Capuchin monkeys, inequity aversion, and the frustration effect.

Each of 4 female capuchin monkeys ("model") was paired with another female capuchin ("witness") in an adjacent cage. In Phases 1 and 3, a model could remove a grape from the experimenter's hand while the witness watched. The witness was then offered a slice of cucumber, a less preferred food. Trials alternated between subjects 50 times, defining a session. In Phases 2 and 4, both were offered cucumber. Witness rejections of cucumber were infrequent and were not dependent on whether models received grape or cucumber. When models were offered cucumber, they rejected it at higher rates than did witnesses. These results fail to support findings of Brosnan and de Waal. An account based on the frustration effect accommodates these results and those of Brosnan and de Waal.