Monkeys Can Tell What's Fair
Both capuchin monkeys and chimpanzees seem to have a concept of fairness, and will become emotionally distressed in unfair situations. If two monkeys receive unequal pay for the same task—one gets a tasty grape while another gets a cucumber slice, for example—the "cheated" party tends to get upset. This behavior was famously documented by primatologist Frans de Waal.
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Key Facts In This Video
Initially, capuchin monkeys are happy to be rewarded with a cucumber for performing the same task. (0:20)
A capuchin monkey will become visibly upset if it sees that its comrade is getting a better reward for a task that both have performed. (0:42)
Chimpanzees have been known to reject a reward if they see that another chimp has received a worse one. (0:58)