Monkeys have a sense of justice. They will protest if they see another monkey get paid more for the same task.
By Dr David Whitehouse
BBC News Online science editor
Researchers taught brown capuchin monkeys to swap tokens for food. Usually they were happy to exchange this "money" for cucumber.
Capuchins: Cooperative and tolerant
But if they saw another monkey getting a grape - a more-liked food - they took offence. Some refused to work, others took the food and refused to eat it.
Scientists say this work suggests that human's sense of justice is inherited and not a social construct.
Differential reward experiment
The research was carried out at Emory University in the US, by Sarah Brosnan and Frans de Waal, and is reported in the journal Nature.
"I'm extremely interested in the evolution of cooperation," Sarah Brosnan told BBC News Online.
"One of the most interesting areas is the recent suggestion that human cooperation is made more effective by a sense of fairness."
She wanted to find out if the human sense of fairness is an evolved behaviour or a cultural construct - the result of society's rules.
So she and her colleagues devised an experiment using capuchin monkeys.
Sarah Brosnan said: "I chose the capuchin because they are very cooperative, and because they come from a very tolerant society.
Aware what the other one gets
"We designed a very simple experiment to see whether or not they react to differential rewards and efforts."
Capuchins like cucumber, but they like grapes even more. So a system was devised whereby pairs of capuchins were treated differently after completing the same task.
"They had never before been in any sort of situation where they were differentially rewarded," she said.
"We put pairs of capuchins side by side and one of them would get the cucumber as a reward for a task."
The partner sometimes got the same food reward but on other occasions got a grape, sometimes without even having to work for it."
'A highly unusual behaviour'
The response was dramatic, the researchers said.
"We were looking for a very objective reaction and we got one. They typically refused the task they were set," Sarah Brosnan said.
"The other half of the time they would complete the task but wouldn't take the reward. That is a highly unusual behaviour.
"Sometimes they ignored the reward, sometimes they took it and threw it down," she added.
The researchers were not surprised that the monkeys showed a sense of fairness, but they were taken aback that they would turn down an otherwise acceptable reward.
They never blamed their partner, say researchers
"They never showed a reaction against their partner, they never blamed them," Sarah Brosnan said.
Commenting on the results, experts in the subject told BBC News Online that the idea of a long evolutionary history for a sense of fairness was an exciting one.
However, they added that they would like to see more research involving more than just the five subjects tested in the Nature study.
So does our instinctive feeling of fairness predate our species?
"It may well," Sarah Brosnan said, and further experiments are planned to see how extensive a sense of justice in the animal world is.
"We are currently repeating the study on chimpanzees, a great ape species, to learn something more about the evolutionary development of the sense of fairness.
"I suspect that there are other non-primate species with tolerant societies that will show the same behaviour."