Capuchin monkeys usually like eating cucumber. But when researchers Sarah Brosnan and Frans de Waal put two of the highly intelligent, cat-sized primates next to one another, gave one monkey watery cucumbers and the other only sugary grapes, the cucumber-receiving monkey responded in a way that might seem awfully similar to a human child. (The monkey repeatedly threw the cucumber at a researcher or onto the floor.)
That experiment, along with several others conducted over the last decade, suggest that our human sense of fairness evolved from monkeys, according to a study published this month in Science. Fairness, goes the theory, evolved from cooperative primate societies. And if some cooperative primates recognize inequality, they’ll protest.
“We can’t ask them why they’re rejecting the cucumber, but I think we’re getting an artifact of how they would respond in the wild,” Brosnan says. She explains that in cooperative societies, primates rely on partners to work with them and share. But if a capuchin monkey sees another consistently receiving better food, it might signal that he’s not working with a cooperative partner. “He would say, ‘That’s it, I’m leaving,’ and would pick up and move to another part of the forest,” Brosnan says. “They can’t do that in the test, so I think they’re getting frustrated.”
So what does this tell us about human inequality? According to Brosnan and de Waal’s research on primates, it triggers an evolutionary values system, including a sense of fairness. “You could also say that fights for equal pay, protests over social causes in general, are directly tied to our evolutionary desire for cooperation,” Brosnan says.
Her research suggests that protest is a sign of a healthy, cooperative system–even if it targets uncooperative individuals. There’s more to a protest than being whiney, or simply wanting grapes over cucumbers. Perhaps that means we ought to start seeing protests against inequality–like fair wages, Occupy Wall Street, or even the recent climate marches–as part of a larger strategy toward cooperation.