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So the final experiment that I want to mention to you is our fairness study. And so this became a very famous study. And there's now many more, because after we did this about 10 years ago, it became very well known. And we did that originally with capuchin monkeys. And I'm going to show you the first experiment that we did. It has now been done with dogs and with birds and with chimpanzees. But with Sarah Brosnan we started out with capuchin monkeys.

So what we did is we put two capuchin monkeys side-by-side. Again, these animals, they live in a group, they know each other. We take them out of the group, put them in a test chamber. And there's a very simple task that they need to do. And if you give both of them cucumber for the task, the two monkeys side-by-side, they're perfectly willing to do this 25 times in a row. So cucumber, even though it's only really water in my opinion, but cucumber is perfectly fine for them. Now if you give the partner grapes -- the food preferences of my capuchin monkeys correspond exactly with the prices in the supermarket -- and so if you give them grapes -- it's a far better food -- then you create inequity between them. So that's the experiment we did.

Recently we videotaped it with new monkeys who'd never done the task, thinking that maybe they would have a stronger reaction, and that turned out to be right. The one on the left is the monkey who gets cucumber. The one on the right is the one who gets grapes. The one who gets cucumber, note that the first piece of cucumber is perfectly fine. The first piece she eats. Then she sees the other one getting grape, and you will see what happens. So she gives a rock to us. That's the task. And we give her a piece of cucumber and she eats it. The other one needs to give a rock to us. And that's what she does. And she gets a grape and she eats it. The other one sees that. She gives a rock to us now, gets, again, cucumber. (Laughter) She tests a rock now against the wall. She needs to give it to us. And she gets cucumber again. (Laughter) So this is basically the Wall Street protest that you see here.

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Original from a TED talk by Frans de Waal, a primatologist, ethologist, and professor of primate behavior at Emory University.

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