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.

如果同种异体也交换代币的首选食物，卷尾猴将接受代币的首选食物。但如果同种异种人交换代币以换取首选食品，他将拒绝首选食品。这些拒绝通常被解释为基于正义感前兆的不平等厌恶的情况。另一种假设提出，正是受试者在整个任务中经历的食物强化类型之间的对比影响了偏爱食物的强化功能。本研究在三个条件下检验了该假设。在第一个和第三个条件下，卷尾猴在代币交换任务中接受了黄瓜（不太受欢迎的食物）作为补强物，而在第二个条件下接受了葡萄（首选食物）。