Bluematter.

Two researchers at the Yerkes National Primate Research Center have found that brown capuchin monkeys have a sense of fairness and will reject inequitable rewards, much as humans do.

Frans de Waal, C.H. Candler professor of primate behavior, said his work with Georgia State University professor Sarah Brosnan was based on a study they did in 2003. In that experiment, monkeys responded negatively when a partner received a superior reward for completing the same task, retrieving a pebble and placing it the researcher’s hand.

“As soon as the partner’s getting something better, like grapes, they don’t want to do it any more,” de Waal said. “They throw the food out of the cage sometimes.”

Brosnan and de Waal conducted a follow-up study to rule out alternative explanations for why monkeys would reject slices of cucumber, a previously acceptable reward.

“The most important one was you could argue that the monkeys reject the cucumber pieces because they see grapes and they want grapes,” de Waal said. “We would show them grapes, but we would put them away, and showing them the grapes didn’t make a difference in our test. It had to do with what partner was getting.”

Brosnan and de Waal also varied the amount of effort required to complete the task to see its effect on the monkeys’ reactions.

They found that when monkeys had to expend more effort, they were more sensitive to inequity and less likely to accept cucumber slices when partners had received grapes for equal or less work. But both would accept grapes even if they completed tasks at different levels of difficulty, de Waal said.

“If you gave them grapes, they were not sensitive to effort,” he said. “The grape is such a good reward that they would do whatever to get the grape.”

De Waal said similar behavior has been shown in chimps, and he “wouldn’t be surprised” if other species, like dogs, would react the same way to inequitable rewards.

According to de Waal, the research illustrates inequity aversion, a concept from the field of behavioral economics, which applies behavioral psychology to economic interactions. Like the monkeys in de Waal’s study, humans do not always act as rational profit maximizers and sometimes turn down good offers if someone else is getting a better deal.

“For a monkey to refuse a perfectly fine food like cucumber just because somebody else is getting something better is an irrational reaction,” de Waal said. “Profit maximizing requires that whenever you can get something you take it.”

Some scholars, however, argue that reactions like the monkeys’ make sense in a social context. The capuchins’ sense of fairness has “evolved within the context of cooperation,” de Waal said, because capuchins live in groups and sometimes hunt squirrels together.

“If you don’t get in accordance to your effort, you should be sensitive to that, or everyone will take advantage of you,” he said. “It’s actually a rational response to make sure you get the right rewards for the right amount of work.”