. Davies. 1978. The power of the classical twin study. Heredity 40:97–116.

                                                                      DOI: 10.1038/hdy.1978.10Save Citation »Export Citation »E-mail Citation »

                                                                      Power calculations suggested that many hundreds, or even thousands, of twin pairs were needed (depending on the hypothesis under investigation), rather than the small sample sizes (typically dozens) that had been used. This result was responsible for the first author’s founding of the Australian Twin Register in 1978, and the development of twin registries elsewhere.

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                                                                      • McGue, Matt. 1992. When assessing twin concordance, use the probandwise not the pairwise rate. Schizophrenia Bulletin 18:171–176.

                                                                        DOI: 10.1093/schbul/18.2.171Save Citation »Export Citation »E-mail Citation »

                                                                        Provides excellent discussion, demonstration, and evaluation of the different methods used for estimating twin concordance for schizophrenia and other psychiatric disorders. Probandwise concordance is identified as the best measure for this purpose.

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                                                                        • McGue, Matt, and Thomas J. Bouchard Jr. 1984. Adjustment of twin data for the effects of age and sex. Behavior Genetics 14:325–343.

                                                                          DOI: 10.1007/BF01080045Save Citation »Export Citation »E-mail Citation »

                                                                          Shared age and sex may inflate resemblance between same-sex cotwins, just as it would between unrelated individuals of the same age and sex. The authors provide the rationale and procedures for statistically removing these effects from twin data prior to analysis.

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                                                                          • Visscher, Peter M., William G. Hill, and Naomi R. Wray. 2008. Heritability in the genomics era: Concepts and misconceptions. Nature Reviews (Genetics) 9:255–266.

                                                                            DOI: 10.1038/nrg2322Save Citation »Export Citation »E-mail Citation »

                                                                            Notes that heritability estimates of height and many other human characteristics are known. However, why these traits have the values that they do remains uncertain. Solutions to such problems may emerge following continued application of genome-wise gene expression studies.

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                                                                            Biology of Twinning

                                                                            The biological bases of twinning cover many topics. Examples include events giving rise to MZ and DZ twins, factors affecting variation within the two types of twins, differences in the frequencies of twinning within and across cultures, and methods for distinguishing MZ from DZ twins. The references included in this section provide the historical background and current findings in each of these areas. Interestingly, despite years of scientific progress, much about human twinning remains unknown. What exactly causes the fertilized egg to divide during the first two gestational weeks to produce MZ twins remains a mystery. Predicting which women are most likely to conceive DZ twins naturally also poses a scientific challenge.

                                                                            Biological Processes

                                                                            It is reasonable to assert that more than two twin types are extant, given the considerable variation among both MZ and DZ twin pairs. The timing of the zygotic division in the case of MZ twins, and the sex of the co-twin in the case of DZ twins, can have important consequences with respect to the twins’ behavioral and physical development. The new assisted reproductive techniques, developed to allow infertile couples to have children, have resulted in a marked increase in twinning rates. The increase involved mostly DZ twins, but a slight increase in the frequency of MZ twins has been linked to manipulation of the fertilized egg prior to implantation in the womb. This section surveys the biology of twinning (Bulmer 1970), implications of X chromosome inactivation for twin similarity (Trejo, et al. 1994) and the physical effects on females of having a male cotwin (Lummaa, et al. 2007). Current conceptions of the biology of twinning (Machin and Keith 1999, Blickstein and Keith 2005) are challenged (Boklage 2010). Factors affecting MZ twinning are also discussed (Moayeri, et al. 2007).

                                                                            • Blickstein, Issac, and Louis G. Keith, eds. 2005. Multiple pregnancy: Epidemiology, gestation and perinatal outcome. 2d ed. London: Parthenon.

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                                                                              A virtual encyclopedia of information on the epidemiology, biological origins, maintenance, and management of multiple birth pregnancies. Written by experts in obstetrics and gynecology, this volume will be valuable to physicians, medical students, psychologists, parents, and others interested in general and specific features of multiple birth.

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                                                                              • Boklage, Charles E. 2010. How new humans are made: Cells and embryos, twins and chimeras, left and right, mind/self/soul, sex, and schizophrenia. Singapore: World Scientific.

                                                                                DOI: 10.1142/9789812835147Save Citation »Export Citation »E-mail Citation »

                                                                                An extensive, somewhat unconventional, overview of the biology of human twinning and reproduction. Challenges conventional wisdom about the biological origins of the different types of twins and related phenomena.

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                                                                                • Bulmer, Martin G. The biology of twinning in man. Oxford: Oxford Univ. Press, 1970.

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                                                                                  The definitive overview of the biological bases of human twinning, one that continues to be cited. Includes information on placentation, twinning rates, and the biological processes giving rise to unusual MZ and DZ twin pairs. A number of helpful diagrams are provided.

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                                                                                  • Lummaa, Virpi, Jenni E. Pettay, and Andrew F. Russell. 2007. Male twins reduce fitness of female co-twins in humans. Proceedings of the National Academy of Sciences 104:10915–10920.

                                                                                    DOI: 10.1073/pnas.0605875104Save Citation »Export Citation »E-mail Citation »

                                                                                    Considers reproductive consequences of the shared intrauterine environment for female twins. Used twins from a preindustrialized sample such that modern medical interventions would not mask these effects. Females with male cotwins had a 25 percent lower rate of having children than female twins with female cotwins. Females with male cotwins had a reduced probability of marrying, compared to females with a female cotwin.

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                                                                                    • Machin, Geoffrey A., and Louis G. Keith. 1999. An atlas of multiple pregnancy. New York: Parthenon.

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                                                                                      An in-depth, scientific look at multiple pregnancies from the perspective of experts in obstetrics and gynecology. Provides detailed descriptions of placentation structure, fetal risk, and maternal hazards associated with multiple births. Includes a large collection of color photos that beautifully illuminate concepts and findings.

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                                                                                      • Moayeri, Sharon E., Barry Behr, Ruth B. Lathi, Lynn M. Westphal, and Amin A. Milki. 2007. Risk of monozygotic twinning with blastocyst transfer decrease over time: An 8-year experience. Fertility and Sterility 87:1028–1032.

                                                                                        DOI: 10.1016/j.fertnstert.2006.09.013Save Citation »Export Citation »E-mail Citation »

                                                                                        Compared MZ twinning frequencies in women who underwent three-day embryo transfer (ET) and women who underwent blastocyst transfer (BT) in 2002–2005, and women who received reproductive assistance before 2002. MZ twinning rates did not differ among women in the 2002–2005 ET group (1.8 percent) and BT group (2.3 percent). Pre-2002 BT women showed a higher rate of MZ twinning (5.6 percent) than 2002–2005 BT women. Methodological improvement may explain.

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                                                                                        • Trejo, V., Catherine Derom, Robert Vlietinck, et al. 1994. Implications for immune relatedness and concordance for autoimmunity. Molecular Medicine 1: 62–70.

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                                                                                          Analysis of X chromosome inactivation in MZ female twins with autoimmune disease and MZ female twin pairs with known placental arrangements. X chromosome inactivation occurs early in gestation, with possible female cotwin differences. Similar X inactivation patterns in monochorionic twins suggested that this process occurs before the twinning event. It was also suggested that X inactivation does not explain MZ female twin discordance for autoimmune disorders.

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                                                                                          Conjoined Twinning

                                                                                          Conjoined twins occur in approximately one in two hundred MZ twin births (see Segal 2000 in Reference Works). Surgical separation of some conjoined twins is becoming increasingly possible, given improvements in detecting and managing these pairs (Votteler and Lipsky 2005). However, not all separations are successful or even viable, and some would argue that conjoined twins represent a rare form of human existence that should be respected and not tampered with (Dreger 2004).

                                                                                          • Dreger, Alice D. 2004. One of us: Conjoined twins and the future of normal. Cambridge, MA: Harvard Univ. Press.

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                                                                                            Considers conjoined twinning as a variant human form, rather than an anatomical aberration that requires correction, thereby challenging current conceptions of normality. Introduces evidence and arguments from a variety of conjoined twin cases, resulting in an important thought-provoking work.

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                                                                                            • Votteler, T. P., and K. Lipsky. 2005. Long-term results of 10 conjoined twin separations. Journal of Pediatric Surgery 40:618–629.

                                                                                              DOI: 10.1016/j.jpedsurg.2004.12.011Save Citation »Export Citation »E-mail Citation »

                                                                                              A scientific review that uniquely describes the biological features of different conjoined twin pairs, the management of these pairs and the outcomes following surgical separation. Inclusion of the twins’ life histories and photographs make this paper informative and compelling.

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                                                                                              Twinning Rates

                                                                                              Following the first successful in vitro pregnancy in 1978, in England, twinning rates have escalated dramatically in Western countries. In contrast, nations in the developing world offer a truer glimpse of the frequency with which human twinning occurs, given the absence of reproductive assistance. Current US twinning rates are summarized here (in Martin, et al. 2012), as are the first worldwide rates from developing countries (as in Smits and Monden 2011). A different approach is taken by Lummaa, et al. 1998, which offers an evolutionary analysis of the different twinning rates in two geographical locations in Finland.

                                                                                              • Lummaa, Virpi, Erkki Haukioja, Risto Lemmetyinen, and Mirja Pokkola. 1998. Natural selection on human twinning. Nature 394:533–534.

                                                                                                DOI: 10.1038/28977Save Citation »Export Citation »E-mail Citation »

                                                                                                Compares natural twinning rates among Caucasian populations on the Finnish archipelago of Åland and Åboland, and in adjacent mainland areas. The higher twinning rate on the archipelago is consistent with life history hypotheses predicting greater reproductive success in areas with more reliable food sources.

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                                                                                                • Martin, Joyce A., B. E. Hamilton, and M. J. K. Osterman. 2012. Three decades of twin births in the United States, 1980–2009. National Center for Health Statistics Data Brief 80, 1–8.

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                                                                                                  A report on 2009 US twinning rates from the Center for Disease Control. Twinning increased by 76 percent since 1980, from 18.9 to 33.3 births per thousand. Reasons include delayed child-bearing, responsible for approximately one-third of the change, and fertility treatments, responsible for approximately two-thirds of the change.

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                                                                                                  • Smits, Jeroen, and Christiaan Monden. 2011. Twinning across the developing world. PLOS One 6.9: e25239.

                                                                                                    DOI: 10.1371/journal.pone.0025239Save Citation »Export Citation »E-mail Citation »

                                                                                                    The first comprehensive report on national twinning rates in developing nations. Low twinning rates characterize South and Southeast Asian areas. Benin boasts the world’s highest twinning rate, a distinction previously held by Nigeria.

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                                                                                                    Zygosity Diagnosis

                                                                                                    Classification of twins as MZ or DZ via objective scientific methods is a vital first step in any research undertaking. That is because misclassification of twin pairs can falsely deflate estimates of genetic influence on a given trait. Most current researchers use comparative DNA analysis for this purpose (see DNA Twin Zygosity Testing), a procedure that is becoming increasingly simple and cost-effective. However, administering a standard physical resemblance questionnaire is an acceptable method when DNA testing is not possible, as shown by Nichols and Bilbro 1966. However, extensive blood-typing, a commonly used procedure in the 1960s through the 1980s, is rarely performed. An unusual historical case involving twin-typing determination was accomplished via reciprocal skin grafting, as discussed in McIndoe and Franceschetti 1949–1950. Also of historical interest is the first successful kidney transplant, which occurred between MZ male twins (Merrill, et al. 1956). Merrill’s work exemplifies the medical advantage enjoyed by genetically identical twins, namely that they are ideal organ donors and recipients for one another.

                                                                                                    • DNA Twin Zygosity Testing.

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                                                                                                      A major laboratory (Affiliated Genetics) describes procedures used for determining twin zygosity and the reasons twins and families might desire this knowledge. Information on pricing and ordering is provided.

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                                                                                                      • McIndoe, Archibald, and Albert Franceschetti. 1949–1950. Reciprocal skin homografts in a medico-legal case of familial identification of exchanged identical twins. British Journal of Plastic Surgery 2:283–289.

