4.2 Cooperation and reciprocal altruism

Kin selection explains helping relatives. But humans cooperate extensively with non-relatives — friends, trading partners, strangers. Explaining this required a second mechanism: reciprocity.

Reciprocal altruism

Robert Trivers showed that altruism toward non-kin can evolve if it is reciprocated: I help you now, you help me later, and both of us end up better off than two non-cooperators. The condition is repeated interaction among individuals who can recognize each other and remember past dealings.

The prisoner's dilemma and tit-for-tat

The problem of cooperation is captured by the prisoner's dilemma, where each individual is tempted to defect even though mutual cooperation pays better. In Robert Axelrod's famous tournaments, the winning strategy was tit-for-tat: cooperate first, then do whatever your partner did last. It is nice, retaliatory, forgiving, and clear — and it stabilizes cooperation among self-interested agents.

Cheater detection and the social emotions

Reciprocity is vulnerable to free-riders, so selection equipped us to police it. We have sharp cheater-detection abilities (recall the Wason task), and a suite of social emotions enforce the system: gratitude rewards cooperators, anger and the urge to punish deter defectors, and guilt repairs ruptured relationships. Trust and reputation become valuable currencies.

A grounding example

Gerald Wilkinson documented reciprocal altruism in vampire bats. A bat that fails to feed on a given night will die within days, but a successful bat can regurgitate blood to share. Bats preferentially share with those who have shared with them in the past, and they remember individual partners. This is reciprocal altruism in a non-human animal: repeated interaction, individual recognition, and help contingent on past cooperation — exactly Trivers's conditions.