People aren't the only animals who know when they've gotten a raw deal. So do monkeys and chimpanzees, according to some clever experiments concluded recently at the Yerkes National Primate Research Center in Atlanta. The findings provide insight into how social environment and relationships sway human decision making, reports Sarah F. Brosnan, who conducted the studies with Frans de Waal at the center.

That nonhuman primates react to social unfairness suggests that such judgment is deeply rooted in evolution. In one experiment, Brosnan and de Waal gave female capuchin monkeys granite pebbles and asked them to hand the pebbles back to the researcher. Capuchins that did so received a sliver of cucumber. The monkeys completed the exchanges 95 percent of the time.

When scientists upped the reward to a coveted grape and gave it only to certain monkeys, the short-changed primates became less cooperative. Their apparent sense of inequity grew much more pronounced when the favored monkeys did not even have to perform a task for a grape; some of the offended animals refused to hand over the pebble or threw their measly cucumber at the researcher.

Social relationships can temper dissatisfaction, however. In a separate study, chimpanzees that had been raised together and lived with one another for 30 years displayed no frustration when unequal rewards were given. But chimps that had met only as adults and lived together for eight years still became agitated at inequity. Brosnan says these reactions closely parallel human behavior. People are more likely, for example, to respond negatively to an unfair situation involving a stranger than they are with good friends or spouses.

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