Capuchins Are Aware of Injustice

Sept. 17, 2003 — Monkeys, like humans, are acutely aware of injustice, which suggests that a sense of equality is an ancestral trait among primates, a study said.

In an unusual two-year experiment, animal behaviorists Sarah Brosnan and Frans de Waal of Emory University in Atlanta, Ga., taught brown capuchin monkeys to receive tokens as a reward, and to barter them for food.

The monkeys were usually quite content to swap the tokens for cucumber, but if the researchers gave one of the monkeys a grape, a more eagerly-sought food, the other animals would become jealous.

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Some of them refused to hand over their tokens. Others would still exchange their token for the cucumber, but scornfully decline to eat it.

If the monkey which got the grape had received the coveted fruit for not doing anything, its colleagues often became incensed.

"People judge fairness based both on the distribution of gains and on the possible alternatives to a given outcome," Brosnan and de Waal wrote for Thursday's issue of Nature, the British science weekly.

"Capuchin monkeys, too, seem to measure reward in relative terms, comparing their own rewards with those available, and their own efforts with those of others," they said. "They respond negatively to previously acceptable rewards if a partner gets a better deal."

The pleasure of reward and anger at unfair treatment are known factors behind the human social hierarchy and cooperation. The researchers added that this evidence suggests the same may be true among non-human primates.