At the first stage, the chimpanzees heard an auditory signal: the same rattling sound coming from the first container. They then received indirect visual evidence: a trail of peanuts leading to a second container. At this point, the chimpanzees chose the first container, presumably because they found the audio evidence more convincing. But then the team removed the stone from the first container. The piece of stone suggested that it was not food that was making the rattling sound. “At this point, a rational agent must conclude, ‘The evidence I followed is now disproved, and I should choose another option,’” Engelmann told Ars. “And that’s exactly what the chimpanzees did.”
The team used 20 chimpanzees in all five experiments, and they followed evidence well above chance—about 80 percent of the time. “At the individual level, about 18 out of 20 chimpanzees followed this expected pattern,” says Engelmann.
He sees this research as one of the first steps towards understanding how rationality developed and when the first sparks of rational thinking appeared in nature. “We're doing a lot of research to answer exactly this question,” Engelmann says.
The team believes that 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 complex and demonstrate a more advanced form of reflective rationality, which I expect only chimpanzees and perhaps bonobos have.”
However, in his opinion, humans are still at least one level above chimpanzees. “Many people say that reflective rationality is the final stage, but I think you can go even further. What people have is what I would call social rationality,” Engelmann argues. “We can discuss and comment on each other’s thoughts and make each other even more rational in the process.”
Sometimes, at least in humans, social interactions can instead reinforce our irrationality. But chimpanzees don't seem to have this problem. Engelman's team is currently conducting a study to see if the choices chimpanzees make influence the choices of their fellow chimpanzees. “Chimps only followed another chimpanzee's decision when the other had better evidence,” Engelmann says. “In this sense, chimpanzees appear to be more rational than humans.”
Science, 2025. DOI: 10.1126/science.aeb7565
