Competing demands of prosociality and equity in monkeys

Prosocial decisions may lead to unequal payoffs among group members. Although an aversion to inequity has been found in empirical studies of both human and nonhuman primates, the contexts previously studied typically do not involve a trade-off between prosociality and inequity. Here we investigate the apparent coexistence of these two factors, specifically the competing demands of prosociality and equity. We directly compare the responses of brown capuchin monkeys (Cebus apella) among situations where prosocial preferences conflict with equality, using a paradigm comparable to other studies of cooperation and inequity in this species. By choosing to pull a tray towards themselves, subjects rewarded themselves and/or another in conditions in which the partner either received the same or different rewards, or the subject received no reward. In unequal payoff conditions, subjects could obtain equality by choosing not to pull in the tray, so that neither individual was rewarded. The monkeys showed prosocial preferences even in situations of moderate disadvantageous inequity, preferring to pull in the tray more often when a partner was present than absent. However, when the discrepancy between rewards increased, prosocial behavior ceased.