Even Chimps Follow the Evidence

Richard Jonathan O. Taduran, Ph.D.  |  20 November 2025

At the Ngamba Island Chimpanzee Sanctuary in Uganda, researchers presented chimps with different kinds of clues about where a piece of fruit was hidden—some weak, some strong, some misleading. The chimps learned the task, but what was more remarkable was that when a clearer or more reliable clue appeared, many of them changed their minds and adjusted their choices. They behaved as if they were weighing evidence, evaluating reliability, and revising earlier conclusions in light of stronger facts.

This skill, previously assumed to be uniquely human, is supposed to be one of our species’ great intellectual strengths—essential not only in science but in everyday reasoning. Yet in some experiments, chimps outperformed humans not in raw intelligence but in the willingness to update beliefs. This study raises the question: if our primate cousins can revise their beliefs with new evidence, why do we—with all our culture, education, and technology—struggle so hard to do the same?

Belief Revision

The study, led by Hanna Schleihauf and colleagues, consisted of a series of cleverly designed tests that probed how chimps evaluate evidence. In the simplest version, researchers used two boxes. A weak clue—like the sound of rattling food that might or might not indicate real contents—pointed to one box. Later, a stronger clue appeared for the other box, such as visibly placing the fruit inside. Many chimps changed their selection, abandoning the weaker hint in favor of the more reliable one.

Other experiments complicated the task. In some trials, chimps saw repeated weak evidence for one box but a new weak hint for another, testing whether novelty affected their thinking. In others, earlier evidence was exposed as false: chimps heard a promising rattle only to later see that the box actually contained a rock. When honest evidence later appeared for a different box, they shifted away from the box that had “lied” to them.

What makes these results significant is not only the behavioral pattern but the computational modeling that followed. The researchers compared the chimps’ choices to simpler strategies—like “always choose the most recent clue” or “always follow the loudest signal.” The chimps did not follow these shortcuts. Their behavior aligned best with rational belief revision: taking in new information, evaluating its strength, and updating earlier assumptions accordingly.

Evolutionarily, this makes sense. Decision-making under uncertainty is a fundamental challenge for primates navigating complex environments. Belief revision—adjusting one’s assumptions as better evidence emerges—is not a human invention but an ancient cognitive tool shaped by the pressures of survival.

Biological anthropology has long shown that capacities we consider uniquely human—memory, planning, social cooperation—have deep roots in the primate lineage. This study adds rational belief revision to the list. The gap between chimpanzee and human reasoning, long described as a vast chasm, may actually be a gradient across which our abilities evolved.

And if belief revision is so embedded in our evolutionary history, why does it feel so rare in our modern societies?

Cultural Forces

Humans possess the biological machinery—our expanded brain size, intricate neural connectivity, and complex cortical structure—for evidence-based reasoning, but we also carry something our primate cousins do not: culture.

Where chimps respond mostly to the reliability of a cue, humans respond to the reliability of a story—our social identity, community narratives, emotional investments, and the pressure to belong. These cultural forces often override our biological inclination to update beliefs.

Confirmation bias is the most familiar culprit. We look for evidence that supports what we already believe and brush aside anything that threatens it. Anchoring ties us to our first impressions. Identity-protective cognition makes us defend beliefs not because they’re true, but because they’re woven into who we are. A person raised in a political echo chamber may reject strong evidence simply because it contradicts their group’s worldview. Someone emotionally invested in a belief—about a partner, a leader, or even a conspiracy theory—may cling to it long after the facts collapse.

Even in scientific and institutional settings, this resistance to reconsidering earlier assumptions is common. Researchers may protect favored hypotheses. Administrators may resist data that challenge organizational narratives. And yes, even fields built on objectivity—like forensic science—are not immune to this. Investigators may cling to early theories of a case despite contradictory evidence, a pattern documented in several wrongful convictions.

The irony is almost poetic: the cognitive capacities we evolved for survival are still present, but they are entangled with social and emotional forces strong enough to override them. The chimps revise their beliefs because nothing gets in the way of the evidence—not ego, not ideology, not group loyalty.

Human reasoning, by contrast, is clouded by meaning-making. Our ability to generate stories, identities, and imagined worlds—arguably our greatest strength—is also our greatest vulnerability. We don’t just process evidence; we negotiate it through the lens of culture, emotion, and belonging.

Cognition Paradox

What this chimpanzee study ultimately reveals is not their brilliance but our fragility. Rationality is not uniquely human. It is older, deeper, and more widespread than we once believed. The chimps remind us that belief revision is an ancestral skill, part of the cognitive toolkit we inherited from the primate lineage.

The paradox is that our advanced cognition also burdens us with narratives that can obstruct clear thinking. As a biological anthropologist, I see this study as evidence that human reasoning did not evolve to achieve perfect truth. It evolved for survival in environments where social cohesion sometimes mattered more than accuracy. Understanding this frees us from the illusion that rationality is automatic or easy.

The chimps show us a simpler model: change your mind when the evidence changes. It’s not a betrayal of intelligence but a practice of it. If anything, their behavior is a reminder that clarity is possible—not because we are more evolved, but because we share an ancient, unfinished lineage of minds that can still learn, still adapt, and still let go of old beliefs when better ones appear.