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Fairness judgment is a fundamental aspect of human cooperation. By carefully balancing the payoffs and efforts with cooperating partner (s) we could either avoid or punish cheaters and stably maintain cooperation. Recent studies investigating the origin of this fairness sentiment have demonstrated that this psychological trait is not unique to humans, but also can be observed in other group-living primates, such as chimpanzees and capuchins, suggesting a convergent evolution of a sense of fairness, with cooperative social life being the selective pressure for it. The current study was designed to test this hypothesis by directly comparing the response to the outcome inequity in two of our closest living relatives, chimpanzees and orangutans, having different social systems, i.e. solitary and patrilocal multi-male multi-female groups.Unlike other inequity experiments, we used a prosocial choice apparatus with different reward distributions (advantageous / disadvantageous) to give subjects an active role of not-sharing foods if they considered it unfair. In addition to the choice, we also recorded the behavioral responses of the apes to the inequity. Throughout the experiments aversive emotional responses toward the disadvantageous inequity were only found in chimpanzees, but not in orangutans, supporting the convergent (or domain-specific) evolution of a sense of fairness. However, this aversion to the inequity did not lead the chimpanzees to actually make selfish choices, indirectly supporting the previous findings that chimpanzees employ a partner choice strategy rather than a punishment for fair cooperation. We also found that hierarchy seems to play an important role in the expression of aversion to inequity and prosocial tendency in chimpanzees.

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