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                                                                                                        A fascinating paper that provided the key evidence for the suspected accidental switching of MZ cotwins at birth. Reciprocal skin grafts between the alleged switched twins (who grew up apart) were successful, while those between the unrelated children (who grew up together) were rejected. This was the first time skin grafts were used to establish twins’ zygosity.

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                                                                                                        • Merrill, John P., Joseph E. Murray, J. Harrison Hartwell, and Warren R. Guild. 1956. Successful transplantation of the human kidney between identical twins. Journal of the American Medical Association 160.4: 277–282.

                                                                                                          DOI: 10.1001/jama.1956.02960390027008Save Citation »Export Citation »E-mail Citation »

                                                                                                          Reports the first successful kidney transplantation between identical twins. This event, which took place in 1954 at the Peter Bent Brigham Hospital in Boston, is still considered a landmark medical success.

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                                                                                                          • Nichols, Robert C., and William C. Bilbro. 1966. The diagnosis of twin zygosity. Acta Genetica et Statistica Medica 16:265–275.

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                                                                                                            A standard self-report physical resemblance questionnaire for assigning zygosity (twin type). The article includes scoring rules that can assign MZ or DZ twinning at three levels of certainty. Shows 93 percent agreement with results from extensive blood group analysis.

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                                                                                                            Twins Reared Apart

                                                                                                            Studies of MZ and DZ twins reared apart from birth (MZA and DZA) and reunited as adults offer scientists pure estimates of genetic influence on behavioral and physical traits. These twins are also the answer to critics’ claims that MZ twins are alike because they are treated alike. Reared apart twins are relatively rare, but subsequent to the first reports published in the 1920s, several major studies were undertaken. This section is organized into “old studies” and “new studies” because of the differences in size and scope among them. For example, in addition to quantitative findings, the older studies provided biographical summaries of the individual twin pairs, something that is prevented by current regulations regarding participant confidentiality.

                                                                                                            Old Studies

                                                                                                            The relatively older reared-apart twin studies focused on the similarities and differences in the twins’ mental abilities and personality traits. The three major studies, Newman, et al. 1937; Shields 1962; and Juel-Nielsen 1965, include a great deal of quantitative and life history data. Both Newman, et al. 1937 and Shields 1962 include informative twin comparison groups in their projects, namely MZ and DZ twins reared together (MZT and DZT), in order to assess genetic (MZ-DZ) and shared environmental effects (MZT-MZA). None of these investigators included DZA twin pairs in their samples, although Shields briefly reports on eleven DZA twin pairs. This omission would be rectified by the new studies that followed. Nevertheless, these articles and book-length treatments remain classics in the twin research field. Some early work included a detailed case study (Muller 1925) and a series of case studies (Burks and Roe 1949). Comprehensive analyses of larger MZA twin samples include a study in Chicago (Newman, et al. 1937), Great Britain (Shields 1962), and Denmark (Juel-Nielsen 1965). Data from these early studies and several case studies were compiled into a single volume in Farber 1981. Findings from a controversial study of purposefully separated twin infants in New York are also available (Abrams 1986).

                                                                                                            • Abrams, Samuel. 1986. Disposition and the environment. In Vol. 41, Psychoanalytic study of the child. Edited by Peter B. Neubauer and Albert J. Solnit, 41–60. New Haven, CT: Yale Univ. Press.

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                                                                                                              A case study of an MZA twin pair that examined associations between parenting and behavioral outcome. Four other MZA twin pairs and one MZA triplet set were also part of this controversial 1960s study in which twins placed for adoption were assigned to different homes. This work provides interesting views of adoption and parenting at that time but remains outside the mainstream of current reared-apart twin research.

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                                                                                                              • Burks, Barbara S., and Anne Roe. 1949. Studies of identical twins reared apart. Psychological Monographs 63, 1–62.

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                                                                                                                A small series of case studies that followed the development of young MZ twins reared apart. The researchers gathered life history information, made personal observations, collected ratings by others, and administered various psychological tests. The material was prepared for publication by the second author, following the death of the first author.

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                                                                                                                • Farber, Susan L. 1981. Identical twins reared apart: A reanalysis. New York: Basic Books.

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                                                                                                                  A complete compendium and synthesis of all life history, behavioral, and medical data from the early reared-apart twin studies and case reports. Includes charts and tables that are helpful for meta-analysis. Farber’s views on the MZA twins’ IQ similarity and on the twins’ degree of separation have been controversial for their openly antigenetic stance.

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                                                                                                                  • Juel-Nielsen, Niels. 1965. Individual and environment: A psychiatric-psychological investigation of monozygous twins reared apart. Acta Psychiatrica Scandinavica Supplementum 183. Copenhagen: Munksgaard.

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                                                                                                                    The third major study of twelve pairs of MZ twins reared apart. Includes analyses of psychological and medical data, in addition to case histories. This study drew participants from the Danish Twin Register, so included the entire population of reared apart MZ twins born between 1870 and 1910. A follow-up study that included the original material was published as a book in 1980 (Individual and environment: Monozygotic twins reared apart, New York: International Universities Press).

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                                                                                                                    • Muller, Hermann J. 1925. Mental traits and heredity. Journal of Heredity 16:443–448.

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                                                                                                                      The second report on the first scientifically documented case of MZ twins reared apart; initial findings were published by Paul Popenoe, in 1922. The twins, Bessie and Jessie, were separated at two weeks of age and reunited at age eighteen. The twins’ behavioral and physical similarities and differences are described, although it was noted that the similarities were more striking than the differences.

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                                                                                                                      • Newman, Horatio H., Frank N. Freeman, and Karl J. Holzinger. 1937. Twins: A study of heredity and environment. Chicago: Univ. of Chicago Press.

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                                                                                                                        The first major psychological study of nineteen pairs of MZ twins reared apart. Includes a historical overview of the field, and quantitative analyses of physical, intellectual, personality, and life history data obtained on the twins when they visited the University of Chicago. Scientifically informative case studies of each reared apart pair are included.

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                                                                                                                        • Shields, James. 1962. Monozygotic twins brought up apart and brought up together. London: Oxford Univ. Press.

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                                                                                                                          The second major psychological study of forty-four pairs of MZ twins reared apart. The twins were identified in Great Britain, in response to an announcement about the study on BBC television. The series of quantitative analyses and comprehensive case histories showed impressive genetic influences across the majority of measures behavioral traits, challenging the environmentalist perspectives of the time.

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                                                                                                                          New Studies

                                                                                                                          The relatively new studies of reared-apart twins variously benefitted from the availability of national twin birth registers, improved assessment instruments, analytical tools, and methods for participant recruitment. These studies, especially Bouchard, et al. 1990 and Pedersen, et al. 1992, were exceedingly comprehensive in that they examined a wide range of behavioral and medical traits. Other published accounts of twins reared apart include the rare sets separated due to accidental switching at (or shortly after) birth. They also include twins whose separated placement was caused indirectly by the Peoples’ Republic of China’s One-Child Policy, which led to the abandonment of thousands of infant girls as couples hoped to conceive boys, as discussed in Segal, et al. 2011. Segal 2012 provides the first complete coverage of the origins, methods, findings, implications, and controversies from the famous Minnesota Study of Twins Reared Apart. The rare subset of twins reared apart, due to accidental switching of infants in the hospital, is described in Segal 2011. Studies on twins reared apart in Japan (Hayakawa, et al. 2002) and Finland (Langinvainio, et al. 1984) are also included.

                                                                                                                          • Bouchard, Thomas J., Jr., David T. Lykken, Matt McGue, Nancy L. Segal, and Auke Tellegen. 1990. Sources of human psychological differences: The Minnesota Study of Twins Reared Apart. Science 250:223–228.

                                                                                                                            DOI: 10.1126/science.2218526Save Citation »Export Citation »E-mail Citation »

                                                                                                                            Analysis of IQ data (based on three different test measures) in the first major paper on general intelligence from the Minnesota Study of Twins Reared Apart. Compares these findings with those from the three previous major reared-apart twin studies. Concludes that genetic factors explain 70 percent of the IQ variance, consistent with the earlier reared-apart twin work. Family background measures made negligible contributions to the twins’ IQ scores and resemblance.

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                                                                                                                            • Hayakawa, K., Tadahiko Shimizu, Kenji Kato, Miyuki Onoi, and Yoko Kobayashi. 2002. A gerontological cohort study of aged twins: The Osaka University Aged Twin Registry. Twin Research 5:387–388.

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                                                                                                                              Presents an analysis of genetic and environmental contributions to physical and intellectual features of aging. The 250 reared-apart twin pairs were identified from among a registry that includes twelve thousand twin pairs born between 1900 and 1935.

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                                                                                                                              • Langinvainio, Heimo, Markku Koskenvuo, Jaakko Kaprio, and Pertti Sistonen. 1984. Finnish twins reared apart. II: Validation of zygosity, environmental dissimilarity and weight and height. Acta Geneticae Medicae et Gemellologiae 3:251–258.

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                                                                                                                                Reports analyses of zygosity, environmental features, height, and weight for 165 reared-apart twin pairs, separated before the age of eleven years. The MZA twins showed greater environmental dissimilarity than the DZA twins. Original zygosity determinations, based on a physical resemblance questionnaire, were validated by blood grouping for ten pairs, with classification changing in only one case.

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                                                                                                                                • Pedersen, Nancy L., Robert Plomin, John R. Nesselroade, and Gerald E. McClearn. 1992. A quantitative genetic analysis of cognitive abilities during the second half of the life span. Psychological Science 3:346–353.

                                                                                                                                  DOI: 10.1111/j.1467-9280.1992.tb00045.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                  Compares cognitive ability similarity in MZ and DZ twins reared apart and together, using data from the Swedish Twin and Adoption Study of Aging. Genetic factors explained 80 percent of the variance in general cognitive ability, a value higher than the widely cited 50 percent figure based on the larger behavioral genetic literature. Average heritability estimates were predictably lower for special cognitive skills, ranging from 38 to 58 percent.

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                                                                                                                                  • Segal, Nancy L. 2011. Someone else’s twin: The true story of babies switched at birth. Amherst, NY: Prometheus.

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                                                                                                                                    A life history of female MZ twins from the Canary Islands, who were accidentally switched at birth in the nursery and raised separately. The twins, reunited at age twenty-eight, showed remarkable behavioral and physical similarities and intriguing differences. Mother-infant identification, hospital procedures for placing infants with mothers, and legal proceedings for addressing hospital errors in this case and others are addressed.

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                                                                                                                                    • Segal, Nancy L. 2012. Born together-reared apart: The landmark Minnesota twin study. Cambridge, MA: Harvard Univ. Press.

                                                                                                                                      DOI: 10.4159/harvard.9780674065154Save Citation »Export Citation »E-mail Citation »

                                                                                                                                      The most recent contribution to reared-apart twin studies. Presents the complete story of the Minnesota Study of Twins Reared Apart, both the quantitative findings and the compelling life histories of the separated and reunited twins.

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                                                                                                                                      • Segal, Nancy L., Joanne Hoven Stohs, and Kara Evans. 2011. Chinese twin children reared apart and reunited: First prospective study of co-twin reunions. Adoption Quarterly 14:61–78.

                                                                                                                                        DOI: 10.1080/10926755.2011.557953Save Citation »Export Citation »E-mail Citation »

                                                                                                                                        A study of twins’ reactions to first meeting with, and departures from, their cotwin in the first prospective study of young Chinese separated twins. The twins were adopted mostly by American and Canadian couples, following abandonment in China, due to the “one-child” policy. Most twins who met at eighteen months or older showed a strong attraction to their cotwin, although leave-taking responses varied.

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                                                                                                                                        Psychological Research

                                                                                                                                        Twins have participated in an enormous array of different psychological studies. The varied categories in this section offer only a glimpse into the many ways that twins have informed our understanding of intelligence, personality, psychopathology, and so much more. Twin studies are not just for twins as some people mistakenly assume; they enable research that allows estimation of genetic and environmental influences on behaviors that apply to the general population.

