Reciprocal Altruism
A behavior whereby an organism acts in a manner that temporarily reduces its fitness while increasing another organism's fitness, with the expectation that the other organism will act in a similar manner at a later time.
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Origin
Evolutionary biologist Robert Trivers, then a graduate student at Harvard studying under Ernst Mayr, introduced the concept in his 1971 paper "The Evolution of Reciprocal Altruism" in the Quarterly Review of Biology. Trivers showed how natural selection could favor cooperation among unrelated individuals — and how cheaters would be detected and excluded over time. Vampire bats sharing regurgitated blood became one of the theory's most cited real-world examples.
Updated February 22, 2026