Study: Monkeys Have Sense of Justice

Sept. 18, 2003 -- It turns out monkeys, like people, are no fools when it comes to equal pay for equal work.

A new study found when brown capuchin monkeys noticed their partners were getting a better reward — a juicy grape — for the same task, and sometimes for no task at all, they became indignant.

The results suggest people and monkeys may have inherited a sense of justice over the course of evolution and it is not something humans simply learn from society.

"There's a theory that people who have a sense of fairness are more likely to cooperate," explained Sarah Brosnan, a researcher at Emory University in Atlanta, Ga. whose study appeared this week in the journal Nature. "We know that capuchins cooperate and we wanted to find out if fairness mattered."

It mattered.

Sour Grapes

In the tests, Brosnan and her colleague, Frans de Waal, handed monkeys a granite stone in the presence of another monkey. The monkeys were trained to then return the stone in exchange for a reward.

If both monkeys got a cucumber slice in return, the animals completed the trade 95 percent of the time. But if one monkey got a grape while the other received a lesser reward of a cucumber chunk, the slighted animal would cooperate only 60 percent of the time. Sometimes it would refuse the cucumber or turn its back to their human subject.

If the animal's partner received a grape without even having to carry out a trade, the partner who had to complete the trade for a cucumber became even more annoyed and cooperated only 20 percent of the time. In some cases, the monkey would throw its cucumber slice back at the human testers.

"It's hard to judge the emotions of non-human animals — you can't ask them," said Brosnan. "But they did show signs of what might be frustration — it's highly unusual for a capuchin to turn down food."

Brosnan used five female monkeys for the study since female capuchin monkeys live in groups in tropical forests and depend on cooperation to share food. Male capuchins, meanwhile, generally live on their own or as alpha males ruling over a group of females.

Cooperation in Our Genes?

But this finding isn't just about female monkeys, says Paul Zak, a leading figure in the relatively new field of neuroeconomics at the Claremont Graduate University in Claremont, Calif. Zak studies human social interaction by testing subjects (both men and women) in scenarios like the so-called Ultimatum Game.

In this exercise, Player 1 is given $10 and then decides whether to send Player 2 (not in the room) some of her money. If she does, the money will be tripled and her partner may then choose to share none, some or all of the cash.

Old mathematical theories would suggest that neither player would share any of the cash, but Zak's and others' studies show that people generally do choose to cooperate and share. Why? Zak believes it has something to do with the close bond all mammals share with their mothers during infancy and childhood.

"Social bonding is an essential feature of mammals," he said. "We're nursed closely and that feeds into our concept of the importance of cooperation."

Living in a social system — as humans and female capuchin monkeys do — then enhances the value placed on cooperation. But, just like the frustrated capuchin monkeys who get cucumbers instead of grapes, people can quickly become less cooperative if they sense things aren't fair.

Cooperative — But Not Gullible

Zak found when he conducted his tests in other countries the results appeared to reflect the level of social equality in each country.

Those living in Peru or Brazil, for example, where there is a large gap between rich and poor, were least willing to cooperate. And in Norway, Sweden and Denmark, where salaries are fairly level and social programs serve most of the population, people were most willing to take a gamble on their anonymous partners and cooperate.

"I think we are very biased toward being cooperative — we stop at stop signs, pay our taxes," said Zak. "But if it's clear things aren't fair, we can turn it off."

After all, he adds, "We're not robots."