Reciprocal Altruism

Reciprocal altruism is a theory in evolutionary biology, proposed by Robert Trivers in a 1971 paper, explaining how altruistic behavior between unrelated individuals, helping another at some cost to oneself, can evolve when the helped individual is likely to reciprocate in the future, so that the net benefit to the altruist over repeated interactions outweighs the initial cost. Trivers illustrated the concept with examples including cleaning symbiosis between certain fish species and their hosts, and applied it to patterns of mutual helping and gratitude in human social behavior. For the strategy to remain evolutionarily stable it generally requires repeated encounters between the same individuals and some capacity to recognize and stop cooperating with cheaters who fail to reciprocate, a dynamic later formalized through game-theoretic models such as Robert Axelrod's work on the iterated prisoner's dilemma in the early 1980s. Reciprocal altruism remains one of the principal explanations, alongside kin selection, for the evolution of cooperative behavior among non-relatives.