Journal papers
Tolerance for inequity may increase with social closeness in chiimpanzees
Capuchin monkey's (Cebus apella) reactions to inequity in an unrestricted barpull situation
Nonhuman species' reactions to inequity and their implications for fairness
During the evolution of cooperation, it may have become worthwhile for individuals to compare their own payoffs to those of others, in an effort to increase relative fitness. Humans do so, frequently rejecting payoffs that are perceived as unfair (even if they are advantageous). While there is some variation, this response is widespread across human populations. If a sense of fairness did evolve to promote cooperation, some nonhuman animals may exhibit inequity aversion as well. This is particularly likely in social species with tolerant societies, such that individuals may reasonably expect some equity between themselves and other group members.
Here we examine the response of five adult female capuchin monkeys and twenty adult chimpanzees (6 male and 14 female) to an unequal distribution of rewards during experimental exchange with a human experimenter. Same-sex pairs alternated exchanges with the experimenter under four conditions: 1) both received the same reward, 2) one received a superior reward, 3) one received a superior reward without exchange (i.e. no work), and 4) a single individual observed a superior reward in the absence of a partner.
Capuchins and chimpanzees are significantly less likely to complete an exchange when their partner receives a higher-value food item than they. Refusals to exchange included passive rejections, such as refusing to either return the token or accept the reward, as well as active rejections, such as throwing the token or the reward out of the testing area.
One significant difference between capuchin monkeys and chimpanzees is that chimpanzees show significant variation in response which apparently depends upon the social environment. Chimpanzees from a short-term group, which had been together less than 8 years, showed a response similar in intensity to the capuchin monkeys. However,chimpanzees from a long-term social group, which had lived together for more than 30 years and in which all subjects but one were born and raised (the exceptional individual was present at group formation), showed virtually no response to inequity. This reflects the responses found by social psychologists in humans. People tend to respond less, if at all, to inequity when they are in a close relationship (e.g. friend or spouse) while they respond strongly to inequity with strangers. Perhaps this long-term group of chimpanzees had formed such close relationships.
In capuchin monkeys, this response is amplified if the partner receives the reward without working for it (e.g. completing the exchange). We do not see this same response to effort in chimpanzees, which may be due to the fact that the amount of effort required to complete the same task in chimpanzees is significantly less than that for capuchin monkeys. This is due to the capuchins' smaller body size.
Whereas the basis of this response is unknown (e.g. social emotions, as proposed for humans), negative responses to this type of situation support a relatively early evolutionary origin of inequity aversion, as well as the evolution of increased complexity in responses somewhere before the split of humans and chimpanzees 5-7 million years ago.
Press Releases:
Yerkes
Press Release (capuchins)
Yerkes
Press Release (chimpanzees)
Nature Press Release
(capuchins)
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