Inequities big and small can lead people to believe that life is indeed not fair. But how humans respond to unfair situations depends on the social circumstances: inequality among friends and family, for instance, is less disturbing than it is among strangers. The results of a new study indicate that the same is true for chimpanzees, a finding that sheds light on how our sense of fairness evolved.

In the fall of 2003 Sarah Brosnan and Frans de Waal of the Yerkes National Primate Research Center in Atlanta determined that capuchin monkeys don't like being subjected to treatment they deem unjust. In the new work, the researchers tested the reactions of pairs of chimpanzees to exchanges of food that varied in quality. The animals received either a grape, which they coveted, or a less appealing cucumber, and they could see what their partner obtained. In pairs of chimps that had lived together since birth, the individual given the cucumber was less likely to react negatively to the situation than was the short-changed member of a pair that did not know each other as well. Indeed, chimps in the short-term social groups refused to work after their partner received a better reward for the same job. "Human decisions tend to be emotional and vary depending on the other people involved," Brosnan says. "Our findings in chimpanzees implies this variability in response is adaptive and emphasizes there is not one best response for any given situation, but rather it depends on the social environment at the time."

Further experiments to investigate reactions to unfair situations are ongoing at the center in the hopes of understanding why we humans make the decisions we do. "Identifying a sense of fairness in two closely related nonhuman primate species implies it could have a long evolutionary history," Brosnan remarks. The findings will be published in the February 7 edition of the Proceedings of The Royal Society B: Biological Sciences.