Getting the short end of the stick tends to tick people off. It turns out the same is true for monkeys. Scientists report today in the journal Nature that capuchin monkeys become upset when they feel they've been treated unfairly. The findings suggest that the animals have an innate sense of justice, a trait previously thought to be unique to humans.

Sarah F. Brosnan and Frans B. M. de Waal of Emory University studied the responses of captive brown capuchin monkeys to rewards from their human handlers. The researchers first trained the animals to swap tokens in exchange for food. Initially, the monkeys were happy to trade a token for a slice of cucumber. But when one monkey in a pair received a more sought-after grape reward instead, the animal offered the cucumber was often less than impressed. Sometimes the slighted animals refused to give up their tokens; on other occasions they took the cucumber but refused to eat it or tossed it out of the cage entirely. "We showed the subjects compared their rewards with those of their partners and refused to accept a lower-value reward if their partners received a higher-value reward," Brosnan says. "This effect is amplified when the partner does not have to work for the reward." Only females were selected for this study, because previous work had shown males to be less attuned to inequality than females.

The researchers are planning experiments with chimpanzees to determine whether they will react in a similar manner to the capuchins and humans. "People often forgo an available reward because it is not what they expect or think is fair," Brosnan remarks. "Such irrational behavior has baffled scientists and economists, who traditionally have argued all economic decisions are rational. Our findings in nonhuman primates indicate the emotional sense of fairness plays a key role in such decision-making."