Tolerance for inequity may increase with social closeness in chimpanzees

Abstract

Economic decision-making depends on our social environment. Humans tend to respond differently to inequity in close relationships, yet we know little about the potential for such variation in other species. We examine responses to inequity in several groups of chimpanzees (Pan troglodytes) in a paradigm similar to that used previously in capuchin monkeys (Cebus apella). We demonstrate that, like capuchin monkeys, chimpanzees show a response to inequity of rewards that is based upon the partner receiving the reward rather than the presence of the reward alone. However, we also found a great amount of variation between groups tested, indicating that chimpanzees, like people, respond to inequity in a variable manner, which we speculate could be caused by such variables as group size, the social closeness of the group (as reflected in length of time that the group has been together) and group-specific traditions.

Figure 1

Mean±s.e.m. of failures to exchange for the chimpanzees across the four test types. The black bars represent the proportion of non-exchanges as a result of refusal to accept the reward, and the hatched bars represent the proportion of non-exchanges as a result of refusal to return the token. ET, equity test; IT, inequity test; EC, effort control; FC, food control.

Figure 2

Sum of the failures to exchange for each subject across sessions within a test type. Lines represent the linear regression of the data. Only those tests in which a higher-value reward was used are included (IT, EC, FC). IT, filled squares; EC, filled triangles; FC, filled circles. (Abbreviatious as in figure 1.)

Figure 3

Mean±s.e.m. of failures to exchange for each of the three housing conditions across the four test types. Grey bars represent subjects from the long-term social group, which had been co-housed for more than 30 years, black bars represent subjects from the short-term social group, which had been co-housed for ca. 8 years, and hatched bars represent subjects who were pair housed. Abbreviations as in figure 1.

Figure 4

Latency to exchange for high- versus low-rate refusers across the four test types. Black bars represent high-rate refusers (refuse to exchange at least 10% of the time) and hatched bars represent low-rate refusers (refuse to exchange less than 10% of the time). Abbreviations as in figure 1.