                                                                                                                                        Cognitive Abilities

                                                                                                                                        Cognitive twin research includes studies on genetic and environmental influences underlying general intelligence, the structure of intelligence, special mental abilities, and information-processing. Studies of twins reared apart have complemented and confirmed findings from studies of twins reared together, as shown clearly by Bouchard and McGue 1981. Genetic components are shown for the recent Verbal-Image-Rotation model (Johnson, et al. 2007), for information processing (Vernon 1989), and for cognition measured via magnetic resonance imaging techniques (Koten, et al. 2009). Twin studies of the elderly reveal genetic and unique environmental effects on cognition (McClearn, et al. 1997), and studies of school-age twins show that twins do not score below nontwins, challenging older data suggesting that they do (Christensen, et al. 2006).

                                                                                                                                        • Bouchard, Thomas J., Jr., and Matt McGue. 1981. Familial studies of intelligence: A review. Science 212:1055–1059.

                                                                                                                                          DOI: 10.1126/science.7195071Save Citation »Export Citation »E-mail Citation »

                                                                                                                                          A meta-analysis examining IQ correlations from 111 studies for kinships varying in genetic and environmental relatedness, including twins (reared apart and together), biological and adoptive siblings, biological and adoptive parents and children, and spouses. IQ correlations generally varied positively with genetic relatedness, providing evidence of genetic contributions to individual differences in intelligence.

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                                                                                                                                          • Christensen, Kaare, Inge Petersen, Axel Skytthe, Anne Maria Herskind, Matt McGue, and Paul Bingley. 2006. Comparison of academic performance of twins and singletons in adolescence: Follow-up study. British Medical Journal 333:1095.

                                                                                                                                            DOI: 10.1136/bmj.38959.650903.7CSave Citation »Export Citation »E-mail Citation »

                                                                                                                                            Early twin studies of IQ suggested that twins scored below nontwins, on average. In contrast, this more recent Danish study did not detect twin-singleton differences in IQ. The difference may be due to the fact that the twins were drawn from a nationwide twin register, thereby avoiding selection biases.

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                                                                                                                                            • Johnson, Wendy, Thomas J. Bouchard Jr., Matt McGue, et al. 2007. Genetic and environmental influences on the Verbal-Perceptual-Image Rotation (VPR) model of the structure of mental abilities in the Minnesota Study of Twins Reared Apart. Intelligence 35:542–562.

                                                                                                                                              DOI: 10.1016/j.intell.2006.10.003Save Citation »Export Citation »E-mail Citation »

                                                                                                                                              A constructive replication of the Verbal-Perceptual-Image Rotation model of intelligence. The model is hierarchical in nature, with g (general intelligence) at the top, followed by broad verbal, perceptual, and image rotation abilities that contribute to eight specialized skills.

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                                                                                                                                              • Koten, Jan Willem, Jr., Guilherme Wood, Peter Hagoort, et al. 2009. Genetic contribution to variation in cognitive function: An fMRI study in twins. Science 323:1737–1740.

                                                                                                                                                DOI: 10.1126/science.1167371Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                A twin study reporting genetic influence on neural networks associated with cognitive functioning. Functional magnetic resonance imaging (fMRI) showed that twins activating frontal-parietal networks reacted more quickly while working on digit memory tasks than twins activating language-related brain regions. Genetic influences on individual differences in brain activation patterns were demonstrated that may result in differences in cognitive processing.

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                                                                                                                                                • McClearn, Gerald E., B. Johansson, S. Berg, et al. 1997. Substantial genetic influence on cognitive abilities in twins 80 or more years old. Science 276:1560–1563.

                                                                                                                                                  DOI: 10.1126/science.276.5318.1560Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                  Reports substantial genetic influence (62 percent) on general cognitive ability in Swedish twins aged eighty years, or older. Environmental experiences unique to individual twins also affected performance in the later years.

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                                                                                                                                                  • Vernon, Philip A. 1989. The heritability of measures of speed of information-processing. Personality and Individual Differences 10.5: 573–576.

                                                                                                                                                    DOI: 10.1016/0191-8869(89)90040-8Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                    A twin study showing genetic influence on eleven speed of information-processing measures. Estimates of genetic influence (heritabilities) ranged from 0.24 to 0.90 for mental speed measures, and from 0.41 to 0.98 for consistency of processing measures. Heritabilities of the information-processing measures correlated positively with general intelligence, based on g factor scores (r = 0.60).

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                                                                                                                                                    Personality

                                                                                                                                                    One of the most important findings to emerge from twin studies of personality over the last twenty years has been that twins raised apart are as similar as twins raised together. This shows that shared genes, rather than shared environments, are responsible for personality resemblance between relatives. These concepts, while seemingly counterintuitive, are variously clarified in the articles and essays included here. Loehlin and Nichols 1976 show that twins treated alike are not more alike behaviorally than those treated differently. Tellegen, et al. 1988 conducted the first four-group design study (MZA, MZT, DZA, DZT), showing similar personality correlations for MZA and MZT twin pairs. Correspondence between personality self-report and report by others was examined by Borkenau, et al. 2001. Associations between personality and the environment were explored by Vernon, et al. 1997, while Lykken and Tellegen 1996 identified a heritable stable personality component. Summaries and syntheses of extant literature on genetic effects on personality (Bouchard 1994) and family environmental effects (Plomin, et al. 2001) are provided.

                                                                                                                                                    • Borkenau, Peter, Rainer Riemann, Alois Angleitner, and Frank M. Spinath. 2001. Genetic and environmental influences on observed personality: Evidence from the German Observational Study of Adult Twins. Journal of Personality and Social Psychology 80:655–668.

                                                                                                                                                      DOI: 10.1037/0022-3514.80.4.655Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                      An analysis of twin similarity in personality based on judges’ ratings of videotaped behavior. Measures included thirty-five adjective scales organized into the Big Five personality traits of openness, conscientiousness, extraversion, agreeableness, and neuroticism. It was estimated that genetic factors accounted for 40 percent of the variance, with 35 percent explained by nonshared environmental factors and 25 percent explained by shared environmental factors.

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                                                                                                                                                      • Bouchard, T. J., Jr. 1994. Genes, environment and personality. Science 264:1700–1701.

                                                                                                                                                        DOI: 10.1126/science.8209250Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                        Reviews the history of twin research on personality, which indicated 50 percent heritability, and acknowledges benefits of larger twin samples and more powerful analytical tools. A table comparing early twin studies, the Minnesota Study of Twins Reared Apart and twin studies conducted by psychologist John Loehlin, with respect to contributions from genetic and environmental factors to the Big Five personality traits, is presented.

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                                                                                                                                                        • Loehlin, John C., and Robert C. Nichols. 1976. Heredity, environment and personality: A study of 850 sets of twins. Austin: Univ. of Texas Press.

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                                                                                                                                                          A landmark study of personality in adolescent twins. Showed that personality traits have a moderate genetic component and that twins’ environmental similarities (e.g., having been dressed alike as children) do not contribute to personality resemblance between them. These findings have been replicated by subsequent studies.

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                                                                                                                                                          • Lykken, David T., and Auke Tellegen. 1996. Happiness is a stochastic phenomenon. Psychological Science 7:186–189.

                                                                                                                                                            DOI: 10.1111/j.1467-9280.1996.tb00355.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                            Reports a reared apart–reared together twin analysis of happiness, using the well-being scale of the Multidimensional Personality Questionnaire. It was concluded that genetic factors explain 44–53 percent of the variance in well-being, and 80 percent of the variance in the stable component, or set-point, of well-being. Various environmental measures, such as income and marital status, accounted for only a negligible portion of individual differences in happiness.

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                                                                                                                                                            • Plomin, Robert, Kathryn Asbury, and Judith Dunn. 2001. Why are children in the same family so different? Nonshared environment a decade later. Canadian Journal of Psychiatry 46:225–233.

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                                                                                                                                                              Elaborates and extends the thesis developed in a 1987 target paper published in Behavioral and Brain Sciences, namely that nonshared environmental factors explain behavioral differences between children in a family. Little progress has, however, been made in identifying the critical nonshared environmental events and the mechanisms by which they work.

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                                                                                                                                                              • Tellegen, Auke, David T. Lykken, Thomas J. Bouchard Jr., Kimerly J. Wilcox, Nancy L. Segal, and Stephen Rich. 1988. Personality similarity in twins reared apart and together. Journal of Personality and Social Psychology 54:1031–1039.

                                                                                                                                                                DOI: 10.1037/0022-3514.54.6.1031Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                The first study to compare personality similarity in MZ and DZ twins reared apart and together. Administered the Multidimensional Personality Questionnaire that assesses eleven primary personality traits and three higher order factors. Estimates of genetic influence ranged from 0.39 to 0.55 for the primary traits, and from 0.40 to 0.58 for the higher order factors.

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                                                                                                                                                                • Vernon, Philip A., Kerry L. Jang, Jennifer A. Harris, and J. M. McCarthy. 1997. Environmental predictors of personality: A twin and sibling study. Journal of Personality and Social Psychology 72:177–183.

                                                                                                                                                                  DOI: 10.1037/0022-3514.72.1.177Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                  Examined MZ and DZ cotwin and nontwin sibling self-reported differences in personality with reference to self-reported environmental background features. Some, but not all perceptions of the environment showed genetic influence. Cotwin differences in personality dimensions showing the lowest heritabilities showed the largest association with environmental difference effects, such as autonomy and conscientiousness.

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                                                                                                                                                                  Bereavement

                                                                                                                                                                  Sibling loss in general, and particularly twin loss, have been largely overlooked by psychological researchers and bereavement counselors. The two works here, Woodward 2010 and Bryan and Higgins 2002, represent significant attempts to correct this oversight. Studies of surviving twins have also been used to test various evolutionary-based hypotheses concerning social relatedness and affiliation.

                                                                                                                                                                  • Elizabeth, Bryan, and Ronald Higgins, eds. 2002. Special issue—Pre Congress Symposium: Twin loss. Twin Research and Human Genetics 5:145–254.

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                                                                                                                                                                    A special issue on the loss of a twin, resulting from a symposium held at the 2001 International Congress of Twin Studies. Covers topics such as parental coping, adult twin bereavement, peer support networks, and cross-cultural perspectives.

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                                                                                                                                                                    • Woodward, Joan. 2010. The lone twin: Understanding twin bereavement and loss. London: Free Association.

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                                                                                                                                                                      A volume that uniquely addresses the psychological consequences of losing a twin before or shortly after birth, as well as during childhood and adulthood. Includes analyses of experiential aspects of twin loss, as well as interviews with surviving twins.

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                                                                                                                                                                      Behavioral and Learning Problems

                                                                                                                                                                      Twin studies of behavioral and learning problems are of interest at two levels. MZ and DZ twin pairs including one or both affected cotwins can be used to assess genetic and environmental contributions to autism, hyperactivity, language delays, and other conditions. In addition, prenatal and postnatal factors specific to MZ and DZ twinships that may contribute to behavioral and learning problems in multiple birth individuals can be explored. The studies included here cover both areas. Topics include language and communication (Thorpe, et al. 2001), ADHD (Mogensen, et al. 2011), learning disabilities (Plomin and Kovas 2005), reading problems (Wadsworth, et al. 2010), and autism (Bailey, et al. 1995).

                                                                                                                                                                      • Bailey, Anthony, Le Couteur, Gottesman Anne, et al. 1995. Autism as a strongly genetic disorder: Evidence from a British twin study. Psychological Medicine 25:63–78.

                                                                                                                                                                        DOI: 10.1017/S0033291700028099Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                        Reassessment of a British twin sample studied previously and a newly recruited British twin sample. The combined sample showed a 60 percent concordance rate for MZ twins and 0 percent for DZ twins for having autism; these figures changed to 92 percent and 10 percent for MZ and DZ twins, respectively, when examined for an autistic spectrum of behavioral symptoms and abnormalities.

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                                                                                                                                                                        • Mogensen, N., H. Larsson, C. Lundholm, and C. Almqvist. 2011. Association between childhood asthma and ADHD symptoms in adolescence: A prospective population-based twin study. Allergy 66:1224–1230.

                                                                                                                                                                          DOI: 10.1111/j.1398-9995.2011.02648.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                          Explores the relationship of childhood asthma to hyperactivity and inattention using a population-based twin sample from Sweden. Children with asthma were at increased risk for developing symptoms of hyperactivity and inattention during adolescence. Genetic factors explained 68 percent of the correlation between these traits.

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                                                                                                                                                                          • Plomin, Robert, and Yulia Kovas. 2005. Generalist genes and learning disabilities. Psychological Bulletin 131:592–617.

