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To find evidence for social reciprocity in primates, three main strategies have been used. The ecological validity of experimental findings on prosociality is supported by the many naturalistic observations of the same tendencies expressed spontaneously among chimpanzees and other nonhuman primates ( 3, 15, 16). After a period during which there was a lack of evidence for nonhuman primate prosociality, including claims of human uniqueness in this regard ( 6, 7), carefully controlled experiments have demonstrated well developed prosocial tendencies in monkeys ( 8– 10) and apes ( 11– 14).
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The main mechanism for human prosociality is assumed to be empathy ( 4), which has been hypothesized to extend to all mammals ( 5). As there is no evidence that animals show altruism while anticipating future repayments-and hence they are unlikely to be motivated by the possibility of repayment-there has been much focus on what drives them to be prosocial in the first place, with prosociality being defined as a motivation to assist others regardless of benefits for the self ( 2, 3). Such reciprocal altruism ( 1) would, first of all, require an altruistic impulse (i.e., altruism needs to be present before it can be reciprocated) and, second, a mechanism by which recipients recognize and repay their benefactors. Repayment of benefits is the most common evolutionary explanation of cooperation among nonrelatives. The outcome of several controls made it hard to explain these results on the basis of reward distribution or learned preferences, and rather suggested that joint action promotes prosociality, resulting in so-called attitudinal reciprocity. Having the partner receive a better reward than the chooser (i.e., 1/2) during the alternating condition increased the payoffs of mutual prosociality, and prosocial choice increased accordingly. As no contingency could be detected between an individual’s choice and their partner’s previous choice, and choices occurred in rapid succession, reciprocity seemed of a relatively vague nature akin to mutualism. When the paradigm was changed, such that both partners alternated making choices, prosocial preference significantly increased, leading to mutualistic payoffs. This indicates that interaction outside of the experimental setting played no role. Capuchins were spontaneously prosocial, selecting the prosocial option at the same rate regardless of whether they were paired with an in-group or out-group partner. Each monkey’s choices with a familiar partner from their own group was compared with choices when paired with a partner from a different group. Twelve capuchins tested in pairs could choose between two tokens, with one being “prosocial” in that it rewarded both individuals (i.e., 1/1), and the other being “selfish” in that it rewarded the chooser only (i.e., 1/0). We investigated whether direct reciprocity could promote prosocial behavior in brown capuchin monkeys ( Cebus apella). The debate about the origins of human prosociality has focused on the presence or absence of similar tendencies in other species, and, recently, attention has turned to the underlying mechanisms.