Someone Got a Grape

In 2003, two researchers at Emory University conducted an experiment with capuchin monkeys.

Sarah Brosnan and Frans de Waal had taught the monkeys a simple transaction. A monkey would hand a small token to a researcher and receive a piece of cucumber in return. It worked perfectly well. The monkey handed over the token. The researcher handed over the cucumber. The monkey ate it.

Then the researchers changed one thing. Two monkeys were placed where they could see each other. The first handed over her token and received the usual cucumber. But when the second monkey completed exactly the same task, the researcher gave her something considerably better: a grape. Capuchins love grapes.

Now the first monkey was offered another cucumber. This time she looked at it, looked at the monkey eating the grape, and wanted nothing to do with it. In the now-famous video of the experiment, she takes the cucumber and throws it back at the researcher.

Nothing about the cucumber had changed.…