                                                                                                                                                                            DOI: 10.1037/0033-2909.131.4.592Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                            Presents an overview of genetic studies on learning disabilities and concludes that most relevant genes exert general as opposed to specific effects. The argument rests on evidence that genes associated with learning disabilities are the same as those associated with normal variation in ability. In addition, these genes affect abilities in more than one area. The implications of this knowledge are discussed with reference to psychological and genetic research.

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                                                                                                                                                                            • Thorpe, Karen, Rosemary Greenwood, Areana Elvers, and Michael Rutter. 2001. Prevalence and developmental course of “secret language.” International Journal of Language and Communication Disorders 36.1: 43–62.

                                                                                                                                                                              DOI: 10.1080/13682820150217563Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                              Examines the varieties and development of private speech in infant and toddler twins and near-in-age nontwins. Unusual verbal exchange between young twins is acknowledged, but little is known about its forms and progression. Two types of “secret language” were identified: shared understanding and private language. Secret speech persisted among a small group of mostly male twins who also showed reduced mental ability and language functioning.

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                                                                                                                                                                              • Wadsworth, Sally J., Richard K. Olson, and John C. DeFries. 2010. Differential genetic etiology of reading difficulties as a function of IQ: An update. Behavior Genetics 40:751–758.

                                                                                                                                                                                DOI: 10.1007/s10519-010-9349-xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                Examines genetic effects on reading disabilities with reference to IQ. Results suggest that reading problems in high IQ children are primarily due to genetic factors and require intensive treatment. The same genetic factors may be operative among low IQ children, but the proportion of variance accounted for may differ.

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                                                                                                                                                                                Developmental Studies

                                                                                                                                                                                Studies that track MZ and DZ twins over time uniquely illuminate factors affecting the timing and expression of behavioral and physical traits. In contrast, research that relies solely on singleton individuals cannot confidently attach genetic or environmental explanations to developmental change. Classic research includes physical and social development (Burlingham 1952), fear and smiling in infant twins (Freedman 1974) and mental development in infants, children, and adolescents (Wilson 1983). An older behavioral study of schoolage twins is still excellent (Koch 1966). Collections of studies on the social experience of young twins (Thorpe and Danby 2006) and twins and families (Bartels 2007) are informative.

                                                                                                                                                                                • Bartels, Meike, ed. 2007. Twin Research and Human Genetics (Special Section on Longitudinal Twin and Family Studies) 10:1–157.

                                                                                                                                                                                  DOI: 10.1375/twin.10.1.1Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                  Includes a series of papers reporting findings from studies tracking the behavioral development of twins and families over time. The establishment of large international twin registries helped pave the way for such large-scale projects. Specific topics include adolescent development, drinking problems, mathematical skills, and child psychopathology.

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                                                                                                                                                                                  • Burlingham, Dorothy. 1952. Twins: A study of three pairs of identical twins with 30 charts. New York: International Univ. Press.

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                                                                                                                                                                                    A small series of case studies following the growth and development of three young MZ twin pairs. Documents aspects of each twin’s relationship with his or her cotwin.

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                                                                                                                                                                                    • Freedman, Daniel G. 1974. Human infancy: An evolutionary perspective. New York: John Wiley.

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                                                                                                                                                                                      A longitudinal analysis of smiling and the fear of strangers in infant MZ and DZ twins during the first year of life. Analyses of filmed samples of behavior gathered monthly for each twin showed that MZ twins were more alike than DZ twins, demonstrating genetic effects. Exceptions emerged among a pair of MZ twins that could not be resolved by birth events or parenting practices.

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                                                                                                                                                                                      • Koch, Helen L. 1966. Twins and twin relations. Chicago: Univ. of Chicago Press.

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                                                                                                                                                                                        A comprehensive study of mental ability, personality traits, physical characteristics, and social relationships in five-year-old MZ, same-sex DZ, and opposite-sex DZ twin pairs. The focus was not on genetic contributions to behavior, but on the differences between twins and nontwins, and the behavioral differences between MZ and DZ twin pairs.

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                                                                                                                                                                                        • Thorpe, Karen, and S. Danby, eds. 2006. Twin Research and Human Genetics (Special Section on the Social Worlds of Children Who Are Twins) 9:90–175.

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                                                                                                                                                                                          Includes a series of papers addressing the special developmental circumstances of twins, for example, rearing with another same-age child and its limiting or enhancing effects on social experience. Topics include relationship negotiation, problem behaviors, social identity, and twin language.

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                                                                                                                                                                                          • Wilson, Ronald S. 1983. The Louisville Twin Study: Developmental synchronies in behavior. Child Development 14:298–316.

                                                                                                                                                                                            DOI: 10.2307/1129693Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                            A landmark study in the history of twin research that tracked the mental development of MZ and DZ twins from infancy through adolescence. MZ twins grew increasingly alike with matching developmental changes, while DZ twins declined in resemblance after age 4 years, eventually showing an intermediate level of resemblance after age 6 years. By age 15 years, their level of resemblance was similar to that of full siblings and parent-child pairs. It was concluded that “spurts” and “lags” in mental development are genetically influenced.

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                                                                                                                                                                                            Interests, Attitudes, and Values

                                                                                                                                                                                            Older twin and adoption research had yielded evidence of genetic influence on interests, but researchers had long believed that attitudes and values were acquired from the rearing family. The prevailing wisdom changed dramatically with the publication of Martin, et al. 1986. The first study on job satisfaction, based on MZ twins reared apart, drew considerable attention (Arvey, et al. 1989). Genetic factors have since been shown to affect social attitudes (Tesser 1993), job satisfaction (Moloney, et al. 1991), and religious views (Waller, et al. 1990; Koenig, et al. 2005). It is suspected, but not confirmed, that the underlying mechanisms by which the relevant genetic factors are expressed operate via personality traits.

                                                                                                                                                                                            • Arvey, Richard D., Thomas J. Bouchard Jr., Nancy L. Segal, and Lauren Abraham. 1989. Job satisfaction: Environmental and genetic components. Journal of Applied Psychology 74:187–192.

                                                                                                                                                                                              DOI: 10.1037/0021-9010.74.2.187Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                              The first twin study of job satisfaction using mostly female participants from the Minnesota Study of Twins Reared Apart. Twins completed the short form of the Minnesota Job Satisfaction Questionnaire that includes items organized into extrinsic, intrinsic, and general satisfaction scales. Genetic influence accounted for 30 percent of the variance in job satisfaction, a result that did not change after controlling for various job characteristics such as complexity and physical demands.

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                                                                                                                                                                                              • Koenig, Laura B., Matt McGue, Robert F. Krueger, and Thomas J. Bouchard Jr. 2005. Genetic and environmental influences on religiousness: Findings for retrospective and current religiousness ratings. Journal of Personality 73:471–488.

                                                                                                                                                                                                DOI: 10.1111/j.1467-6494.2005.00316.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                Twins were assessed (retrospectively) with reference to their attitudes in adolescence, as well as in the current time frame. MZ and DZ twins showed similar levels of resemblance during adolescence, demonstrating little genetic influence and a stronger role for shared environmental factors. In contrast, MZ twins showed greater resemblance than DZ twins in the current (adult) time frame. Heritability of religiosity appears to increase from adolescence to adulthood.

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                                                                                                                                                                                                • Martin, Nicholas G., Lindon J. Eaves, Andrew C. Heath, Rosemary Jardine, Lynn M. Feingold, and Hans J. Eysenck. 1986. Transmission of social attitudes. Proceedings of the National Academy of Sciences 83:4364–4368.

                                                                                                                                                                                                  DOI: 10.1073/pnas.83.12.4364Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                  Presents findings from studies that assessed twins from registries in Australia and Great Britain. Australian twins completed the Wilson-Patterson Conservatism Scale and the British twins completed a Public Opinion Inventory. Sex differences in the contribution of genetic and environmental factors to social attitudes were only found in the British sample: genetic factors contributed more strongly to radicalism in males and to tough-mindedness in females.

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                                                                                                                                                                                                  • Moloney, Daniel P., Thomas J. Bouchard Jr., and Nancy L. Segal. 1991. A genetic and environmental analysis of the vocational interests of monozygotic and dizygotic twins reared apart. Journal of Vocational Behavior 39:76–109.

                                                                                                                                                                                                    DOI: 10.1016/0001-8791(91)90005-7Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                    Examines genetic and environmental influences on vocational interests using MZ and DZ twins reared apart who completed the Strong Vocational Interest Black-Strong Campbell Interest Inventory and the Jackson Vocational Interest Survey. Genetic influence explained 45–50 percent of the individual differences in vocational interests, while associations between the interest measures and family environment measures were negligible.

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                                                                                                                                                                                                    • Tesser, Abraham. 1993. On the importance of heritability in psychological research: The case of attitudes. Psychological Review 100:129–142.

                                                                                                                                                                                                      DOI: 10.1037/0033-295X.100.1.129Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                      A seminal essay by a social psychologist that describes findings of genetic influence on attitudes. Upon organizing attitudes as a function of degree of heritability, the author found that those with higher heritabilities were harder to change, were responded to more rapidly, and were more significant for interpersonal attraction based on attitude resemblance. This paper was important for acquainting social psychologists with the effects of genetic factors on social attitudes.

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                                                                                                                                                                                                      • Waller, Niels G., Brian A. Kojetin, Thomas J. Bouchard Jr., David T. Lykken, and Auke Tellegen. 1990. Genetic and environmental influences on religious interests, attitudes and values: A study of twins reared apart and together. Psychological Science 1:138–142.

                                                                                                                                                                                                        DOI: 10.1111/j.1467-9280.1990.tb00083.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                        Information on religious interests and values was available from five different inventories for Minnesota Study of Twins Reared Apart participants, and from two of these inventories for Minnesota Twin Registry participants. Genetic influence explained 50 percent of the variance in religiosity. This paper challenged the prevailing wisdom that religious behaviors were acquired mostly from the rearing family, leading to additional twin studies in this area.

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                                                                                                                                                                                                        Twin Relationships

                                                                                                                                                                                                        Twin studies teach us a great deal about factors affecting social relatedness within families. The greater social closeness between MZ than DZ twins is well known, but the reasons are less clear. Studies in this section address cooperation and competition (Von Bracken 1934, Loh and Elliott 1998), adult twin closeness (Neyer 2002), attachment (Tancredy and Fraley 2006), and choice of friends and mates (Rushton and Bons 2005).

                                                                                                                                                                                                        • Loh, C. Y., and J. M. Elliott. 1998. Cooperation and competition as a function of zygosity in 7- to 9-year-old twins. Evolution and Human Behavior 19:397–411.

                                                                                                                                                                                                          DOI: 10.1016/S1090-5138(98)00034-8Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                          The cooperative and competitive behaviors of young twins in Singapore were compared in game situations that either assured equality of outcome or did not. MZ twins were more cooperative than DZ twins when reward equality was uncertain. In contrast, MZ twins competed to a greater degree than DZ twins when matched outcomes were provided. An equal rewards situation may have offered MZ twins a chance for dominance testing.

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                                                                                                                                                                                                          • David T., and Auke Tellegen. 1993. Is human mating adventitious or the result of lawful choice? A twin study of mate selection. Psychological Science 65:56–68.

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                                                                                                                                                                                                            MZ and DZ reared together twins completed personality and interest inventories, and indicated their initial attraction to their cotwin’s spouse or partner. Partner similarity across seventy-four mostly psychological variables could not explain partner choice. Few MZ twins were attracted to their cotwin’s mate and few nontwin spouses were attracted to their partner’s MZ twin.

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                                                                                                                                                                                                            • Neyer, Franz J. 2002. Twin relationships in old age: A development perspective. Journal of Personality and Social Relationships 19:155–177.

                                                                                                                                                                                                              DOI: 10.1177/0265407502192001Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                              Compared degree of social relatedness between elderly MZ and DZ twins and between twins and their nontwin siblings. Contact and closeness decreased over time for both types of twins during early adulthood but increased in later years. Twin relations were closer between twins than nontwins and closer between MZ twins than DZ twins. Social closeness was independent of contact frequency in MZ twins only.

