Chimpanzees rationally revise their beliefs when presented with new evidence, according to research by an international team of behavioural scientists, challenging the assumption that this aspect of rational thought is uniquely human.
The study, published in Science, demonstrates that chimpanzees judge the quality of evidence when making decisions, a cognitive ability previously believed to be exclusive to humans.
Researchers from Utrecht University in the Netherlands, working alongside colleagues from the University of St Andrews, University of Portsmouth, and University of California Berkeley, investigated how chimpanzees (Pan troglodytes) update their initial beliefs about reward locations when faced with conflicting evidence.
In the experiments, food was concealed in two boxes and shown to the chimpanzees. Researchers then provided either strong or weak evidence about one box’s contents, such as shaking it to produce sound or directly revealing what was inside. Each chimpanzee made an initial choice before receiving fresh evidence about the second box that was either stronger or weaker than the first clue.
Results showed that when the initial clue was stronger, the chimpanzees maintained their original choice. However, when it was weaker, they switched boxes, indicating they were updating their beliefs according to the evidence.
Crucially, when researchers presented misleading evidence—such as revealing that apparent ‘food’ was merely a photograph—the chimpanzees recognised the initial evidence as invalid.
Dr Esther Herrmann, senior author from the University of Portsmouth, stated: “Our findings reveal that humans aren’t the only ones who make rational decisions based on evidence—chimpanzees, our closest living relatives, also weigh the strength and quality of the evidence before making decisions.”
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Professor Josep Call from the Global Research Centre for Diverse Intelligences at the University of St Andrews added: “This study opens a new avenue of research into the psychological mechanisms underlying decision-making in chimpanzees. Together with previous work on bargaining games, it contributes to portray chimpanzees as rational decision-makers.”
The research suggests that certain elements of rational thought existed before the emergence of our species.
