Frans de Waal, a Dutch primatologist and ethologist has contributed to research on primate social behavior, including conflict resolution, cooperation, food-sharing, and inequity aversion. De Waal was the first to introduce the thinking of Machiavelli to primatology and has inspired the field of primate cognition from his writings of primate emotions and intentions. De Waal has proven “reconciliation” to be a part of animal behavior and has focused on cooperation, altruism, and fairness in order to reveal the origins of animal morality. When studying fairness and equality with capuchin monkeys, De Waal hypothesized that giving a capuchin a lesser reward for a task while his peer received a greater reward for the same task would create inequality and produce a negative reaction from the capuchin.

In Frans de Waal’s fairness study, capuchin monkeys were used to study the rejection of unequal pay. Two capuchin monkeys from the same social group were placed in separate cages side-by-side and were assigned a simple task to complete, to deliver a rock to the observer. When both were rewarded with cucumber for the task, the capuchins were satisfied, however when one capuchin was given a grape instead of the cucumber as a reward for the same task, inequity was created.

Beginning the experiment, a capuchin was rewarded with a cucumber for delivering a rock to the observer. After the capuchin saw her peer receive a grape, a better reward than a cucumber, for the same task, however, she completed the task once more. After still receiving a cucumber as a reward for the task, the capuchin rejected the vegetable by throwing it at the observer and began reaching her arms out of the holes in the enclosure and grabbing on to the cage and shaking about. Once the capuchin witnessed her peer receive another grape for the same task again, she then hesitated in completing the task and banged the rock on the metal wall of her cage. She then delivered the rock to the observer, and when given a cucumber as a reward again, reacted in a similar fashion by throwing the vegetable, reaching her arms out of the cage, and thrashing about.

It would appear at first that De Waal’s hypothesis was correct in that the capuchin sensed a feeling of inequality and was negatively affected. Because the capuchin only rejected the lesser reward after seeing his peer receive a greater reward, it would seem that the capuchin wanted equal pay for equal work. De Waal believed that the capuchin’s distressed behavior showed his understanding of equality and fairness. He believed that this experiment helped demonstrate that animals have a sense of morality and that they can understand social expectations. Alternatively, the capuchin could have simply desired the greater reward after discovering it, regardless of whether or not his peer received the greater reward. Instead of the capuchin wanting a grape because his peer received a grape, he may have simply preferred the grape over the cucumber.

In order to test these hypotheses and better understand the factors causing the monkey’s “tantrum” behaviors, a new study could be conducted. By removing the capuchin’s peer from the study, one can better understand if the monkey is reacting in a social context. In a similar study, the capuchin must deliver a rock to the observer in exchange for a food reward. At first the capuchin will be given cucumber slices as a reward, but after a few exchanges, a grape, a more-desired reward, could be granted. After delivering grapes as a reward for some amount of time, cucumber slices could be introduced again. Based upon the capuchin’s reaction to receiving cucumber slices for the same task that he received grapes, one can more accurately determine whether a peer monkey affects the capuchin’s behavior.

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