CONCEPT
Reciprocal Altruism
Robert Trivers’s 1971 proof that cooperation among unrelated agents is evolutionarily stable—not through sentiment but through the mechanical logic of repetition, memory, and the detection and punishment of defection.
Reciprocal altruism is the evolutionary mechanism that explains how cooperation among strangers survives natural selection. The puzzle Darwin left open was vivid: why would any creature pay a cost to help another to whom it shares no special genetic stake? Kin selection covers the altruistic parent and the colonial insect, but it cannot cover the grooming primate or the human who keeps a promise to someone he may never see again.
Trivers’s 1971 paper supplied the missing mechanism: when interactions repeat, when partners can recognize one another and remember past behavior, and when the benefit to the receiver exceeds the cost to the giver, natural selection can favor a disposition to help—because help is reciprocated, and over many rounds the reciprocators outcompete the cheats. Cooperation is not altruism in disguise. It is enlightened self-interest stabilized by the shadow of the future. Robert Axelrod’s computer tournaments of the repeated
prisoner’s dilemma confirmed the prediction in code: the winning strategy was tit-for-tat—nice, retaliatory, forgiving, and legible. Strip away the