In recent years a large number of experimental studies have documented the existence of
strong reciprocity among humans. Strong reciprocity means that people willingly repay gifts
and punish the violation of cooperation and fairness norms even in anonymous one-shot
encounters with genetically unrelated strangers. We provide ethnographic and experimental
evidence suggesting that ultimate theories of kin selection, reciprocal altruism, costly
signalling and indirect reciprocity do not provide satisfactory evolutionary explanations of
strong reciprocity. The problem of these theories is that they can rationalize strong reciprocity
only if it is viewed as maladaptive behavior whereas the evidence suggests that it is an
adaptive trait. Thus, we conclude that alternative evolutionary approaches are needed to
provide ultimate accounts of strong reciprocity.