Recent experimental research has revealed forms of human behavior involving interaction among unrelated individuals that have proven difficult to explain in terms of kin or reciprocal altruism. One such trait, strong reciprocity is a predisposition to cooperate with others and to punish those who violate the norms of cooperation, at personal cost, even when it is implausible to expect that these costs will be repaid. We present evidence supporting strong reciprocity as a schema for predicting and understanding altruism in humans. We show that under conditions plausibly characteristic of the early stages of human evolution, a small number of strong reciprocators could invade a population of self-regarding types, and strong reciprocity is an evolutionary stable strategy. Although most of the evidence we report is based on behavioral experiments, the same behaviors are regularly described in everyday life, for example, in wage setting by firms, tax compliance, and cooperation in the protection of local environmental public goods.