Humans often respond negatively when receiving a less good outcome than another (inequity), a behavior which is hypothesized to be a mechanism to support successful cooperation. First documented by Brosnan & de Waal (2003, Nature) in capuchin monkeys, we have since found evidence that several primate species respond negatively if they receive a less good reward than a social partner for completing the same task. This requires the individuals to take in to account both their own and their partners' rewards or procedures, and make assessments of their outcome based upon these parameters. We have found in particular that this response is contingent upon a task, and does not occur when rewards are provided for free. Moreover, among most primates, while effort seems to enhance the response, individuals are more sensitive to different rewards than to different levels of effort.
While this indicates that the behavior is not unique to humans, it does not provide an evolutionary explanation for the emergence of inequity responses due to the behavioral similarities among the initial species studied, capuchin monkeys and chimpanzees. Thus the inequity response could be due to either an evolutionary homology or a convergence based on one or more of these traits. To address this, we have recently tested several additional primate species (orangutans, squirrel monkeys, owl monkeys and common marmosets) which differ on these dimensions, using the same paradigm as in previous work in my lab. We find that these other species do not show responses to inequity, indicating that the response is a convergent behavior which likely emerged in the context of cooperation amongst non-kin not from the same family group. Knowing the conditions under which inequity responses evolved allows for a better understanding of the behavior and how it may influence cooperation.
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