In recent tests designed to assess monkeys' sense of fairness, a group of brown capuchin monkeys "went on strike" and refused to perform routine tasks when they saw others receiving greater rewards for the same tasks.
The more effort the primates used to earn a reward, the more upset they appeared to be at the inequity, according to scientists who conducted the research.
"In human terms it doesn't matter how hard you have to work for a million dollars," said lead researcher Sarah Brosnan of Georgia State University in Atlanta.
"But there's a pretty low cutoff point on what you'll do for five."
Building on previous research, Brosnan's team tested six pairs of monkeys on a simple task: handing a token to a human examiner in return for a food reward.
When monkeys noticed that their partners received better rewards for the same task—a cherished grape instead of a bit of cucumber—they became likely to refuse participation, the study showed.
The behavior, called inequity aversion, might have its roots in activities like food gathering, in which primates can suffer if they cooperate with others who do not do their share of work, Brosnan said.
Brosnan stressed that the primates' response wasnt one of simple greed or wanting a bigger payoff just because they knew one was available.
"What really mattered was if someone else got a better reward," she said, "not [just] that they wanted a better reward."
The team's findings appear in the new online version of the Proceedings of the National Academy of Sciences.
Grapes of Wrath?
Brosnan's team studied monkeys' sense of fairness in a similar test conducted in 2003.
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