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.
When the first piece of evidence was weak, the chimpanzees were way more likely to change their choice when they received strong counter-evidence later. When the order was reversed and the strong evidence was followed by the weak one, they usually stuck to their initial belief. “Quite frankly, I think many other animals would pass this test,” Engelmann says. “But then we moved on to making things more complicated for the chimpanzees.”
In the third experiment, the team tested whether the chimpanzees really think about the options they have. The setup was changed to include three containers. At the beginning, the chimpanzees heard rattling sounds from the first container, which was a piece of relatively weak evidence in this experiment. Then they received strong evidence pointing at the second container: they were shown the food in there through a glass pane. The team provided them with no evidence for the third container. Finally, the strong evidence container was removed, and the chimpanzees were allowed to make their choice. “We wanted to know if they stand and think like, ‘Hmm, the food could be here, but it also could be there,’” Engelmann says.
The chimpanzees apparently did organize the evidence they had in a hierarchy. When the strong evidence container was gone, they consistently went along with the weak evidence and chose the container with the rattling noise.
The fourth experiment showed the chimps most likely worked with mental representations of the evidence they had. When the team offered them the same auditory clue repeatedly, it did not increase the likelihood of them choosing the container it pointed to—they knew they had already heard the rattling, so they considered the evidence redundant.
“And then we went for the fifth experiment, which was sort of the big finale: We found chimpanzees understand second-order evidence,” Engelmann says.
Sparks of rationality
The goal of the final experiment was to see if chimpanzees can figure out if the evidence they’ve just received supports or contradicts other pieces of evidence they had. “They understand evidence about the evidence—more colloquially, they understand that the evidence can be misleading,” Engelmann explains.
In the first step, the chimps got the auditory evidence, the same rattling sound coming from the first container. Then, they received indirect visual evidence: a trail of peanuts leading to the second container. At this point, the chimpanzees picked the first container, presumably because they viewed the auditory evidence as stronger. But then the team would remove a rock from the first container. The piece of rock suggested that it was not food that was making the rattling sound. “At this point, a rational agent should conclude, ‘The evidence I followed is now defeated and I should go for the other option,’” Engelmann told Ars. “And that’s exactly what the chimpanzees did.”
The team had 20 chimpanzees participating in all five experiments, and they followed the evidence significantly above chance level—in about 80 percent of the cases. “At the individual level, about 18 out of 20 chimpanzees followed this expected pattern,” Engelmann claims.
He views this study as one of the first steps to learn how rationality evolved and when the first sparks of rational thought appeared in nature. “We’re doing a lot of research to answer exactly this question,” Engelmann says.
The team thinks rationality is not an on/off switch; instead, different animals have different levels of rationality. “The first two experiments demonstrate a rudimentary form of rationality,” Engelmann says. “But experiments four and five are quite difficult and show a more advanced form of reflective rationality I expect only chimps and maybe bonobos to have.”
In his view, though, humans are still at least one level above the chimps. “Many people say reflective rationality is the final stage, but I think you can go even further. What humans have is something I would call social rationality,” Engelmann claims. “We can discuss and comment on each other’s thinking and in that process make each other even more rational.”
Sometimes, at least in humans, social interactions can also increase our irrationality instead. But chimps don’t seem to have this problem. Engelmann’s team is currently running a study focused on whether the choices chimps make are influenced by the choices of their fellow chimps. “The chimps only followed the other chimp’s decision when the other chimp had better evidence,” Engelmann says. “In this sense, chimps seem to be more rational than humans.”
Science, 2025. DOI: 10.1126/science.aeb7565
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