One of the more durable mysteries in evolutionary biology is why, across every human culture ever studied, approximately 90 percent of the population prefers to use their right hand. The pattern is, by every available measure, an outlier among primates. No other primate species shows population-level handedness anywhere close to this scale. Chimpanzees, our closest relatives, show only mild handedness preferences, and the preferences are roughly evenly split between left and right across the population. Humans are, in this respect, structurally anomalous.
The wider research community has spent decades trying to figure out where the anomaly comes from. The candidate explanations have ranged across genetics, brain lateralization, language development, social transmission, and various combinations of these. None of the candidates, on close examination, has produced a fully satisfying answer. The 90 percent figure has remained, in some real way, one of the more persistent open questions in human evolutionary biology.
A new study published in April 2026 in PLOS Biology has, on the available evidence, finally produced a strong candidate explanation. The explanation, somewhat counterintuitively, has nothing structural to do with the hands themselves. The explanation has to do with the legs, and specifically with what happened to human anatomy when our ancestors started walking on two of them.
What the study actually examined
The research, led by Thomas Püschel and Rachel Hurwitz of the University of Oxford’s School of Anthropology and Museum Ethnography, in collaboration with Chris Venditti of the University of Reading, took a comparative approach to the handedness question. The researchers brought together data on 2,025 individual primates across 41 species of monkeys and apes, including humans. The comparative scope of the study is, on close examination, the structural feature that makes its findings worth attending to.
The team tested the major existing hypotheses for human handedness against this comparative dataset. The hypotheses included diet, habitat, body mass, social structure, tool use, and locomotion. The standard approach to handedness research had been to identify a single factor and test whether it correlated with handedness within a particular species. The Oxford team’s approach was structurally different. They tested all the major hypotheses simultaneously against data from 41 different species, looking for which combination of factors actually explained the cross-species pattern in which humans are the extreme outlier.
The result, on close examination, was that no single factor explained the pattern. Diet did not explain it. Habitat did not explain it. Social structure did not explain it. Tool use, which had been one of the more popular candidate explanations for decades, did not on its own explain it. Each of these factors had been proposed as the explanation for human handedness at various points in the research literature, and each, on the available comparative data, turned out to be insufficient.
What the data actually pointed at
The combination that did explain the pattern was, on close examination, somewhat unexpected. The Oxford team found that two factors together accounted for the human anomaly. The first factor was brain size, specifically the dramatic expansion of brain size that occurred in the human lineage over the last several million years. The second factor was the ratio between arm length and leg length, which is a standard anatomical marker of bipedal locomotion.
The second factor is the surprising one. The arm-to-leg ratio is, in some real way, an indirect measurement of how committed an animal is to walking on two legs. Animals that walk on four legs, or that use their arms substantially for locomotion through trees, have different arm-to-leg ratios than animals that have committed to bipedal walking. Humans have, across millions of years of evolution, developed legs that are considerably longer relative to their arms than the legs of our quadrupedal or arboreal relatives. The ratio is, accordingly, a measurement of how thoroughly our lineage has become bipedal.
When the Oxford team factored arm-to-leg ratio and brain size into their model, the human handedness anomaly disappeared. The 90 percent right-handedness figure was no longer an unexplained outlier. The figure was, more accurately, what their model predicted for an animal with our specific combination of brain size and bipedal commitment. The other primates, with smaller brains and more arboreal locomotion, were predicted by the model to have weaker handedness preferences, which is what the data show.
Why the legs actually matter
The structural mechanism the researchers propose is, on close examination, worth attending to. The proposal is that bipedalism freed the hands from the task of locomotion. In quadrupedal or arboreal primates, the hands are doing significant amounts of locomotor work. They are, in some real way, structurally constrained to be locomotor organs first and manipulative organs second. The constraint limits how specialized the hands can become for non-locomotor tasks.
When the human lineage committed to bipedal walking, the hands were released from the locomotor constraint. The hands became, more specifically, organs whose primary function could be manipulation rather than movement. The freeing-up of the hands made it possible for them to specialize in ways that quadrupedal animals could not afford. The specialization could include, among other things, the development of strong handedness, in which one hand becomes the dedicated tool-using and fine-manipulation hand while the other becomes the assisting hand.
The brain expansion, the second factor in the model, then provided the cognitive substrate for this specialization to be expressed and maintained. The larger brain, with its more elaborate hemispheric lateralization, allowed the handedness preference to become more strongly encoded in the neural infrastructure of the species than it could be in primates with smaller brains. The combination of the two factors, on the available evidence, produced the extreme right-handedness that distinguishes humans from the rest of the primate family.
What the fossil record supports
The fossil record, as documented by the Oxford team and by previous researchers, supports the proposed sequencing of these evolutionary changes. The earliest hominins, including the various australopithecines, showed only mild handedness preferences, consistent with their mixed bipedal-and-arboreal locomotion and their relatively modest brain sizes. The pattern intensified across the Homo lineage. Homo ergaster, Homo erectus, and Neanderthals all show increasing evidence of population-level right-handedness, with the pattern reaching its modern extreme in Homo sapiens.
One particularly informative exception is Homo floresiensis, the small-brained “hobbit” species from Indonesia. Floresiensis appears to have had a body adapted to a mix of upright walking and climbing, and a relatively small brain. The species also shows, on the available evidence, weaker handedness preferences than other Homo species. The exception, on close examination, fits the pattern the Oxford model predicts. An animal with mixed locomotion and a smaller brain should, by the model, show weaker handedness, regardless of being in the Homo genus. Floresiensis does show weaker handedness. The model is, in this respect, consistent with the available fossil evidence.
The sequencing is also worth noting. The bipedalism came first, on the available fossil record, with brain expansion following later. The hands were freed from locomotion before the brain became large enough to fully exploit the freeing. The handedness intensified gradually across the Homo lineage as both factors continued to develop. The modern 90 percent figure is, accordingly, the cumulative product of millions of years of these two evolutionary trajectories operating in combination.
The acknowledgment this article wants to leave
The mystery of why 90 percent of humans are right-handed has, on the available evidence from the April 2026 PLOS Biology study, finally been given a plausible structural answer. The answer is not, on close examination, located in the hands themselves. The answer is located in the legs, and specifically in what happened to human anatomy when our ancestors committed to bipedal walking. The bipedal commitment freed the hands from locomotion. The free hands could then specialize for manipulation in ways that quadrupedal hands could not. The simultaneous expansion of the brain provided the cognitive substrate for the manipulative specialization to be strongly encoded. The combined effect, accumulated across millions of years of human evolution, was the extreme right-handedness that distinguishes Homo sapiens from every other primate species.
The structural lesson, on close examination, is that the most distinctive features of human biology are, in many cases, not explicable by reference to the features themselves. The features are, more accurately, the cascading consequences of earlier evolutionary changes that the wider register has not previously connected to them. Walking on two legs would not, on first inspection, appear to have anything to do with handedness. The connection is, however, on the available evidence, real and structurally fundamental. The hands became what they are because the legs became what they are. The wider cultural register has, for decades, been looking in the wrong place for the answer to the handedness question. The answer, all along, was located in the legs.
