Fair is fair

Natural History, Dec, 2003 by Stephan Reebs

For more than two years Sarah F. Brosnan and Frans B.M. de Waal have been bartering with brown capuchin monkeys. Sometimes the animals get a fair deal, sometimes not. It's all part of a study the two primatologists are conducting at the Yerkes National Primate Research Center in Atlanta, on the evolutionary origins of the sense of fairness. Brosnan and de Waal propose that an aversion to inequity, regarded as a cornerstone of human cooperation, may have evolved in our primate ancestors.

In a recent experiment with pairs of captive capuchins, Brosnan handed a familiar token (a small rock) to one of the monkeys, then turned her own hand palm up. If the capuchin returned the token to Brosnan's hand within a minute, it got a reward. The same basic procedure was repeated nonstop with both monkeys, alternating between them, for twenty-five cycles.

The reward setup had four variations: (1) The reward was a piece of cucumber ("boring" food) for both monkeys in the pair--equal treatment for equal "work." (2) The "subject" got cucumber and its partner got a yummy grape, even though both monkeys did the same work. (3) The partner was absent, but a grape was placed in the partner's area as the subject watched. Then the subject not only had to work but also got cucumber in return. (4) The partner was given a grape without having done any work; the subject did the work but got the cuke--outrageously unequal treatment.

When treated unfairly, the rive subjects often refused to return the rock token or tossed the cuke across the room. Occasionally a monkey settled for inequality, although sometimes it became more outraged as the unequal treatment persisted. ("Monkeys reject unequal pay," Nature 425:297-99, September 18, 2003)