Reciprocal altruism

Reciprocal altruism is cooperation in which an individual pays a cost to help another, with benefits returned later. It can evolve when individuals interact repeatedly and future returns outweigh the initial cost.

Iterated prisoner's dilemma: A repeated version of the prisoner's dilemma in which players choose actions across multiple rounds. It models how future encounters can make cooperation worthwhile despite immediate incentives to defect.
Altruism (biology): Behavior that increases another individual's fitness while reducing the actor's fitness. Reciprocal altruism is a special case in which a costly benefit may later be repaid.
Robert Trivers: An American evolutionary biologist known for influential theories of social behavior and evolution. His 1971 paper formulated reciprocal altruism as an explanation for cooperation beyond kinship.
Vampire bat food sharing: The sharing of regurgitated blood meals among common vampire bats, including nonrelatives. Long-term observations found that bats often share with individuals that previously shared with them.
Indirect reciprocity: Cooperation in which help is returned by someone other than the original recipient, often through reputation. It differs from direct reciprocity because repayment need not come from the individual helped.
Tit for tat: A strategy that cooperates first, then copies the other player's previous move. Its success in repeated games illustrates how reciprocity can reward cooperation and answer defection.
Inclusive fitness: An individual's evolutionary success measured through effects on its own reproduction and that of relatives. It distinguishes benefits to relatives from the later personal returns central to reciprocity.
The Selfish Gene: Richard Dawkins's 1976 book explaining evolution through the differential propagation of genes. It helped popularize reciprocal altruism as one route by which apparently selfless behavior can evolve.
Cleaner fish: Fish that remove ectoparasites and dead tissue from other fish at cleaning stations. Repeated client visits and partner choice create conditions for reciprocal service exchanges.
Mutualism (biology): An interaction between species in which both partners benefit. Mutualism can yield immediate benefits to both partners rather than delayed repayment.

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