Humans often respond negatively when receiving a less good outcome than another person (inequity), a behavior which is hypothesized to be a mechanism to support successful cooperation. First documented by me and Frans de Waal 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 into 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. Finally, there is variation among individuals within a species, which seems to be based upon individual differences, rather than differences in relationship quality.
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, rhesus 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 that likely emerged in the context of cooperation among non-kin not from the same family group. This response is likely a partner choice mechanism by which individuals recognize when they would do better in a new cooperative relationship.
Of course, a true sense of fairness involves two components: responding when one receives less and responding when others do. The inequity studies just described address only the first of these: how subjects respond when their partners get more than them. Recent evidence indicates that chimpanzees (and likely other cognitively advanced social species) also change their behavior when they get more than a partner, at least when their partner has some recourse. This indicates that the evolution of fairness involves two steps. In first order fairness, species evolve to respond negatively to inequity as a way for individuals to recognize the value of their cooperative partners, and therefore increase their payoffs from cooperation. In second order fairness, species evolve to recognize when they receive more than a cooperative partner, and act to ameliorate this inequity in order to maintain a beneficial cooperative relationship. Humans, with our advanced abilities at foresight and our ability to delay gratification for a long-term gain, then developed the full-blown sense of fairness that we see today.