The Evolution of Strong Reciprocity

Where genetically unrelated members of a group benefit from mutual adherence to a social norm, agents may obey the norm and punish its violators, even when this behavior cannot be justified in terms of self-regarding, outcome-oriented preferences. We call this strong reciprocity. We distinguish this from weak reciprocity, namely reciprocal altruism, tit-for-tat, exchange under complete contracting, and other forms of mutually beneficial cooperation that can be accounted for in terms of self-regarding outcome-oriented preferences. We review compelling evidence for the existence and importance of strong reciprocity in human society. However, where benefits and costs are measured in fitness terms and where the relevant behaviors are governed by genetic inheritance subject to natural selection, it is generally thought that, as a form of altruism, strong reciprocity cannot invade a population of non-reciprocators, nor can it be sustained in a stable population equilibrium. We show that this is not the case, and offer an evolutionary explanation of the phenomenon. As the late Pleistocene is the only period long enough to account for a significant development in modern human gene distributions, we base our model on the structure of interaction among members of the small hunter-gatherer bands that constituted most of the history of Homo sapiens, as revealed by historical and anthropological evidence.