Twin Studies

by

Nancy L. Segal

• LAST REVIEWED: 19 March 2013
• 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/nrg932

  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.

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

  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.

• 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.x

  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.

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

  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.

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

  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.

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

  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.

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

  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.

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

  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.