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                                                                                                                                                                                                              • Rushton, J. Philippe, and Trudy A. Bons. 2005. Mate choice and friendship in twins: Evidence for genetic similarity. Psychological Science 16:555–559.

                                                                                                                                                                                                                DOI: 10.1111/j.0956-7976.2005.01574.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                Reports evidence of personality, attitudinal, and demographic similarities between the friends and spouses of MZ twins, thus challenging Lykken and Tellegen’s findings reported directly above. Partner similarity for specific measures increased as a function of the heritability of those measures.

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                                                                                                                                                                                                                • Tancredy, Caroline M., and R. Christopher Fraley. 2006. The nature of adult twin relationships: An attachment-theoretical perspective. Journal of Personality and Social Psychology 90:78–93.

                                                                                                                                                                                                                  DOI: 10.1037/0022-3514.90.1.78Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                  Proposes that attachment processes differ between twins and nontwins in that cotwins are more likely to serve as attachment figures for one another than are nontwin siblings. The effects of genetic relatedness and shared experiences on twin relationships are also considered, although greater distinction between the types of relationships characteristic of the two twin types could have been made.

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                                                                                                                                                                                                                  • Von Bracken, Helmut. 1934. Mutual intimacy in twins. Character and Personality 2:293–309.

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                                                                                                                                                                                                                    Twin children were observed working on arithmetic and coding tasks, both in close proximity to one another and in isolation. MZ twins coordinated their behavior when they were together, whereas DZ twins either competed strongly or did not put forth much effort. The effects of genetic relatedness on partner interaction were demonstrated.

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                                                                                                                                                                                                                    Sexual Behavior and Orientation

                                                                                                                                                                                                                    The roots of sexual behavior and orientation are timely and controversial topics. Twin research conducted in the early 1990s estimated that genetic factors explained about 50 percent of individual differences in sexual orientation. Recent studies have revised that figure downward, while still showing greater genetic influence among males than females (Bailey, et al. 2000; Långström, et al. 2010). This section also includes studies of age at first intercourse (Waldron, et al. 2007) and sexual reassignment in a unique case of twin infants (Diamond and Sigmundsson 1997).

                                                                                                                                                                                                                    • Bailey, J. Michael, Michael P. Dunne, and Nicholas G. Martin. 2000. Genetic and environmental influences on sexual orientation and its correlates in an Australian twin sample. Journal of Personality and Social Psychology 78:524–536.

                                                                                                                                                                                                                      DOI: 10.1037/0022-3514.78.3.524Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                      An anonymous study of genetic and environmental influences on sexual orientation. Sexual orientation was assessed by the Kinsey Scale and childhood gender nonconformity was assessed by items drawn from published scales. MZ twin concordance rates for sexual orientation were lower in the present study (males: 0.20, females: 0.24) than in previous studies (males: 0.47, females: 0.48).

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                                                                                                                                                                                                                      • Diamond, Milton, and Keith Sigmundsson. 1997. Sex reassignment at birth: Long-term review and clinical implications. Archives of Pediatrics and Adolescent Medicine 15:298–304.

                                                                                                                                                                                                                        DOI: 10.1001/archpedi.1997.02170400084015Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                        The parents of an MZ male twin who was accidentally castrated at age six months following circumcision were advised by Johns Hopkins psychologist John Money to change their son into a girl. Brief reports of this case in the ensuing years suggested that the treatment was successful, but the opposite was true. Sigmundsson, who worked with the child, and his family collaborated with Diamond to tell the true story in full.

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                                                                                                                                                                                                                        • Långström, Niklas, Qazi Rahman, Eva Carlström, and Paul Lichtenstein. 2010. Genetic and environmental effects on same-sex sexual behavior: A population study of twins in Sweden. Archives of Sexual Behavior 39:75–80.

                                                                                                                                                                                                                          DOI: 10.1007/s10508-008-9386-1Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                          The largest twin study of sexual orientation conducted to date using a population-based Swedish sample. Genetic effects were larger for males (0.34–0.39) than for females (0.18–0.19), consistent with most previous studies. Nonshared environmental influences played a large role for both sexes, whereas shared environmental effects were absent for males and minimal for females.

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                                                                                                                                                                                                                          • Waldron, Mary, Andrew C. Heath, Eric Turkheimer, et al. 2007. Age at first intercourse and teenage pregnancy in Australian female twins. Twin Research and Human Genetics 10.3: 440–449.

                                                                                                                                                                                                                            DOI: 10.1375/twin.10.3.440Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                            Greater genetic effects on sexual behaviors were observed among an older cohort (born 1893–1964), and were common to early pregnancy and age at intercourse. In contrast, shared environmental effects on sexual behaviors were more important for a younger cohort (born 1964–1971) and were shared by early pregnancy and intercourse.

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                                                                                                                                                                                                                            Psychiatric Disorders

                                                                                                                                                                                                                            The conventional wisdom of the 1960s was that atypical family dynamics were responsible for psychopathology in children and other family members. Twin studies, backed up by adoption strategies, have significantly changed that view, such that few would deny the important contribution of genetic factors to schizophrenia (Gottesman and Shields 1972, Gottesman and Bertelsen 1989), anxiety (Tambs, et al. 2011), alcohol (Prescott and Kendler 1999), personality disorders (Reichborn-Kjennerud, et al. 2007), suicide (Voracek and Loibl 2007), and many other conditions. Nevertheless, MZ twin similarity is far from perfect; only about 40–50 percent of MZ twins with a schizophrenic cotwin are themselves affected. These discordant pairs give scientists opportunities to study nongenetic factors that trigger the disease in one twin and silence it in the other. This information can enlighten the greater population of nontwin individuals with psychiatrically diagnosed relatives.

                                                                                                                                                                                                                            • Gottesman, Irving I., and Axel Bertelsen. 1989. Confirming unexpressed genotypes for schizophrenia: Risks in the offspring of Fischer’s Danish identical and fraternal discordant twins. Archives of General Psychiatry 46:867–872.

                                                                                                                                                                                                                              DOI: 10.1001/archpsyc.1989.01810100009002Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                              Disorder was examined in the children of MZ and DZ twins discordant for schizophrenia. The children of MZ twins were equally at risk regardless of being born to a well or affected parent, whereas only the children born to an affected DZ twin were at the same high-risk level seen in the offspring of identicals. Unexpressed genetic predispositions can be transmitted to children, and rearing by an affected parent is not requisite to disease occurrence in a child.

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                                                                                                                                                                                                                              • Gottesman, Irving I., and James Shields. 1972. Schizophrenia and genetics: A twin study vantage point. New York: Academic Press.

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                                                                                                                                                                                                                                An analysis of schizophrenia in twenty-four MZ and thirty-three DZ twin pairs that has been called the definitive book in the field. This study, which began in 1962, included all same-sex twins admitted to London’s Maudsley and Bethlem Hospitals over a sixteen-year period. Evidence of genetic and environmental influences on the onset and progression of the disorder are examined.

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                                                                                                                                                                                                                                • Prescott, Carol A., and Kenneth S. Kendler. 1999. Genetic and environmental contributions to alcohol abuse and dependence in a population-based sample of male twins. American Journal of Psychiatry 156:34–40.

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                                                                                                                                                                                                                                  The first population-based twin study of alcohol abuse. Data were obtained from structured interviews administered to 3,012 males from twin, triplet, and quadruplet sets. Genetic influence on alcohol abuse was estimated as 48–58 percent and, contrary to some previous studies, the shared environment played little role.

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                                                                                                                                                                                                                                  • Reichborn-Kjennerud, T., N. Czajkowski, Svenn Torgersen, et al. 2007. The relationship between avoidant personality disorder and social phobia: A population-based twin study. American Journal of Psychiatry 164:1722–1728.

                                                                                                                                                                                                                                    DOI: 10.1176/appi.ajp.2007.06101764Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                    A twin study examining genetic and environmental factors common to avoidant personality disorder and social phobia. The twin participants were members of same-sex female pairs from the Norwegian Institute of Public Health Twin Panel. Genetic factors common to the two conditions were detected but relevant environmental factors were unique to each.

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                                                                                                                                                                                                                                    • Tambs, Kristian, S. Kendler Kenneth, T. Reichborn-Kjennerud, et al. 2011. Genetic and environmental contributions to the relationship between education and anxiety disorders: A twin study. Acta Psychiatrica Scandinavica 125: 203–212.

                                                                                                                                                                                                                                      DOI: 10.1111/j.1600-0447.2011.01799Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                      Examines the extent to which the negative relationship between educational level and anxiety disorders may be explained by genetic factors. Participants included 3,339 twin pairs from the Norwegian Twin Registry. A genetic link between low education and anxiety disorders was indicated.

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                                                                                                                                                                                                                                      • Voracek, M., and L. M. Loibl. 2007. Genetics of suicide: A systematic review of twin studies. Wiener Klinsche Wochenschrift 119.15–16: 463–475.

                                                                                                                                                                                                                                        DOI: 10.1007/s00508-007-0823-2Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                        Reviews nineteen reports, five twin-register studies, four epidemiological studies, and four studies of surviving co-twins with reference to suicidal behavior. Comparison of concordance rates for MZ and DZ twins revealed a significant genetic component, estimated to fall between 30 and 55 percent for the population-based studies. Nonshared environmental influences were also considered important for possibly contributing to suicidal risk.

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                                                                                                                                                                                                                                        Neurological Disorders

                                                                                                                                                                                                                                        Neurological disorders involve conditions including the brain, nerves, and spinal cord. Twin studies have examined genetic and environmental effects on Alzheimer’s disease (Gatz, et al. 2005), Parkinson’s disease (Goldman, et al. 2012), cerebral palsy (Pharoah, et al. 2002), and various sleep disorders (Desai, et al. 2004).

                                                                                                                                                                                                                                        • Desai, A. V., S. F. Cherkas, T. D. Spector, and A. J. Williams. 2004. Genetic influences in self-reported symptoms of obstructive sleep apnea and restless legs: A twin study. Twin Research 7:589–595.

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                                                                                                                                                                                                                                          A study of sleep apnea and restless leg syndrome in female twins. Genetic effects on both conditions were suggested by the higher concordance rates for MZ than DZ twins pairs. Heritability estimates were 52 percent for disruptive snoring and 54 percent for restless legs.

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                                                                                                                                                                                                                                          • Gatz, M., L. Fratiglioni, B. Johansson, et al. 2005. Complete ascertainment of dementia in the Swedish Twin Registry: The HARMONY study. Neurobiology of Aging 26:439–447.

                                                                                                                                                                                                                                            DOI: 10.1016/j.neurobiolaging.2004.04.004Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                            A study of dementia in Swedish Twins (HARMONY). Concordance rates for Alzheimer’s disease were 59 percent for monozygotic twins, 32 percent for like-sexed, and 24 percent for unlike-sexed dizygotic twins. Among monozygotic twins where both twins had Alzheimer’s disease, the within-pair difference in age of onset ranged from zero to seven years difference in onset.

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                                                                                                                                                                                                                                            • Goldman, S. M., P. J. Quinlan, W. Ross, et al. 2012. Solvent exposure and Parkinson disease risk in twins. Annals of Neurology 71: 776–784.

                                                                                                                                                                                                                                              DOI: 10.1022/ana.22629Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                              The first population-based study of possible links between solvent exposure and Parkinson’s Disease (PD), using twins. Risk for PD was highest among individuals with exposure to selected solvents, as determined by occupational histories.

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                                                                                                                                                                                                                                              • Pharoah, P. O., T. S. Price, and R. Plomin. 2002. Cerebral palsy in twins: A national study. Archives of Diseases 87:122–124.

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                                                                                                                                                                                                                                                Twins are more likely to suffer from cerebral palsy (CP) than nontwins. Like-sex twin survivors are somewhat more likely to exhibit the disease than members of opposite-sex twin pairs. Monozygotic twins, especially those with single chorions, may be especially susceptible to CP.

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                                                                                                                                                                                                                                                Economic and Political Behavior

                                                                                                                                                                                                                                                The usefulness of twin studies has been recently recognized by researchers interested in economics and politics. The specific concerns are factors affecting choices in decision-making situations posed by various non-zero sum games (Cesarini, et al. 2008), tacit coordination tasks (Segal, et al. 2008), the bases of political attitudes and beliefs (Alford, et al. 2005; Settle, et al. 2009). Applying standard twin designs and analytical methods, researchers have detected genetic influence on behaviors long thought to be acquired from the rearing family.

