In This Article Expand or collapse the "in this article" section Kin Selection

  • Introduction
  • General Overviews
  • Historical Background
  • Hamilton’s Rule
  • Refinements of the Theory
  • Evolution of Eusociality
  • Haplodiploidy Hypothesis
  • Cooperative Breeding in Vertebrates
  • Kin Discrimination
  • Kin-Selected Conflict
  • Sex Ratios
  • Major Transitions
  • Humans
  • Other Applications
  • Controversy

Ecology Kin Selection
Andrew F. G. Bourke
  • LAST REVIEWED: 08 September 2021
  • LAST MODIFIED: 30 March 2015
  • DOI: 10.1093/obo/9780199830060-0051


According to Hamilton’s kin selection theory (also known as “inclusive fitness” theory), kin selection is the process by which social evolution occurs in nature. The theory extends the genetical theory of natural selection to social behaviors and finds that their evolution is affected by the likelihood that individuals share genes (relatedness). In biology, a social behavior occurs when one individual (the actor) behaves so as to affect the direct fitness (number of offspring) of itself and another individual (the recipient). For example, altruism occurs when the actor’s behavior decreases the actor’s direct fitness and increases the recipient’s direct fitness. Conversely, selfishness occurs when the actor’s behavior increases the actor’s direct fitness and decreases the recipient’s. Social behaviors are widespread in nature. A classic example is the altruism shown by the sterile workers of social insects such as ants, which sacrifice their own reproduction in order to rear the queen’s offspring. At first sight, altruism poses a problem for the genetical theory of natural selection, which seems to preclude the spread of a gene for reduced reproduction. Kin selection was devised by William Hamilton in the early 1960s to address this “problem of altruism.” The basic principle behind kin selection had been hinted at by Darwin, Fisher, and Haldane, but it was Hamilton who provided the first general model. Hamilton called his idea “inclusive fitness” theory, and it was later dubbed “kin selection” by Maynard Smith in 1964. For most purposes, the two can be considered identical, although inclusive fitness theory technically includes kin selection theory because the relatedness it invokes need not involve kin (genealogical relatives). Kin selection theory solved the problem of altruism by showing that a gene for altruism can spread if altruism is directed at individuals likely to bear the same gene. By definition, kin are likely to share genes. So, a gene for altruism can spread if altruism is directed at kin and the loss of gene copies through the actor’s decreased reproduction is more than offset by the gain in gene copies through the increased reproduction of the recipient. The algebraic version of this condition is termed “Hamilton’s rule.” Although kin selection theory was devised to explain altruism, it also applies to the other forms of social behavior such as selfishness. The theory is therefore now widely used to investigate and explain many kinds of social behavior in living organisms as diverse as bacteria and human beings.

General Overviews

Hamilton’s initial publication on kin selection was a short note in 1963, but Hamilton 1964 represents the first full exposition of the theory. Although there was some early support, it took several years for the theory to achieve widespread recognition and acceptance and to begin to be used in empirical studies. Such studies focused on explaining eusociality in insects (i.e., societies of insects in which there is a reproductive division of labor between reproductive forms such as queens and sterile or semisterile altruistic forms such as workers) and cooperative breeding in vertebrates (i.e., societies of vertebrates in which some individuals fail to breed, at least temporarily, and instead help rear offspring of other group members). West-Eberhard 1975 presents a comprehensive review of kin selection and its applications both to insects and vertebrates from the end of the early phase of the theory’s development and application. West, et al. 2007 surveys the state of the field in its maturity. There is no single, comprehensive monograph or textbook devoted solely to theoretical and empirical aspects of kin selection. Some of the closest approaches are found in Bourke 2011; Davies, et al. 2012; Dugatkin 1997; and Wilson 2000. Dugatkin 1997 concentrates on cooperative interactions between nonrelatives, which according to some researchers come under the scope of kin selection theory, representing the special case that arises when relatedness takes a zero value. Gardner and West 2014 is a special issue of review articles on kin selection theory written to commemorate the fiftieth anniversary of the original presentation of the theory in Hamilton 1964.

  • Bourke, A. F. G. 2011. Principles of social evolution. Oxford: Oxford Univ. Press.

    DOI: 10.1093/acprof:oso/9780199231157.001.0001

    A short book that includes an introductory primer in kin selection (inclusive fitness) theory. Seeks to apply the theory systematically to the evolution of the hierarchical structure of life: that is, to the “major transitions” in evolution such as the evolution of cells, of multicellularity, and of eusociality (see Major Transitions).

  • Davies, N. B., J. R. Krebs, and S. A. West. 2012. An introduction to behavioural ecology. 4th ed. Oxford: Wiley-Blackwell.

    The most recent edition of the classic textbook of behavioral ecology, containing several chapters covering both the theory of kin selection and empirical applications in invertebrates and vertebrates. Particularly suitable for undergraduates.

  • Dugatkin, L. A. 1997. Cooperation among animals: An evolutionary perspective. New York: Oxford Univ. Press.

    Covers the central ground of kin selection theory (social interactions between relatives) while focusing on social interactions between nonrelatives.

  • Gardner, A., and S. A. West, eds. 2014. Special issue: Inclusive fitness; 50 years on. Philosophical Transactions of the Royal Society B: Biological Sciences 369.1642.

    DOI: 10.1098/rstb.2013.0356

    A theme issue of the journal, dedicated to kin selection (inclusive fitness), with review articles from multiple authors covering theory, empirical tests, and applications.

  • Hamilton, W. D. 1964. The genetical evolution of social behaviour I, II. Journal of Theoretical Biology 7.1: 1–52.

    DOI: 10.1016/0022-5193(64)90038-4

    Seminal twin research articles (I, pp. 1–16; II, pp. 17–52) first setting out kin selection theory (following Hamilton’s note of 1963). The method of derivation has been superseded by other methods (see Hamilton’s Rule). Applies the theory to insect sociality and other social phenomena. Part II also proposed the haplodiploidy hypothesis (see Haplodiploidy Hypothesis).

  • West, S. A., A. S. Griffin, and A. Gardner. 2007. Evolutionary explanations for cooperation. Current Biology 17.16: R661–R672.

    DOI: 10.1016/j.cub.2007.06.004

    A punchy review article that sets out the state of kin selection theory in its fifth decade, with consideration of the relationship between kin selection theory and alternative frameworks for analyzing social evolution.

  • West-Eberhard, M. J. 1975. The evolution of social behavior by kin selection. Quarterly Review of Biology 50.1: 1–33.

    DOI: 10.1086/408298

    An early review article notable for emphasizing that Hamilton’s rule for the evolution of social behavior requires both genetic conditions (affecting relatedness) and ecological conditions (affecting benefit and cost) to be fulfilled. For altruism, benefit and cost mean gains and losses in offspring number in the recipient and actor, respectively.

  • Wilson, E. O. 2000. Sociobiology: The new synthesis. 25th anniversary ed. Cambridge, MA: Harvard Univ. Press.

    An encyclopedic, well-illustrated survey of social systems in nature, first published in 1975. Endorsed kin selection and sparked controversy through applying kin selection and similar ideas to social evolution in humans. The author has since become a critic of kin selection theory (see Controversy).

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