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.
“But no one has ever looked into their ability to revise beliefs in rational ways,” Engelmann explains. He views revision of beliefs as the hallmark of rationality, a perspective that’s consistent with our best knowledge in philosophy, psychology, and cognitive science. “There are so many irrational ways of responding to counter evidence—you know, ‘I’m gonna keep believing what I believe, I’m not gonna switch my mind,’” Engelmann claims.
Engelmann and his colleagues tested whether chimpanzees can revise their beliefs rationally based on the same setup, where the animals chose between two containers. “It turned out, when they first got the evidence that the food was in one of those containers and they made their choice based on that, they could later change their mind when we offered them evidence to the contrary,” Engelmann explains.
Complicated choices
The team started by classifying evidence presented to the chimpanzees as either weak or strong. Weak evidence included things like crumbs around one of the containers. Strong evidence pointed to the food more directly, like the rattling sounds used in previous studies. The first two experiments relied on giving chimpanzees weak evidence pointing at one container, strong evidence pointing at the other one, and manipulating the order in which the evidence was received.