Fairness: product of evolution

Attention bosses: Even monkeys seem to know the value of equal pay for equal work.

When rewarded similarly for the same task - in this case, exchanging a small rock with a scientist - capuchin monkeys worked happily for a slice of cucumber. But after they witnessed a partner getting a coveted, succulent grape for the bit of granite, the cucumber-paid monkeys took offense.


Some went on strike. Some kept halfheartedly doing the work, but refused to accept the cucumber.

"There were none that didn't care," said Sarah Brosnan, a graduate student at Emory University in Atlanta. A description of her experiment, conducted with Emory's Frans de Waal, appeared in the journal Nature.


If primates offer a window to the origin of human behavior, the findings suggest that people may be born with a strong, primal reaction to unfair rewards. The researchers have a term for it: inequity aversion.

The monkeys "respond negatively to previously acceptable rewards if a partner gets a better deal," they wrote in the journal. "Although our data cannot elucidate the precise motivations underlying these responses, one possibility is that monkeys, similarly to humans, are guided by social emotions."

de Waal and his colleagues have studied capuchin monkeys before, finding, among other things, that profit-sharing is a strong motivator in monkey work situations.

For the new experiment, the scientists divided a group of female monkeys into pairs. For the first few rounds, both monkeys in each pair were rewarded with cucumbers. To get the prize, the monkey had to place the rock token in Brosnan's hand. More than 95 percent of the time, each did so readily.

She then began paying one monkey partner with a grape. The rate of successful exchange for the monkeys paid in cucumbers quickly fell to 60 percent.

Sometimes the slighted monkey wouldn't give up the token. Other times, in a more dramatic show of disgust, it would throw the cucumber out of the testing area. The latter reaction was especially striking to the researchers - that a payoff that had been perfectly fine earlier suddenly became unacceptable.

The worst rate of token exchange occurred when the monkey witnessed a partner getting a better reward for performing less work. (Brosnan would simply hand the other monkey a grape, with no need for a token.) In that part of the experiment, the overworked, underpaid monkey would hand over the rock or take the cucumber only about 20 percent of the time.

"This is the first experimental test to look at a sense of fairness in any animal," Brosnan said. The results suggest that fairness may be an artifact of evolution, something that has soaked into the primate psyche over eons.


Many researchers have looked at reactions to inequity in humans, finding that cucumber-style wages in a grape-paying workplace can affect turnover, morale and worker health.

"I think we all react to it in a negative way," said Jason Shaw of the University of Kentucky in Lexington, who has studied fairness in pay issues.

But how each person responds is a complicated picture, he said, influenced by the employee's life situation and budget. Some employees, like the monkeys who stopped handing over the tokens, look for other jobs. Others with more limited options keep working, but often find their health suffering, Shaw said.

Brosan said she hopes experiments like hers will help social scientists understand the deeply emotional reactions to unfairness. She is hoping that similar experiments in chimpanzees, humans' closest living relatives, might add insight to this behavior.