Kin Selection Explains Altruism

The gene-centered view of Hamilton's inclusive fitness theory solves Darwin's problem of how a gene that reduces the fitness of its carrier can evolve. One of the main aims of Hamilton's work was to explain altruistic behavior but in fact, the theory is much more general and applies to any social behavior (see Table 1). Examples of animals behaving altruistically toward one another are all around: animals feed one another, groom one another, build homes for one another, defend one another, babysit for one another, and even die for one another. If there is no direct fitness benefit to a helping behavior, then kin selection is the only explanation for the behavior. Crucially, the beneficiary of an altruistic act must have a higher probability of sharing genes in common with the altruist than a random member of the population. Helping relatives is simply the most common way in which this can be achieved.

The spread of a gene for altruism was formalized by Hamilton in what is known as Hamilton's rule. The rule states that a gene will be favored if the following condition is met:

Table 1 Categories of social behaviors based on the effect on the fitness of the actor and the recipient

Effect on recipient

Effect on actor




Mutual benefit





where r = relatedness between the actor of a behavior and the beneficiary, b = benefit to the recipient, and c = cost to the actor. The effect of the behavior to the actor's own lifetime reproductive success is -c and this must be outweighed by the positive effect on the recipient rb.

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