Chimpanzees may be more like human thinkers than we ever imagined.
A new study published in Science shows that chimpanzees can rationally change their beliefs when they receive new evidence—a mental skill once thought to be uniquely human.
The study, titled “Chimpanzees rationally revise their beliefs,” was carried out by an international research team that included UC Berkeley psychologist Emily Sanford, UC Berkeley professor Jan Engelmann, and Utrecht University professor Hanna Schleihauf.
Their findings reveal that chimpanzees can update their decisions based on the strength of information they receive, demonstrating a key aspect of rational thought.
The research took place at the Ngamba Island Chimpanzee Sanctuary in Uganda, where scientists worked with chimps in a simple decision-making game.
The animals were shown two boxes, one of which contained food.
At first, they were given a hint suggesting which box had the treat. Later, they were given a new clue—this time a stronger one—that pointed to the other box. When presented with better evidence, the chimps often switched their choice.
“Chimpanzees were able to revise their beliefs when better evidence became available,” says Sanford, a postdoctoral researcher in UC Berkeley’s Social Origins Lab.
“This kind of flexible reasoning is something we usually associate with young children. It was exciting to show that chimps can do this too.”
To ensure that the chimps weren’t simply reacting to new signals or following habits, the researchers designed a series of tightly controlled experiments and used computational models to analyze their decisions.
These models ruled out simpler explanations, such as the chimps just choosing whichever clue they saw last. Instead, the data showed that their decisions matched rational strategies similar to human belief revision.
“We tracked their first and second choices and used computer models to see which reasoning strategy best fit their behavior,” Sanford explains. “The evidence clearly showed that chimps were acting in ways consistent with rational updating.”
The results challenge the long-held idea that rational thinking—the ability to weigh evidence and revise beliefs—is uniquely human.
“The difference between humans and chimpanzees isn’t a sharp divide,” says Sanford. “It’s more of a continuum.”
Sanford believes the findings have broader implications for how we understand learning, child development, and even artificial intelligence. “By studying how primates revise their beliefs, we can gain insights into how humans reason, how children learn, and how to build smarter AI systems,” she says.
Her next step is to test the same tasks with young children between the ages of two and four to see how their reasoning compares to the chimps’. “It’s fascinating to adapt a task made for chimps to one that toddlers can understand,” she says.
For Sanford, the takeaway is simple: animals—and especially primates—are capable of more complex thinking than we give them credit for. “They may not know what science is,” she says, “but they’re already doing their own experiments on the world around them.”
