The influence of reward quality and quantity and spatial proximity on the responses to inequity and contrast in capuchin monkeys (Cebus [Sapajus] apella)

Abstract

Recent evidence within the field of comparative psychology has demonstrated that small differences in procedure may lead to significant differences in outcome. Therefore, failing to fully explore the impact of different contexts on a behavior limits our ability to fully understand that behavior. A behavior that has exhibited substantial variation, both within and across studies, is animals’ responses to violations of their expectations, either when expectations were based on another’s outcome (inequity) or one’s own previous outcome (contrast). We explored this further in capuchin monkeys, focussing on two factors that often vary in such tests but have not yet been rigorously explored: the relative values of the food rewards and the degree of separation of the subjects. Concerning the first, we examined responses to violation of expectations when the difference between what was expected (or what the partner got) and what was received differed in either quality or quantity. Concerning the second, we compared responses when the two individuals were separated by a clear partition (barrier condition) versus sharing the same enclosure without the partition (no barrier condition). Our results suggest that responses to inequity are most likely to emerge when the food received is low-value food, regardless of the difference between the actual and expected outcome. However, capuchins did not respond differently to different quantities of rewards, nor did the degree of separation between subjects significantly impact results. We consider the implications of this work for both studies of violation of expectation and other cognitive and behavioral tasks.

Talbot CF, Parrish A, Watzek J, Essler JL, Leverett KL, Paukner A, Brosnan SF (2018) The influence of reward quality and quantity and spatial proximity on the responses to inequity and contrast in capuchin monkeys (Cebus [Sapajus] apella). J Comp Psychol 132:75-87. doi:10.1037/com0000088

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