When Aristotle claimed that humans differ from other animals because they have the ability to be rational, he understood rational to mean that we could form our views and beliefs based on evidence, and that we could reconsider that evidence. “You know—ask ourselves if we should really believe that based on the evidence we’ve got,” says Jan M. Engelmann, an evolutionary anthropologist at the University of California, Berkeley.

Engelmann says that from the beginning of the Western intellectual tradition, people thought that only humans are rational. So, he designed a study to see if rationality shows up in chimpanzees. It turned out that they’re almost as rational as we are.

Food puzzles

“There was quite a bit of research showing that chimpanzees can form their beliefs in response to evidence,” Engelmann says. The experiments usually involved chimpanzees deciding which of the two boxes contained a snack. When the researchers shook both boxes and there was a rattling sound coming from one of them, the chimps almost always chose the box where the rattling came from.

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