Kin Selection

The puzzle of altruism and Hamilton's rule

Since Darwin, altruistic behavior, such as worker bees that forgo reproduction to help others, was a puzzle for natural selection. Fisher (1930) and Haldane (1955) left hints; Haldane famously quipped that he would lay down his life for two brothers or eight cousins. W. D. Hamilton (1964) systematized the idea. He defined "inclusive fitness" as an individual's own offspring plus the effects of its behavior on relatives' reproduction, weighted by relatedness, and proposed "Hamilton's rule": altruism is favored when rB > C. Here r is the relatedness between actor and recipient (the probability of sharing a gene, 0.5 for full siblings and 0.125 for first cousins), B is the reproductive benefit to the recipient and C the reproductive cost to the actor. John Maynard Smith (1964) named this "kin selection," and Richard Dawkins's The Selfish Gene (1976) popularized the gene's-eye view.

Empirical evidence

Hamilton explained the repeated evolution of eusociality in Hymenoptera such as bees, ants and wasps through haplodiploid sex determination. Because males are haploid, full-sister workers share a relatedness of 0.75, higher than to their own daughters (0.5), making it advantageous to raise the queen's daughters. Trivers and Hare (1976) presented data supporting the prediction that workers bias investment toward females. Yet diploid eusocial animals such as termites show haplodiploidy is not sufficient, and Hughes et al. (2008) showed phylogenetically that eusociality evolved only in lineages with monogamous ancestors, underscoring high relatedness. Sherman (1977) found that Belding's ground squirrels give alarm calls more often when kin are nearby, and Emlen and Wrege (1988) showed that white-fronted bee-eater helpers preferentially assist closely related nests. In humans, Daly and Wilson (1988) interpreted higher abuse and homicide risks for stepchildren than for genetic children through a kin-selection lens.

Debate and significance

In 2010, Martin Nowak, Corina Tarnita and Edward O. Wilson argued that inclusive fitness theory is unnecessary and of limited generality, calling for eusociality to be explained with standard natural selection models and group-level accounts. Abbot and 136 co-authors (2011) replied that inclusive fitness theory is backed by extensive empirical work and is mathematically general, and the debate continues. West, Griffin and Gardner (2007) noted that confusion over terms such as altruism, cooperation, mutualism and group selection fuels such disputes. Kin selection remains a core framework for the evolution of social behavior, alongside reciprocal altruism among non-relatives, research on the evolution of cooperation and evolutionary game theory.

FAQ

What is kin selection in simple terms?

Helping family members can pay off for your genes even if you have fewer offspring, because relatives who share your genes survive and reproduce more.

What does Hamilton's rule rB > C mean?

If the benefit to the helped relative (B) multiplied by relatedness (r) exceeds the cost to the helper (C), the altruistic behavior can spread by natural selection.

Sources reviewed: 2026-10-26

References

1. Hamilton, W. D. (1964). The genetical evolution of social behaviour. I & II. Journal of Theoretical Biology, 7(1), 1–52.
2. Maynard Smith, J. (1964). Group selection and kin selection. Nature, 201(4924), 1145–1147.
3. Haldane, J. B. S. (1955). Population genetics. New Biology, 18, 34–51.
4. Fisher, R. A. (1930). The Genetical Theory of Natural Selection. Oxford: Clarendon Press.
5. Trivers, R. L., & Hare, H. (1976). Haploidploidy and the evolution of the social insects. Science, 191(4224), 249–263.
6. Dawkins, R. (1976). The Selfish Gene. Oxford: Oxford University Press.
7. Sherman, P. W. (1977). Nepotism and the evolution of alarm calls. Science, 197(4310), 1246–1253.
8. Emlen, S. T., & Wrege, P. H. (1988). The role of kinship in helping decisions among white-fronted bee-eaters. Behavioral Ecology and Sociobiology, 23(5), 305–315.
9. Hughes, W. O. H., Oldroyd, B. P., Beekman, M., & Ratnieks, F. L. W. (2008). Ancestral monogamy shows kin selection is key to the evolution of eusociality. Science, 320(5880), 1213–1216.
10. Nowak, M. A., Tarnita, C. E., & Wilson, E. O. (2010). The evolution of eusociality. Nature, 466(7310), 1057–1062.
11. Abbot, P., et al. (2011). Inclusive fitness theory and eusociality. Nature, 471(7339), E1–E4.
12. West, S. A., Griffin, A. S., & Gardner, A. (2007). Social semantics: Altruism, cooperation, mutualism, strong reciprocity and group selection. Journal of Evolutionary Biology, 20(2), 415–432.
13. Daly, M., & Wilson, M. (1988). Homicide. New York: Aldine de Gruyter.