Inclusive fitness is a term in evolution that describes the total proportion of genes that an individual organism contributes to the next generation. Fitness is usually divided into two categories Direct fitness or Darwinian fitness and indirect fitness. For many years after Charles Darwin published his theory most people focused on direct fitness, which was just a measurement of direct descendants. However, in 1964 W.D. Hamilton published a book that outlined how indirect fitness could be just as important if not more important than direct fitness. Hamilton realized that an organism's genetic relatives could also contribute that organism's genes into the next generation so any aid given to ones relatives to aid in their reproduction could increase fitness indirectly. The likelihood that aid given to a relative would translate into a fitness benefit is proportional to how many genes that relative shares with you. So offspring of a brother or sister is a larger fitness benefit then the offspring of a half-sibling or cousin. This led to the development of Hamilton's rule to mathematically describe the relationship that would allow nepotism to evolve. This is commonly written as c<rb with c correspond to the cost of an act, r the co-efficient of relatedness (the proportion of genes shared in common), and b is the benefit to the relative. So if the cost to an organism is less than the benefit to the relative time the co-efficient of relatedness it would be adaptive to aid ones relative.

This rule has been used in evolutionary biology, behavioral ecology, and evolutionary psychology to describe a wide range of behaviors. Most famous is Robert Triver's parent-offspring conflict theory which attempts to explain the conflict between parents and their children using Hamilton's rule. Triver's pointed out that to a parent each of one's offspring is related by 0.5 so all are viewed equally so the parent wants benefits to be distributed to each offspring equally. However, each individual offspring views them selves (r=1) as twice as valuable as its siblings (r=0.5) so it wants twice the investment as its siblings get. This sets the stage for conflict to emerge.


  • Campbell, N., Reece, J., et al. 2002. Biology. 6th ed. San Francisco, California. pp. 1145-1148.
  • Hamilton, W. D. 1964 The Genetical Evolution of Social Behaviour I and II, J. Theor. Biol. v7, pp 1-16, and 17-52
  • Hamilton, W. D. 1975, Innate Social Aptitudes of Man: an Approach from Evolutionary Genetics, in Robin Fox (ed.), Biosocial Anthropology, Malaby Press, London, 133-153 (IF including altruism to fellow altruists among strangers discussed herein)
  • Buss, D.M. (2004). Evolutionary Psychology: The New Science of the Mind. Boston: Pearson Education, Inc.

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