Study: Monkeys Have Sense of Justice

Sept. 18 -- 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.