                                                                                                                                                                                                                                                • Alford, John R., Carolyn L. Funk, and John R. Hibbing. 2005. Are political orientations genetically transmitted? American Political Science Review 99:153–167.

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                                                                                                                                                                                                                                                  A twin study of political attitudes and affiliations. Genetic influence was found for political attitudes (43 percent), with a more modest role played by genetic factors on party affiliation (14 percent). A genetic perspective was generally not considered by political scientists at that time, but they were urged to incorporate this view into subsequent research programs. Genetic perspectives are gaining attention in current political science research.

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                                                                                                                                                                                                                                                  • Cesarini, David, Christopher T. Dawes, James H. Fowler, Magnus Johannesson, Paul Lichtenstein, and Björn Wallace. 2008. Heritability of cooperative behavior in the trust game. Proceedings of the National Academy of Sciences 105:3721–3726.

                                                                                                                                                                                                                                                    DOI: 10.1073/pnas.0710069105Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                    A study of cooperative behavior in American and Swedish twins during a widely used experimental game. Results showed greater resemblance among MZ than DZ twins in decisions regarding investment and reciprocal investment, demonstrating genetic influence. However, cotwins were paired with unrelated individuals, not with each other, precluding analysis of twin relationships in this experimental game situation.

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                                                                                                                                                                                                                                                    • Segal, Nancy L., Shirley A. McGuire, Steven Miller, and June Havlena. 2008. Tacit coordination in monozygotic twins, dizygotic twins and virtual twins: Effects and implications of genetic relatedness. Personality and Individual Differences 45:607–612.

                                                                                                                                                                                                                                                      DOI: 10.1016/j.paid.2008.06.022Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                      Reports genetic influence on tacit coordination (non-negotiated consensus), that is, ability for partners to provide the same solution when so instructed, but in the absence of communication. MZ, DZ, and virtual twins (VTs, same-age unrelated siblings) completed twenty questions individually and with the instruction to “produce the same answer as your co-twin.” MZ twins outperformed DZ twins who, in turn, outperformed VT twins under both conditions.

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                                                                                                                                                                                                                                                      • Settle, Jaime E., Christopher T. Dawes, and James H. Fowler. 2009. The heritability of partisan attachment. Political Research Quarterly 62:601–613.

                                                                                                                                                                                                                                                        DOI: 10.1177/1065912908327607Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                        A twin study of the strength of partisan attachment or party identification. Twins were identified at the annual 2006 and 2007 Twins Days Festival in Twinsburg, Ohio. Partisanship strength was assessed by questionnaire and yielded a 46 percent estimate of genetic influence. Such findings urge revision of the origin and progression of political ideologies and behavior.

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                                                                                                                                                                                                                                                        Medicine and Health

                                                                                                                                                                                                                                                        Twin studies are ideal for assessing genetic and environmental contributions to a wide range of medical, health, and other physical characteristics. Many people suppose that MZ twins are exactly alike in such measures, but this is not the case. MZ twins discordant for medical and health-related traits (such as disease, diet, and body build) allow a look into nongenetic factors that trigger or suppress a given condition. In addition, advances in our understanding of heritable changes in gene expression and the structure and function of genes at the molecular level have given rise to new studies involving affected and unaffected twins. Outcomes from these investigations promise to further understanding and prevention of detrimental diseases among the general population.

                                                                                                                                                                                                                                                        Body Size and Structure

                                                                                                                                                                                                                                                        This section on body size and structure surveys factors affecting standard (e.g., body mass index) and novel measures (e.g., fluctuating asymmetry) in a variety of ways. A novel kinship, that is, virtual twins (same-age unrelated siblings reared together) is included in some analyses for the first time. Specific topics include effects of overeating on body weight (Bouchard, et al. 1990), factors affecting body mass index (Segal, et al. 2008; Allison, et al. 1996; Mustelin, et al. 2009), and the causes of obesity (Naukkarinen, et al. 2011). Other work on body size and structure include twin studies of fluctuating asymmetry (Johnson, et al. 2008), longevity (Hjelmborg, et al. 2006), and handedness (Medland, et al. 2006).

                                                                                                                                                                                                                                                        • Allison, David B., Jaakko Kaprio, M. Korkeila, Marku Koskenvuo, Michael C. Neale, and Kazuo Hayakawa. 1996. The heritability of BMI among an international sample of monozygotic twins reared apart. International Journal of Obesity 20:501–506.

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                                                                                                                                                                                                                                                          Analysis of body mass index (BMI) for fifty-three MZA twin pairs. Participants were twins from registries in Finland and Japan, and from published case studies. BMI showed an estimated heritability of 0.79 for all twins, and ranged between 0.63 (Finland) and 0.85 (published case studies) for the three subsamples. Findings were consistent with those from previous MZA studies.

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                                                                                                                                                                                                                                                          • Bouchard, Claude, Angelo Tremblay, Jean-Pierre Deprés, et al. 1990. Response to long-term overfeeding in identical twins. New England Journal of Medicine 322:1477–1482.

                                                                                                                                                                                                                                                            DOI: 10.1056/NEJM199005243222101Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                            Analysis of weight gain in twelve Canadian identical male twin pairs maintained on a high-calorie diet and no exercise for twelve weeks. The average weight gain was 17.8 pounds but ranged between 9.5 pounds and 29.2 pounds for individual pairs. Importantly, weight gain was most similar within pairs, not between pairs, demonstrating genetic influence on the body’s response to overeating.

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                                                                                                                                                                                                                                                            • Hjelmborg, Jacob V. B., Ivan Iachine, Axel Skytthe, et al. 2006. Genetic influence on human lifespan and longevity. Human Genetics 119.3: 312–321.

                                                                                                                                                                                                                                                              DOI: 10.1007/s00439-006-0144-ySave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                              A study of genetic influence on life-span using Danish, Swedish, and Finnish twins. Mean life span for MZ and DZ twins increased by 0.39 and 0.21, respectively, for every year that the cotwin survived past age sixty, demonstrating genetic influence. Having a cotwin live to an elderly age increases the chance of the cotwin doing so, but to a greater degree for MZ twins.

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                                                                                                                                                                                                                                                              • Johnson, Wendy, Steven W. Gangestad, Nancy L. Segal, and Thomas J. Bouchard Jr. 2008. Heritability of fluctuating asymmetry in a human twin sample: The effect of trait aggregation. American Journal of Human Biology 20:651–658.

                                                                                                                                                                                                                                                                DOI: 10.1002/ajhb.20788Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                A study of fluctuating asymmetry (FA or departure from body symmetry in bilateral traits, for example, wrist breadth and knee height) in twins reared apart. Most studies had reported low heritability for FA, but did so using single traits. The present study reported a 0.30 heritability for a ten-trait composite, challenging previous findings.

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                                                                                                                                                                                                                                                                • Medland, Sarah E., David L. Duffy, M. J. Wright, G. M. Geffen, and Nicholas G. Martin. 2006. Handedness in twins: Joint analysis of data from 35 samples. Twin Research and Human Genetics 9:46–53.

                                                                                                                                                                                                                                                                  DOI: 10.1375/twin.9.1.46Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                  Hand preference is one of the few physical measures not suited to analysis by conventional twin studies because some factors affecting left-handedness in twins (especially MZ twins, e.g., zygotic splitting) do not occur among nontwins. It is estimated that about 20–25 percent of both MZ and DZ twin pairs differ in hand preference. This study summarizes a large number of existing studies in order to report more reliable figures.

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                                                                                                                                                                                                                                                                  • Mustelin, L., K. Silventoinen, K. Pietiläinen, A. Rissanen, and Jaakko Kaprio. 2009. Physical activity reduces the influence of genetic effects on BMI and waist circumference: A study in young adult twins. International Journal of Obesity 33.1: 29–36.

                                                                                                                                                                                                                                                                    DOI: 10.1038/ijo.2008.258Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                    The first study to use the Purcell type of moderation models to demonstrate that physical activity influences heritable effects on BMI. Large meta-analyses have now convincingly shown that physical activity reduces the association of FTO (the strongest obesity gene) with BMI by 27 percent; see the International Journal of Epidemiology, 2011.

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                                                                                                                                                                                                                                                                    • Naukkarinen, J., A. Rissanen, Jaakko Kaprio, and K. H. Pietiläinen. 2011. Causes and consequences of obesity: The contribution of recent twin studies. International Journal of Obesity 8: 1,017–1,024

                                                                                                                                                                                                                                                                      DOI: 10.1038/ijo.2011.192Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                      A review of approximately ten years of work that identifies and characterizes a rare set of MZ pairs discordant for obesity. The twins were investigated using a wide range of modern clinical medical and omics tools. Ongoing work not included in the paper covers genomewide epigenetics and the microbial metagenome of the gut, showing the value of twin studies in the postgenomic era.

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                                                                                                                                                                                                                                                                      • Segal, Nancy L., Rui Feng, Shirley A. McGuire, David B. Allison, and Steven Miller. 2008. Genetic and environmental contributions to body mass index: Comparative analysis of monozygotic twins, dizygotic twins and same-age unrelated siblings. International Journal of Obesity 33:37–41.

                                                                                                                                                                                                                                                                        DOI: 10.1038/ijo.2008.228Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                        A study of body mass index using 929 individuals from MZ, DZ, and virtual twin (same-age unrelated siblings) pairs. Nonadditive genetic factors and shared environmental factors were significant in the analytical model. Virtual multiples were considered important in distinguishing nonadditive genetic variance from common environmental variance.

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                                                                                                                                                                                                                                                                        Disease Susceptibility

                                                                                                                                                                                                                                                                        Twin studies have become a common research design for studying medical and dental disease susceptibility. It is sometimes surprising, but highly informative, to note the less-than-perfect MZ twin concordance. Such results offer confidence that by identifying the relevant nongenetic triggering mechanisms, researchers will be able to check the onset and progression of debilitating and tragic human conditions. Genetic factors, possibly associated with antibody combinations, may affect the course and development of type 1 diabetes (Hawa, et al. 1997). Different factors appear to affect onset of leukemia in young and older twins (Greaves, et al. 2003). Adult periodontal disease may reflect genetic factors, mainly host genetic effects (Michalowicz, et al. 1991). Genetic factors are also considered in twin studies of breast cancer by Hamilton and Mack 2003 and in heart disease by Su, et al. 2012. The role of genetic and environmental factors has been examined in Alzheimer’s Disease (Gatz, et al. 2006) and in tuberculosis (Van der Eijk, et al. 2007).

                                                                                                                                                                                                                                                                        • Ann, Hamilton, and Thomas Mack. 2003. Puberty and genetic susceptibility to breast cancer in a case control study in twins. New England Journal of Medicine 343:2313–2322.

                                                                                                                                                                                                                                                                          DOI: 10.1056/NEJMoa021293Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                          Presents within-pair comparisons of MZ and DZ female twins, one or both of whom had breast cancer. When both twins were affected the twin with the earlier onset of puberty was diagnosed first, but was not at increased risk. When one twin was affected a later first pregnancy, fewer pregnancies and later menopause were associated with increased disease risk.

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                                                                                                                                                                                                                                                                          • Gatz, Margaret, Chandra A. Reynolds, L. Fratiglioni, et al. 2006. Role of genes and environments for explaining Alzheimer’s disease. Archives of General Psychiatry 63:168–174.

                                                                                                                                                                                                                                                                            DOI: 10.1001/archpsyc.63.2.168Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                            A study of Alzheimer’s disease (AD) in elderly twins from the Swedish Twin Registry. Genetic influence on AD was estimated to be 50–79 percent, and was constant for males and females. Age at onset was more alike for concordant MZ than DZ twin pairs, demonstrating genetic effects on the timing of the disease.

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                                                                                                                                                                                                                                                                            • Greaves, M. F., A. T. Maia, J. L. Wiemels, and A. M. Ford. 2003. Leukemia in twins: Lessons in natural history. Blood 102:2321–2333.

                                                                                                                                                                                                                                                                              DOI: 10.1182/blood-2002-12-3817Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                              A review of the prenatal and postnatal origins of leukemia in twins and the implications of these findings for nontwins. Disease concordance observed in the majority of infant twin pairs is explained by an early role for chromosomal translocation in one twin and transfer to the other twin via shared placental membranes. Discordance in most older affected twins may reflect leukemia of postnatal origin.

