Monkeys Share The Roots Of Human Geometric Thinking, Study Finds

The Human Mind’s Geometric ‘Superpower’ May Not Be So Human After All

For decades, scientists believed humans exclusively held the mental gift of encoding geometric shapes through abstract, rule-based thinking. Only humans, the theory held, could represent a square as discrete properties: equal sides, right angles, perfect symmetry. Monkeys were thought to rely mainly on raw visual pattern-matching instead. A new study in PNAS challenges that assumption.

Researchers tested monkeys, preschoolers, and adults from an indigenous community on a shape-matching task and found monkeys’ choices were best explained by the same basic mix of geometric representations seen in humans. Rather than a sharp divide, the study reveals a smooth, overlapping continuum. Monkeys aren’t as sharp as adults, but their geometric thinking is far more sophisticated than credited, overlapping considerably with young children’s.

Most provocative: on tasks requiring recognition of tilted shapes, some monkeys leaned on abstract geometric reasoning even more than preschoolers did on the easier version of the same task. That’s a comparison across different conditions, not proof the monkeys are better at geometry overall, but it’s still a striking reversal of what scientists expected.

A 2021 Study First Suggested Humans Alone Grasp Geometric Rules

Scientists called the leading explanation the “symbolic singularity” view: humans have a unique mental system for encoding geometric rules almost like a checklist, parallel sides, right angles, equal lengths. This checklist lets humans instantly recognize a square no matter how it’s drawn. Other primates, lacking this system, were thought to depend only on continuous perceptual features rather than discrete rules.

A widely cited 2021 study backed this up, showing baboons performing worse than humans at picking the “odd one out” among geometric shapes. Baboons showed no advantage when the odd shape was highly regular; humans did, and researchers read this as evidence that symbolic geometric thinking was uniquely human. This new study takes direct aim at that interpretation.

Monkeys, Kids, and Adults Faced the Same Shape-Matching Test

Monkeys, preschoolers ages 3 to 6, and adults completed a matching task: view a sample shape, then pick which of two options matched. Correct answers earned a food pellet for monkeys, a star sticker for kids, or verbal confirmation for adults.

Eight adult monkeys participated: four rhesus macaques at Carnegie Mellon University and four olive baboons at the Seneca Park Zoo in Rochester, New York. Fifty-eight American preschoolers were recruited through Carnegie Mellon, along with 79 adults from Tsimane’ communities near San Borja, Bolivia, a farming and foraging society with little formal schooling. That last group mattered because it let researchers test whether geometric thinking depends on schooling, or runs deeper than that.

Rather than tallying right and wrong answers, researchers used statistics to determine how each group mentally represented shapes, comparing behavior against three models: raw pixel similarity, holistic shape recognition, and a symbolic model encoding properties like symmetry and right angles.

Across every group, behavior was best explained by the two sophisticated models, not the primitive one. Monkeys were not limited to basic visual matching, as the old theory predicted; they showed reliable sensitivity to the symbolic model too, though less strongly than adults. Monkeys and preschoolers, in fact, landed in largely overlapping territory, with no meaningful gap between the species.

“Our findings reveal that monkeys share geometric representations with humans, suggesting the origin of geometric intuition is shared among primates,” the authors write.

When Rotating Shapes Turned the Findings Upside Down

Its most arresting result came from rotated shapes: participants saw options tilted clockwise or counterclockwise and had to identify which still matched the sample, testing whether a shape reads as the same once turned on its side.

A subset of five monkeys tripled their reliance on symbolic-style reasoning once shapes were rotated, and preschoolers showed a similar jump. The monkeys’ jump was even bigger, suggesting it was the harder task, not the species, driving the shift.

This led researchers to question what “symbolic” thinking actually is. It turns out the symbolic model behaves almost identically to a simpler idea: recognizing a shape as itself no matter which way it’s turned. What looked like a uniquely human rulebook for geometry may really just be a sturdier version of that same basic skill, one monkeys have too, just less developed.

“Rotation invariance, rather than biological species, induced categorical restructuring of representational space,” the authors conclude.

The Roots of Geometric Intuition May Predate Humans

In these tasks, researchers found no clean break between the groups, only differences of degree that shifted depending on the task. Adult humans still showed the sharpest use of these skills overall, and the exact mental code behind it remains an open question. But the basic ingredients, it turns out, are not uniquely human, an ability shared far enough back in the primate family tree that geometric intuition looks less like a human invention and more like an inheritance.

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