Hamilton's Rule

Content of the rule and inclusive fitness

Hamilton (1963, 1964) argued that helping relatives who are likely to share the same genes contributes to a gene's spread just as producing one's own offspring does. He defined inclusive fitness as an individual's own reproductive success plus its effects on relatives weighted by relatedness. On this view altruism is favored when rB > C. Relatedness r is 0.5 between parents and offspring and between full siblings, and 0.125 between first cousins. Haldane's (1955) quip that he would risk his life for two brothers or eight cousins is often cited as anticipating the logic.

Kin selection and generalization

Maynard Smith (1964) called the process kin selection and distinguished it from group selection. Price (1970) introduced the Price equation, which partitions selection into the covariance between a trait and fitness, and Grafen (1984) and Queller (1992) used it to generalize Hamilton's rule with relatedness defined as a regression coefficient. Frank (1998) and Gardner, West and Wild (2011) showed that the framework applies broadly even when the structure of interactions is complex. West, Griffin and Gardner (2007) clarified terms such as altruism, cooperation and mutualism within the theory.

Tests and controversy

Hamilton's rule has been used to explain eusociality in haplodiploid insects, cooperative breeding in birds and public goods production in microbes. Nowak, Tarnita and Wilson (2010), however, argued that inclusive fitness theory is unnecessary and that eusociality can be explained with standard natural selection models. Abbot and 136 co-authors (2011) responded that inclusive fitness theory has had extensive empirical success. Bourke (2011) organized the major transitions in social evolution within the framework of Hamilton's rule.

FAQ

What is Hamilton's rule in simple terms?

Helping behavior can evolve even at a cost to the helper if the benefit to the relative is large enough and the relative shares enough of the helper's genes.

Is the inclusive fitness debate settled?

Many researchers continue to support inclusive fitness theory after the critique by Nowak, Wilson and colleagues, but discussion of its scope and mathematical generality continues.

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. Hamilton, W. D. (1963). The evolution of altruistic behavior. American Naturalist, 97(896), 354–356.
3. Haldane, J. B. S. (1955). Population genetics. New Biology, 18, 34–51.
4. Maynard Smith, J. (1964). Group selection and kin selection. Nature, 201(4924), 1145–1147.
5. Price, G. R. (1970). Selection and covariance. Nature, 227(5257), 520–521.
6. Queller, D. C. (1992). A general model for kin selection. Evolution, 46(2), 376–380.
7. Grafen, A. (1984). Natural selection, kin selection and group selection. In J. R. Krebs & N. B. Davies (Eds.), Behavioural Ecology (2nd ed., pp. 62–84). Oxford: Blackwell.
8. Frank, S. A. (1998). Foundations of Social Evolution. Princeton: Princeton University Press.
9. 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.
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. Bourke, A. F. G. (2011). Principles of Social Evolution. Oxford: Oxford University Press.
13. Gardner, A., West, S. A., & Wild, G. (2011). The genetical theory of kin selection. Journal of Evolutionary Biology, 24(5), 1020–1043.