Reciprocal altruism
Helping others now in the expectation that the help will be returned later.
What it means
Reciprocal altruism is cooperative behavior in which an individual incurs a cost to benefit another with the expectation of future reciprocation, and it provides an evolutionary explanation for how cooperation among non-relatives can be stable despite its apparent cost to the helper. Trivers's key insight was that altruism toward non-kin can be favored by natural selection when interactions are repeated and individuals can recognize one another, because the long-run benefit of receiving help in return outweighs the short-run cost of giving it. The mechanism requires several conditions — repeated encounters, the ability to recognize and remember partners, and a way to detect and withhold help from non-reciprocators (cheaters) — which is why it is often accompanied by psychological adaptations for tracking fairness, gratitude, and cheating. It is the evolutionary substrate of the human norm of reciprocity and is closely modeled by repeated games such as the iterated prisoner's dilemma, where strategies like tit-for-tat embody it. Classic biological examples include blood-sharing among vampire bats and grooming among primates. It matters because it grounds human cooperation, trust, and the sense of obligation to return favors in evolutionary logic, linking moral intuitions to adaptive advantage.
Examples
Vampire bats regurgitate and share blood meals with hungry roost-mates who have shared with them before and are likely to return the favor.
Colleagues cover each other's shifts on the unspoken understanding that it will be repaid; the one who always takes and never covers quietly stops being asked.
Hunter-gatherers share a large kill across the whole camp rather than hoard it, because today's lucky hunter is next week's unlucky one and the sharing runs both ways.
First described in Robert Trivers (1971).