Kin selection stands among W. D. Hamilton's most influential ideas. The purpose of this review is to assess the impact of Hamilton's ideas about kin selection on studies of social recognition. Kin selection theory predicts that animals should direct aid-giving behaviour to closely related animals, provided that a positive net benefit in inclusive fitness is achieved from the altruistic act. Kin recognition is the key proximate mechanism by which animals can sort more related from less related interactants in a population. Kin recognition also has the potential to allow fine distinctions among animals based on identity by descent. Following the publications of Hamilton's 1964 papers on kin selection, studies of kin recognition focused on four disparate behavioural contexts: identification of group membership, inbreeding avoidance, alarm calls and other forms of aid-giving, and parent–offspring interactions. Investigations of eusocial insects have focused on identification of group membership by phenotypes that are shared among all members of a colony. In birds and mammals, social structure is often based on individual recognition. Some species, particularly rodents, have the ability to make discriminations based on relatedness among animals that they have not previously met. Future studies of kin recognition should be less reliant on assumptions that all forms of societal closure are due to factors related to kin selection. These studies should acknowledge that the role of individual recognition in vertebrate societies is key to understanding the full texture of social interactions, and that individual recognition may be equally important in many other types of animals. Of particular interest will be the discovery of how information about kinship is integrated with information about individual identity.
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