Date of Award


Degree Name

Master of Arts



First Advisor

Reichard, Ulrich


Inequity aversion, the negative response to receiving an unequal reward, has been intensely studied and is well established in humans. However, why humans developed a profound sense of equity is still enigmatic, and the evolutionary roots of this interesting phenomenon are still largely unknown. The little research that has been completed on nonhuman primates indicates that some species, like humans, are inequity averse, while others are not. Brosnan and de Waal (2003) suggested that an aversion to inequitable outcomes coevolved as a response to an increased emphasis on cooperative relationships, where individuals would respond negatively when their rewards differed from those of a social partner. Chen and Santos (2006), however, suggested that inequity aversion evolved in response to contrast effects, or individual expectations, in which individuals would respond negatively when their rewards differed from those previously received by the individual; this suggests that an animal forms expectations that are irrespective of rewards received by a social partner. This study aimed to test these two hypotheses by examining responses to inequitable outcomes in three yet untested primate genera (Gorilla, Nomascus, and Papio) and one genus which has been previously tested (Pongo). To investigate responses to inequitable outcomes, an established inequity paradigm was used following Brosnan and de Waal (2003), in which primate subjects were required to complete a task before receiving a reward. Because only responses to differences in reward quality had been tested with nonhuman primates using this paradigm, this study introduced an additional test condition to determine how reward quantity differences would affect individual responses to unequal offerings. It was found that some olive baboons, western-lowland gorillas, and white-cheeked gibbons responded negatively to both individual expectations and social expectations. Orangutans, however, responded to individual expectations, but not to social expectations. This study suggests that there is individual variation in inequity responses of olive baboons, western-lowland gorillas, and white-cheeked gibbons; this is similar to the individual variation in inequity aversion that has been proposed for chimpanzees and bonobos. This study also suggests that orangutans are not inequity averse, which supports results found in previous studies of orangutan inequity aversion.




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