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                                                                                                                                                                                                                                                                              • Hawa, M., R. Rowe, M. S. Lan, et al. 1997. Value of antibodies to islet protein tyrosine phosphatase-like molecule in predicting type 2 diabetes. Diabetes 46:1270–1275.

                                                                                                                                                                                                                                                                                DOI: 10.2337/diabetes.46.8.1270Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                MZ twin pairs selected from the British Diabetic Twin Study were genotyped for HLA-DQB1 and -DRB1 and for diabetes-associated autoantibodies. All antibodies predicted type 1 diabetes, but were also detected in twins at low disease risk. Genetic factors may affect combinations of disease-associated antibodies and the development of type 1 diabetes.

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                                                                                                                                                                                                                                                                                • Michalowicz, Bryan S., Dorothy P. Aeppli, John G. Virag, et al. 1991. Periodontal findings in adult twins. Journal of Periodontology 62:293–299.

                                                                                                                                                                                                                                                                                  DOI: 10.1902/jop.1991.62.5.293Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                  The first study of periodontal measures in twins reared together and apart. Significant genetic components were found for gingivitis, probing depth, attachment loss, and plaque. The role of bacteria in periodontal disease is recognized, yet this paper highlights the important contribution from host genetic influences.

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                                                                                                                                                                                                                                                                                  • Su, S., J. Votaw, T. Faber, et al. 2012. Measurement of heritability of myocardial blood flow by positron emission tomography: The Twins Heart Study. Heart 98: 495–499

                                                                                                                                                                                                                                                                                    DOI: 10.1136/heartjnl-2011-301080Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                    A twin study of genetic influence on myocardial blood flow (MBF) and coronary flow reserve (CFR). Participants included middle-aged MZ and DZ male twin pairs. Genetic influence was detected for MBF, both at rest and during adenosine stress, and for CFR. Future directions include finding the genetic variants for early atherosclerosis.

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                                                                                                                                                                                                                                                                                    • Van der Eijk, E. A., E. van de Vosee, J. P. Vandenbroucke, and J. T. van Dissel. 2007. Heredity versus environment in tuberculosis in twins: The 1950s United Kingdom Prophit Survey: Simonds and Comstock revisited. American Journal of Respiratory and Critical Care Medicine 175:1281–1288.

                                                                                                                                                                                                                                                                                      DOI: 10.1164/rccm.200703-435OCSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                      A paper challenging findings of higher MZ than DZ twin concordance for tuberculosis. Claims that previous work did not consider confounding environmental factors.

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                                                                                                                                                                                                                                                                                      Epigenetics

                                                                                                                                                                                                                                                                                      Epigenetics is the study of heritable changes in gene expression that are unrelated to changes in DNA. That is, events such as methylation and histone modification can alter the way genes are expressed while leaving the DNA itself intact. Epigenetics has given researchers a new domain for understanding medical and behavioral differences between MZ cotwins. This information can then be used to understand what triggers various conditions in nontwins. Studies using twins to assess observed differences possibly linked to epigenetic differences include Fraga, et al. 2005; Wong, et al. 2005; and Kaminsky, et al. 2009. A large-scale epigenetic research program to assess such relationships in newborn twins is described by Saffery, et al. 2011.

                                                                                                                                                                                                                                                                                      • Fraga, Mario F., E. Ballestar, M. F. Paz, et al. 2005. Epigenetic differences arise during the lifetime of monozygotic twins. Proceedings of the National Academy of Sciences 102.30: 10604–10609.

                                                                                                                                                                                                                                                                                        DOI: 10.1073/pnas.0500398102Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                        A study of epigenetic differences in MZ twins that attracted considerable attention to this source of within-pair variation in observable traits. DNA methylation and histone acetylation were compared in forty MZ twin pairs of varying age. The greatest epigenetic differences were found between older twins and twins living apart.

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                                                                                                                                                                                                                                                                                        • Kaminsky, Z. A., T. Tang, S. C. Wang, et al. 2009. DNA methylation profiles in monozygotic and dizygotic twins. Nature Genetics 41:240–245.

                                                                                                                                                                                                                                                                                          DOI: 10.1038/ng.286Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                          The first annotation and comparison of epigenetic changes in the genomic regions of MZ and DZ twins. Greater epigenetic discordance was observed in the buccal epithelial cells of DZ than MZ twins. It is likely that this difference reflects epigenetic variability as well as differences in DNA. Examination of twins’ white blood cells and gut biopsies was also performed. Chorion type was an important determinant of MZ similarity.

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                                                                                                                                                                                                                                                                                          • Saffery, Richard, Ruth Morley, B. John, et al. 2011. Cohort profile: The peri/post-natal epigenetic twins study. International Journal of Epidemiology 41: 55–61.

                                                                                                                                                                                                                                                                                            DOI: 10.1093/ije/dyr140Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                            An antenatally recruited longitudinal twin cohort study of early life environmental effects on the epigenomes of various tissues collected at birth and subsequently. The focus is on epigenetic mechanisms associated with low birth weight and postnatal risk factors for chronic disease. The epigenetic state of genes involved in metabolism, growth, and cardiovascular function correlate with birth weight, suggesting a mechanistic link with programmed risk for later chronic disease.

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                                                                                                                                                                                                                                                                                            • Wong, A. H. C., Irving I. Gottesman, and Art Petronis. 2005. Phenotypic differences in genetically identical organisms: The epigenetic perspective. Human Molecular Genetics 14 (suppl. 1): 11–18.

                                                                                                                                                                                                                                                                                              DOI: 10.1093/hmg/ddi116Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                              Reviews research showing the importance of epigenetic variation as a possible explanation of MZ cotwin differences in measured traits, especially complex diseases. The authors call for reconciliation of these findings with those from the Minnesota Study of Twins Reared Apart that found similar levels of resemblance in personality, social attitudes, and other behaviors in MZ twins reared apart and together.

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                                                                                                                                                                                                                                                                                              Molecular Genetics

                                                                                                                                                                                                                                                                                              Molecular genetics is the study of the structure and function of genes at the molecular level. Following completion of the Human Genome Project in 2003, scientists began more intensive searches for genes linked to specific diseases and other characteristics, a pursuit for which twin participants are well suited. Advanced techniques and international collaborations among twin researchers have furthered this effort, although progress has been slower than anticipated initially. The papers in this section cover the search for genes linked to dizygotic twinning (Painter, et al. 2010), monozygotic twin differences (Bruder, et al. 2008), molecular genetic research designs (Boomsma, et al. 2008), associations between candidate genes linked to clinical traits (Busjahn, et al. 1999), and relationships between violence exposure and telomere erosion in young twins (Shalev, et al. 2012).

                                                                                                                                                                                                                                                                                              • Boomsma, Dorret I., G. Willemsen, J. M. Vink, et al. 2008. Design and implementation of a twin-family database for behavior genetics and genomics studies. Twin Research and Human Genetics 11:342–348.

                                                                                                                                                                                                                                                                                                DOI: 10.1375/twin.11.3.342Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                Describes features of a database to accommodate extended families that include multiple twin sets, complex genetic and environmental relationships and changing associations among the subjects. Additional features include options for data storage, contact methods, and procedures for advanced queries.

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                                                                                                                                                                                                                                                                                                • Bruder, C. E. G., A. Piotrowski, A. A. C. J. Gijsbers, et al. 2008. Phenotypically concordant and discordant monozygotic twins display different DNA copy-number-variation profiles. American Journal of Human Genetics 82:763–771.

                                                                                                                                                                                                                                                                                                  DOI: 10.1016/j.ajhg.2007.12.011Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                  Presents an analysis of copy number variations (CNVs or changes in DNA leading to atypical numbers of copies of DNA segments) in nine MZ twin pairs discordant for Parkinson’s disease and ten pairs of normal unaffected MZ twin pairs. CNVs were found in both twin groups, suggesting a link between disease discordance and CNVs, but also cautioning that CNVs may be identified in routine analyses of single tissues.

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                                                                                                                                                                                                                                                                                                  • Busjahn, Andreas, H. Knoblauch, H. D. Faulhaber, et al. 1999. QT interval is linked to 2 long-QT syndrome loci in normal subjects. Circulation 99:3161–3164.

                                                                                                                                                                                                                                                                                                    DOI: 10.1161/01.CIR.99.24.3161Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                    Established the heritability of a relevant clinical trait and also provided evidence for linkage with candidate genes. The study showed that monogenic disease genes can be quantitative trait loci (QTLs) for related complex traits.

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                                                                                                                                                                                                                                                                                                    • Painter, Jodie N., Gonneke Willemsen, Dale Nyholt, et al. 2010. A genome wide linkage scan for dizygotic twinning in 525 families of mothers of dizygotic twins. Human Reproduction 25:1569–1580.

                                                                                                                                                                                                                                                                                                      DOI: 10.1093/humrep/deq084Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                      An investigation of possible candidate genes associated with familial tendencies toward dizygotic twinning. Participants were families from Australia, New Zealand, and the United States in which sisters had naturally conceived DZ twins and families in which non-twins had one or more third-degree relatives who had naturally conceived DZ twins. A link between DZ twinning and gene variants identified in two Utah families was suggested.

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                                                                                                                                                                                                                                                                                                      • Shalev, I., T. E. Moffit, K. Sudgen, et al. 2012. Exposure to violence during childhood is associated with telomere erosion from 5 to 10 years of age: A longitudinal study. Molecular Psychiatry.

                                                                                                                                                                                                                                                                                                        DOI: 10.1038/mp.2012.32Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                        A longitudinal twin study examining associations between telomere length and exposure to violence. Twins experiencing two or more kinds of violence showed greater telomere erosion than those who experiences less. However, the small number of twins discordant for violence exposure prevented within-pair comparisons.

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                                                                                                                                                                                                                                                                                                        Brain Imaging

                                                                                                                                                                                                                                                                                                        Brain imaging techniques allow a close look at the structure and functioning of the human brain. Twin research has revealed that MZ twins’ brains are not exactly the same, a fascinating finding given that twins are closely matched on many measured cognitive traits. At the same time, differences in brain images can be tied to MZ cotwin differences in cognition and abnormal behaviors. Such findings enhance understanding of the origins of such traits in non-twins. A review of research in this area shows that genetic factors affect brain volume and other brain structures (Peper, et al. 2007). Mirror-imaging of cerebral brain functions was demonstrated in a case study of hand-discordant MZ twins (Sommer, et al. 1999). The behavioral implications of a shared brain structure in a set of conjoined twins are also described (Dominus 2011).

                                                                                                                                                                                                                                                                                                        • Dominus, Susan. 2011. Could conjoined twins share a mind? New York Times Magazine, 25 May.

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                                                                                                                                                                                                                                                                                                          A close look at the behaviors and rearing of a young pair of conjoined female twins, joined at the head (craniopagus). Physicians have identified a unique connection between the thalmus of each twin, a structure allowing transfer of sensory input between them. Inspection of brain images made of the twins may provide new information about neural mechanisms.

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                                                                                                                                                                                                                                                                                                          • Peper, Jiska S., R. M. Brouwer, D. I. Boomsma, R. S. Kahn, and H. E. Hulshoff Pol. 2007. Genetic influence on human brain structure: A review of brain imaging studies in twins. Human Brain Mapping 28:464–473.

                                                                                                                                                                                                                                                                                                            DOI: 10.1002/hbm.20398Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                            Reviews brain imaging studies with twins. Genetic factors vary across brain regions, from 90–95 percent for frontal lobe volumes to 40–65 percent in the hippocampus. Current research is considering specific genes linked to the regulation of brain volume across the life span.

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                                                                                                                                                                                                                                                                                                            • Sommer, I. E. G., N. F. Ramsay, A. Bouma, and R. S. Kahn. 1999. Cerebral mirror-imaging in a monozygotic twin. The Lancet 354:1445–1446.

                                                                                                                                                                                                                                                                                                              DOI: 10.1016/S0140-6736(99)04130-6Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                              A case study of hand-discordant MZ twins who underwent magnetic resonance imaging during language and mental rotation tasks. Mirroring was found for both functions. Hand-discordance occurs in about 25 percent of MZ twin pairs, suggesting possible underestimation of genetic contributions to diseases involving brain lateralization.

