Ninety percent of humans favor their right hand. The University of Oxford figure that reorders the story sits much lower, at 0.28, the predicted hand-preference index for a metre-tall, small-brained Indonesian “hobbit” species that lived until roughly 50,000 years ago. The same model places modern Homo sapiens at 0.76. It places Homo floresiensis closer to a chimpanzee.
That gap is the spine of a paper published in PLOS Biology by a team led by Dr. Thomas A. Püschel, Wendy James Associate Professor in Evolutionary Anthropology at Oxford. It also explains why the left-handed minority just became a harder problem, not an easier one.
The Numbers Behind the Oxford Finding
The 90/10 split has held across every population ever measured. What changed last month is the explanation. Püschel, with co-authors Rachel M. Hurwitz at Oxford and Chris Venditti, an evolutionary biologist at the University of Reading, ran a Bayesian phylogenetic meta-analysis on 2,025 individual primates across 41 anthropoid species. The model isolated which traits predict population-level right-hand bias.
The strongest predictors were not the obvious candidates. Tool use did not carry the signal. Social structure did not. Diet did not. The two variables that did the work were endocranial volume (a brain-size proxy) and the intermembral index (a leg-versus-arm ratio that captures how committed a species is to walking upright).
Stripped of those two variables, humans look like an evolutionary glitch. Plug them in, and the rightward bias becomes a slope predictable from bone shape and skull capacity.
How a Bayesian Model Made 2,025 Primates Talk
The dataset pulled together the two largest recent compilations of primate handedness scoring, by Soto and colleagues in 2022 and Caspar and colleagues the same year. Each individual was rated on two metrics: MHI, the direction of hand preference, and MABSHI, the strength of that preference regardless of which hand dominates.
The phylogenetic signal in MABSHI ran at h² = 0.66; in MHI it ran at h² = 0.48. In plainer terms: the strength of a primate’s hand bias tracks its evolutionary lineage more tightly than which hand it favors. Strength is older than direction.
That distinction is what opens the rest of the paper. The team did not need fossils to assign predicted values to extinct hominins. Once the model knew how brain volume and intermembral index mapped onto present-day primates, it could read those traits off the bones of Ardipithecus or Homo erectus and back-calculate where each species sat on the MHI axis.
That is the move that re-frames the field. Handedness, until now treated as a behavioral artifact recoverable only from stone-tool flake scars or tooth striations, becomes a number the skeleton itself reports. A 2010 study on chimpanzee bipedal tool use and hand preference had already shown that standing up sharpens lateralisation; what was missing was a way to extend that link across deep time. Oxford built it.
Six Extinct Species, One Predicted Slope
The paper’s table of predicted indices reads as a slow climb across roughly four million years.
| Species | Approximate Age | Predicted MHI (direction) | Predicted MABSHI (strength) |
|---|---|---|---|
| Ardipithecus ramidus | 4.4 million years | 0.16 | 0.73 |
| Australopithecus afarensis | 3.2 million years | 0.32 | 0.64 |
| Homo floresiensis | 100,000 to 50,000 years | 0.28 | 0.61 |
| Homo ergaster | 1.9 million years | 0.50 | 0.74 |
| Homo erectus | 1.9 million to 110,000 years | 0.54 | 0.76 |
| Homo neanderthalensis | 400,000 to 40,000 years | 0.64 | 0.81 |
| Homo sapiens (modern) | present | 0.76 | 0.94 |
Read the MHI column top to bottom, setting the Indonesian anomaly aside for now. The rightward bias goes from a faint nudge in Ardipithecus to a dominant pattern in Neanderthals to the near-universal modern human signal. That trajectory aligns with the fossil record’s two big trends: a steady commitment to bipedal locomotion and a roughly fourfold increase in brain size from australopithecines to sapiens.
It also lines up with archaeology. Work on a German Neanderthal toolkit, applying flake-scar geometry to recover the knapper’s hand preference, found at least 85% of the tools were knapped by right-handers. The El Sidrón cave site in Asturias produced tooth striations on a six-to-eight-year-old Neanderthal child that pointed the same way. Two independent methods, one direction, decades before this Oxford framework existed.
The Hobbit That Breaks the Pattern
Then there is the Indonesian outlier. The Flores species, recovered from Liang Bua cave on the island of Flores, sits at MHI 0.28. That places it below Australopithecus afarensis on the rightward-bias scale, despite belonging to the genus Homo and surviving until roughly 50,000 years ago, contemporary with anatomically modern humans.
The model’s reasoning is mechanical. The Liang Bua lineage had a small brain (around 400 cubic centimetres) and an upper-limb-to-lower-limb ratio that suggests it had not fully committed to bipedal locomotion in the way later Homo species had. By the two variables the model trusts, it reads as a hominin that branched off the lineage before the right-handedness threshold was crossed.
Our results suggest it is probably tied to some of the key features that make us human, especially walking upright and the evolution of larger brains.
That is Püschel, quoted in the Oxford release. The Flores fossil is the species that lets him say it cleanly: a member of genus Homo that lacks both qualifying traits, and lacks the bias to match.
Why Left-Handedness Just Got Harder
If brain size and bipedalism explain the rightward majority, those same variables cannot account for the left-handed 10%. Every modern human shares the brain capacity and the gait. Variables that do not vary within a species cannot generate within-species variation. So what does?
The Oxford paper does not answer that question. It narrows it, which is the harder kind of progress. Four lines of evidence now have less explanatory room than they did a month ago:
- Genetics carries some of the load, but not most of it. A 2024 whole-exome-sequencing analysis in UK Biobank put the heritability of left-handedness due to rare coding variants at 0.91%. Genome-wide association studies have identified 41 loci that together explain roughly 6% of heritability. The bulk of the variance sits elsewhere.
- Brain-asymmetry mapping across 31,864 individuals in a large PNAS study tied left-handedness to structural asymmetries in language-related cortex, but the direction of causation, and what flips the switch, is still not resolved.
- Cross-species comparison offers no template. Kangaroos lean left at the population level. