Abstract
A capuchin monkey will accept a preferred food in exchange for tokens if a conspecific also exchanges tokens for the preferred food; but he will reject a less-preferred food if the conspecific exchanges tokens for preferred food. These rejections are usually interpreted as a case of an inequity aversion based on a precursor of a sense of justice. An alternative hypothesis proposes that it is the contrast between the types of food reinforcement experienced by the subject throughout the task that affects the reinforcing function of the less-preferred food. The present study tested this hypothesis with three conditions. Capuchin monkeys received cucumbers (the less-preferred food) as the reinforcer in a token-exchange task during the first and third conditions, and received grapes (the preferred food) in the second condition. The difference in the proportions of acceptances between the first and the third conditions provided evidence of negative contrast in this species, a type of contrast that can at least partially account for the rejections documented in the inequity-aversion literature.