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                                                                                                                                                                                                                                                                                                              Controversies and Challenges

                                                                                                                                                                                                                                                                                                              Twin studies have had a mixed reception despite their proliferation. Critics have variously claimed that MZ twins’ environmental similarities and not their common genes explain their observed resemblance, that recruitment of twins is biased, and that the results have been exaggerated to suggest that genetic influence does not allow for environmental contributions. The papers in this section both advance (Palmer 2011, Wyatt and Midkiff 2007) and refute (Mitchell, et al. 2007) these arguments. Other noted controversies in twin research include the alleged fabrication of reared-apart twin data by Sir Cyril Burt (Hearnshaw 1981) and the horrific medical experiments conducted on twins in the Auschwitz-Birkenau concentration camp (Posner and Ware 2000).

                                                                                                                                                                                                                                                                                                              • Hearnshaw, Leslie S. 1981. Cyril Burt, psychologist. New York: Vintage.

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                                                                                                                                                                                                                                                                                                                Tells the story of the alleged falsifying of reared-apart twins and twin data by British psychologist, Sir Cyril Burt. Burt’s guilt has never been established conclusively, but his findings are no longer cited in the relevant literature; their omission leaves extant findings and conclusions unaffected.

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                                                                                                                                                                                                                                                                                                                • Mitchell, Karen S., Suzanne E. Mazzeo, Cynthis M. Bulik, Steven H. Aggen, Kenneth S. Kendler, and Michael C. Neale. 2007. An investigation of a measure of twins’ equal environments. Twin Research and Human Genetics 10:840–847.

                                                                                                                                                                                                                                                                                                                  DOI: 10.1375/twin.10.6.840Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                  An analysis of a self-report measure of twins’ equal environments, with the aim of eventually including the equal environments construct in analytical models. The equal environments assumption, that is, that factors affecting measured traits are similar for MZ and DZ twins, is fundamental to the twin research design. Most analyses of the equal environments assumption have failed to find violation.

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                                                                                                                                                                                                                                                                                                                  • Palmer, Brian. 2011. Double inanity: Twin studies are pretty much useless. Slate, 24 August.

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                                                                                                                                                                                                                                                                                                                    Outlines supposed flaws in twin research methodology, all of which have been refuted successfully. The article was extremely controversial, drawing numerous responses from readers. A response by twin research colleagues was reprinted in Twin Research and Human Genetics, 2011.

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                                                                                                                                                                                                                                                                                                                    • Posner, Gerald L., and John Ware. 2000. Mengele: The complete story. New York: Cooper Square.

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                                                                                                                                                                                                                                                                                                                      Tells the life history of Dr. Josef Mengele, the infamous Auschwitz physician, who performed unthinkable medical experiments on twins and individuals with various physical anomalies.

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                                                                                                                                                                                                                                                                                                                      • Wyatt, W. J., and D. M. Midkiff. 2007. Psychiatry’s thirty-five year, non-empirical reach of biological explanations. Behavior and Social Issues 16:197–213.

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                                                                                                                                                                                                                                                                                                                        The third in a series of articles asserting that research supporting biological underpinnings of behavioral and mental disorders has been overstated. The authors challenge findings showing that the rearing environments of twins separated at birth are dissimilar. They argue that MZ cotwins’ similar physical appearance is likely to evoke similar treatment responses and that adoption agencies routinely place separated twins in similar home settings.

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                                                                                                                                                                                                                                                                                                                        Twin-Based Perspectives

                                                                                                                                                                                                                                                                                                                        The unique features of being or raising a twin have sometimes been lost in human developmental research and debates. However, individuals working closely with twins are sensitive to the unusual developmental features of twins and the ways twins can inform artistic and scientific creativity. The papers in the subsections here consider twins’ special educational and parenting concerns, unique life histories, and effects on artistic and scientific accomplishments.

                                                                                                                                                                                                                                                                                                                        Education and Parenting

                                                                                                                                                                                                                                                                                                                        The rearing and educating of twins is a growing concern, given that twinning rates have recently escalated. Researchers are turning attention to such issues, such as different teacher effects on twins’ literacy (Byrne, et al. 2010) and the effects of classroom environments on problem behaviors (DiLalla and Mullineaux 2008). Preedy 2010 provides an overview of the unique educational concerns regarding twins, and Fraser 2009 addresses parenting concerns.

                                                                                                                                                                                                                                                                                                                        • Byrne, B., William L. Coventry, Richard K. Olson, et al. 2010. “Teacher effects” in early literacy development: Evidence from a study of twins. Journal of Educational Psychology 102:32–42.

                                                                                                                                                                                                                                                                                                                          DOI: 10.1037/a0017288Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                          A study of the extent to which having different teachers affects twins’ early literacy achievement. Participants included 355 MZ twin pairs and 356 same-sex DZ twin pairs. Twins assigned to the same teacher showed greater resemblance than twins assigned to different classes, but the effect was quite modest. Teacher effects may, however, be confounded with curriculum and peer factors.

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                                                                                                                                                                                                                                                                                                                          • DiLalla, Lisbeth F., and Paula Y. Mullineaux. 2008. The effect of classroom environment on problem behaviors: A twin study. Journal of School Psychology 46:107–128.

                                                                                                                                                                                                                                                                                                                            DOI: 10.1016/j.jsp.2007.02.001Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                            Examines agreement between teachers and parents regarding twins’ behavioral problems as a function of whether twins were in the same or separate classrooms. Parent and teacher ratings were most alike when twins were in the same classroom, although parents’ ratings of problem behaviors were lower when twins were placed together. The findings challenge the conventional wisdom that young twins should be routinely separated in school.

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                                                                                                                                                                                                                                                                                                                            • Fraser, Erika. 2009. Multiple failings: Parents’ of twins and triplets experience of pre and post natal NHS care. Guilford, UK: Twins & Multiple Births Association.

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                                                                                                                                                                                                                                                                                                                              Presents results from a health and lifestyle survey completed by British mothers of multiples. Most mothers were satisfied with the care they received during pregnancy, but problem areas remained. Difficulties included the increased rate of postnatal depression among multiple birth mothers, the relatively high frequency of special needs multiple birth children necessitating early mother-infant separation, insufficient information about nutrition and inadequate parental education.

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                                                                                                                                                                                                                                                                                                                              • Preedy, Pat. 2010. Are multiple birth children different from singletons? Meeting the educational needs of multiple birth children on school entry. London: Lambert Academic.

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                                                                                                                                                                                                                                                                                                                                Reports results from a national British study concerning support for parents and twins when twins first attend school. The twin sample was identified via PIPS (Performance Indicators in Primary Schools), a program of early educational assessment. The research yielded a theoretical model based on various social and emotional characteristics of families with multiple birth children. Additional information can be found at a related website.

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                                                                                                                                                                                                                                                                                                                                Life Histories

                                                                                                                                                                                                                                                                                                                                Twins tell us a great deal about behavior just by acting naturally. Their life histories offer a rich domain for developing hypotheses that may be tested with large twin samples. Twins’ life histories also identify unique aspects of being a twin and a parent of twins. Accounts of twelve twin, triplet, and quadruplets sets combined the scientific merit and human fascination with twins (Segal 2007). Schein and Bernstein 2007 is an autobiographical work by reared-apart twins that describes the experience of discovering a twin, meeting a twin, and learning that the separation was purposeful and unethical. The parental heartache of discovering the accidental switching of one twin infant with another is also described in Joye 1954.

                                                                                                                                                                                                                                                                                                                                • Joye, Madeleine. 1954. He was not my son. New York: Rinehart.

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                                                                                                                                                                                                                                                                                                                                  A moving memoir of a Swiss mother who discovered that one of her “fraternal twins” was not her child, but had been switched with one of her identical twin sons. The relatedness of the three children was established by genetic studies. The local judge determined that the two boys in question should be returned to their biological families.

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                                                                                                                                                                                                                                                                                                                                  • Schein, Elysse, and Paula Bernstein. 2007. Identical strangers: A memoir of twins separated and reunited. New York: Random House.

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                                                                                                                                                                                                                                                                                                                                    A revealing personal story of identical twins separated at birth and, unknown to their families, followed as part of a child development study. The book describes the twins’ reunion, their similarities and differences, and their quest to confront the investigator who conducted the project.

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                                                                                                                                                                                                                                                                                                                                    • Segal, Nancy L. 2007. Indivisible by two: Lives of extraordinary twins. Cambridge, MA: Harvard Univ. Press.

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                                                                                                                                                                                                                                                                                                                                      A compendium of case studies of twelve unusual twin, triplet, and quadruplet sets that combine scientific findings and human interest. The cases include, but are not limited to, twins separated at birth, twins concordant for selective mutism, twins who survived imprisonment in the Auschwitz concentration camp, triplets discordant for sexual orientation, identical twins who married identical twins and a twin who lost her sister on 9/11.

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                                                                                                                                                                                                                                                                                                                                      Arts and Sciences

                                                                                                                                                                                                                                                                                                                                      Twins are often found in the arts and sciences because they are convenient devices for exploring issues of identity and individuality. Twins’ appearance in Shakespeare’s plays prompted an analysis of his knowledge of twinning by two physicians (Groesbeck and Pool 1940). The physical mirroring of some traits in identical twins inspired a surgeon/artist to examine this phenomenon via photographs (Teplica 2009). The recent possibility of human cloning has also led to the use of twins as human models for exploring the behavioral consequences of such a procedure (Levick 2004). The phenomenology of being and losing a twin is sensitively presented in Van der Wee 2011, a documentary film made by a surviving twin. The physical fascination and visual interest of identical twins is captured in Abbe and Gill 1980, which is a timeless collection of photographs taken by identical twin photographers.

                                                                                                                                                                                                                                                                                                                                      • Abbe, Kathryn M., and Frances M. Gill. 1980. Twins on twins. New York: Clarkson N. Potter.

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                                                                                                                                                                                                                                                                                                                                        A compendium of photographs and interviews with thirty twin pairs. Authors are identical twin photographers. The book was digitized in 2008.

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                                                                                                                                                                                                                                                                                                                                        • Groesback, W., and R. M. Pool. 1940. Shakespeare’s knowledge of twins and twinning. Southern Medicine and Surgery (102, April): 173–176.

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                                                                                                                                                                                                                                                                                                                                          Reviews Shakespeare’s knowledge of twinning as expressed in two of his plays, Twelfth Night and Comedy of Errors. Shakespeare was the father of opposite-sex twins, Judith and Hamnet, explaining his interest in twinning phenomena. The possibility that Viola’s (Twelfth Night) strong character and eagerness to assume the role of male reflected Shakespeare’s awareness of the freemartin effect (masculinization of female cattle with male cotwins) was raised.

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                                                                                                                                                                                                                                                                                                                                          • Levick, Stephen E. 2004. Clone being: Exploring the psychological and social dimensions. New York: Rowman & Littlefield.

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                                                                                                                                                                                                                                                                                                                                            Addresses the psychological and social consequences of possible human reproductive cloning for parents and children. This is accomplished by using behavioral findings from studies of MZ twins and other kinships that mimic the situation of human clones and donors.

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                                                                                                                                                                                                                                                                                                                                            • Teplica, David. 2009. Recapturing science in clinical photography. Journal of Biocommunication 35:E31–E32.

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                                                                                                                                                                                                                                                                                                                                              Argues for the increased use of digital photographic analysis for addressing complex anatomical questions. The paper includes photographs of mirror-imaging (physical reversal) anatomic effects in MZ twins showing concordant, but mirrored, body shape. Mirroring or nonmirroring in this pair agree with the orientation of secondary skin findings reported for MZ twins thus far.

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                                                                                                                                                                                                                                                                                                                                              • Van der Wee, Anna, dir. 2011. The lone twin. Brussels: Wildheart Productions.

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                                                                                                                                                                                                                                                                                                                                                A documentary on the psychological consequences of twin loss and twin relationships. Explores the unique social relationships of twins with a focus on fraternal twins who are often overlooked. The film was inspired by the director’s loss of her twin brother due to a tragic accident. Commentary on twin-related topics by experts in the field is provided.

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