Samuel Bowles, Herbert Gintis

Paper #: 03-05-031

How do human groups maintain a high level of cooperation despite a low level of genetic relatedness among group members? We suggest that many humans have a predisposition to punish those who violate group-beneficial norms, even when this reduces their fitness relative to other group members. Such altruistic punishment is widely observed to sustain high levels of cooperation in behavioral experiments and in natural settings. It is known that if group extinctions are sufficiently common, altruistic punishment may evolve through the contribution of norm adherence to group survival. Additionally, those engaging in punishment of norm violators may reap fitness benefits if their punishment is treated as a costly signal of some underlying but unobservable quality as a mate, coalition partner, or opponent. Here we explore a different mechanism in which neither signaling nor group extinctions plays a role. Rather, punishment takes the form of ostracism or shunning, and those punished in this manner suffer fitness costs. We offer a model of this behavior, which we call strong reciprocity: where members of a group benefit from mutual adherence to a social norm, strong reciprocators obey the norm and punish its violators, even though they receive lower payoffs than other group members, such as selfish agents who violate the norm and do not punish, and pure cooperators who adhere to the norm but free-ride by never punishing. Our agent-based simulations show that, under assumptions approximating some likely human environments over the 100,000 years prior to the domestication of animals and plants, the proliferation of strong reciprocators when initially rare is highly likely, and that substantial frequencies of all three behavioral types can be sustained in a population.