In it, capuchins exchanged a piece of rock with their human handlers in return for a morsel of food. (See a photo of capuchin monkeys.)
Monkeys that witnessed their partners getting grapes often refused to conduct future exchanges, would not eat the cucumbers they received, and in some cases, threw their rewards at the researchers.
In the new study, the scientists tried to rule out alternate explanations for such behavior, including the possibility that the primates knew the grapes were available and were simply holding out for a better reward.
The monkeys were sometimes shown a grape before completing their task, but at other times they were unaware a grape was available. There was no discernible difference in the monkeys' responses, Brosnan said.
Researchers also distributed rewards evenly among the monkeys, so that no one animal was consistently rewarded or shortchanged.
The scientists found that the capuchins didn't become frustrated by expecting a grape simply because they had previously received one for doing the same task.
Laurie Santos, a Yale University psychologist, said, "The original study was met with much controversy in the field, including a number of now published claims that the original effect could not be replicated using slightly different tasks.
"Given this level of controversy, it's nice to see that their findings hold up when other alternative explanations are controlled," she added.
Evolution of Justice
Brosnan said her team's research scratches the surface of a philosophical quandary: Is the human sense of fairness instilled by social institutions like religion, or is it the product of a long genetic evolution?
Even if the primates are really displaying a sense of social justice in the experiment, it remains primitive in important ways, Brosnan said.
"We aren't seeing a whole lot of response [when the monkeys] are the better rewarded ones," Brosnan said.
"In humans we've expanded [our sense of justice] to [include] situations where another is treated badly."
Like humans, many monkeys live and interact in groups much larger than the study pairs. Exploring the complex dynamics of those social groups may be a next step for Brosnan and her colleagues.
"We'd like to study those relationships and how they affect their responses to inequity," she said.
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