One explanation for altruism is the "Kin selection theory" proposed by Hamilton (1964, as cited in Krebs & Davies, 1997). The theory concerns the conditions under which genes for social actions spread through populations. This theory suggests that traits, which directed an individual's altruism towards its relatives, but not to others, would evolve. Only those sharing genes would benefit, thus promoting the survival of related individuals, who are also likely to be altruistic. This means the altruist gains via "inclusive fitness". This is because fitness is actually measured in terms of the number of genes passed on to subsequent generations, not just the number of offspring produced. As well as passing on genes directly to their own offspring, they can do so indirectly by aiding the reproduction of other individuals who are likely to carry the same gene. The argument Hamilton put forward was that a gene for altruism could evolve under Darwinian selection if the altruist's behaviour allowed a genetic relative that shared the same gene to reproduce more than it would otherwise have done. An implication of this is that an individual should always prefer to aid close kin over more distant kin. .
This theory can be nicely summarised as a formula known as "Hamilton's Rule". The rule is as follows: a gene for altruism will evolve whenever "rB > C" where B is the benefit to the recipient of the altruistic act, C is the cost to the actor and r is the coefficient of relatedness (% of genes shared between beneficiary and actor). Therefore altruistic behaviour should only occur when this inequality is satisfied. .
An example that supports this theory is the helping at the nest in "Florida Scrub Jays". (Woolfenden & Fitzpatrick, 1984 as cited in Alcock, 1998). Nonbreeding birds help rear both nestlings and fledglings. These nonbreeding adults may be 2 or 3 years old and are physiologically capable of producing offspring of their own, but instead defend the breeding pairs territory and feed the nestlings.
