In the first phase, the chimpanzees received auditory evidence, the same loud sound coming from the first container. Then, they found indirect visual evidence: a trail of peanuts leading to another container. At this point, the chimpanzees picked up the first container, possibly because they considered the auditory evidence to be stronger. But then the team will first remove a rock from the container. The piece of rock revealed 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 turn to another option,'” Engelman told Ars. “And that’s exactly what chimpanzees did.”
The team had 20 chimpanzees who participated in all five experiments, and they followed the evidence well above chance levels in about 80 percent of the cases. Engelmann claims, “At the individual level, about 18 out of 20 chimpanzees followed this expected pattern.”
He considers this study as one of the first steps in knowing how rationality evolved and when the first sparks of rational thought appeared in nature. “We are doing a lot of research to accurately answer this question,” Engelman says.
The team thinks that rationality is not an on/off switch; Instead, different animals have different levels of rationality. Engelmann says, “The first two experiments demonstrate a rudimentary form of rationality.” “But experiments four and five are quite difficult and show a more advanced form of reflective rationality than I would expect only from chimpanzees and perhaps bonobos.”
However, in his view, humans are still at least one level above chimpanzees. “A lot of people say reflective rationality is the last stage, but I think you can go even further than that. What humans have is what I would call social rationality,” Engelman claims. “We can discuss and comment on each other’s thinking and make each other even more rational in that process.”
Sometimes, at least in humans, social interactions can even increase our irrationality. But chimpanzees don’t seem to have this problem. Engelmann’s team is currently running a study focused on whether chimpanzees’ choices are influenced by the choices of their fellow chimpanzees. “Chimpanzees followed other chimpanzees’ decisions only when the other chimpanzee had better evidence,” Engelman says. “In this sense, chimpanzees appear to be more rational than humans.”
Science, 2025. doi: 10.1126/science.aeb7565