Initially, the chimps were presented with auditory proof, the identical rattling noise emanating from the first container. Subsequently, they were given indirect visual proof: a peanut trail leading to the second container. At this juncture, the chimpanzees chose the first container, likely because they regarded the auditory proof as more compelling. However, the researchers later removed a rock from the first container. The rock indicated that it was not food causing the rattling sound. “At this juncture, a logical agent should deduce, ‘The evidence I followed has now been invalidated, and I ought to consider the alternative,’” Engelmann mentioned to Ars. “And that is precisely what the chimpanzees accomplished.”
The research group included 20 chimpanzees across all five experiments, and they adhered to the evidence significantly above the chance level—in approximately 80 percent of instances. “At the individual level, around 18 out of 20 chimpanzees exhibited this anticipated behavior,” claims Engelmann.
He regards this investigation as one of the initial strides toward understanding how rationality has evolved and when the earliest manifestations of rational thought emerged in nature. “We are conducting extensive research to specifically address this inquiry,” states Engelmann.
The researchers believe that rationality is not a binary concept; rather, various animals exhibit varying degrees of rationality. “The first two experiments illustrate a basic form of rationality,” Engelmann notes. “However, experiments four and five are considerably challenging and reveal a more sophisticated type of reflective rationality that I expect only chimpanzees and perhaps bonobos to possess.”
In his perspective, humans still operate at least one level beyond that of chimpanzees. “Many assert that reflective rationality marks the ultimate phase, but I contend that it can progress further. What humans possess is something I would term social rationality,” Engelmann asserts. “We can engage in dialogue and critique each other’s reasoning and, through that process, enhance each other’s rationality.”
Occasionally, at least among humans, social interactions can also exacerbate our irrational tendencies. However, chimps do not appear to encounter this issue. Engelmann’s team is currently conducting a study aimed at determining whether the decisions made by chimpanzees are swayed by the choices of their fellow chimpanzees. “The chimps only adhered to another chimp’s choice when that chimp had better evidence,” Engelmann states. “In this regard, chimps seem to demonstrate greater rationality than humans.”
Science, 2025. DOI: 10.1126/science.aeb7565
