Social comparison mediates chimpanzees' responses to loss, not frustration

Abstract

Why do chimpanzees react when their partner gets a better deal than them? Do they note the inequity or do their responses reflect frustration in response to unattainable rewards? To tease apart inequity and contrast, we tested chimpanzees in a series of conditions that created loss through individual contrast, through inequity, or by both. Chimpanzees were tested in four social and two individual conditions in which they received food rewards in return for exchanging tokens with an experimenter. In conditions designed to create individual contrast, after completing an exchange, the chimpanzees were given a relatively less-preferred reward than the one they were previously shown. The chimpanzees' willingness to accept the less-preferred rewards was independent of previously offered foods in both the social and individual conditions. In conditions that created frustration through inequity, subjects were given a less-preferred reward than the one received by their partner, but not in relation to the reward they were previously offered. In a social context, females were more likely to refuse to participate when they received a less-preferred reward than their partner (disadvantageous inequity), than when they received a more-preferred reward (advantageous inequity). Specifically, the females' refusals were typified by refusals to exchange tokens rather than refusals to accept food rewards. Males showed no difference in their responses to inequity or individual contrast. These results support previous evidence that some chimpanzees' responses to inequity are mediated more strongly by what others receive than by frustration effects.

Fig. 1

Percentage of trials, and standard error of the mean for each, in which the female (black) and male (gray) chimpanzees refused to accept or exchange the token with experimenter in each of the four social conditions. The horizontal lines represent significant differences between conditions, where P<0.05

Fig. 2

Percentage of trials in which the female chimpanzees refused when tested as both the subject and partner. Error bars show the standard error of the mean for each. The horizontal lines across roles indicate significant differences, where P<0.05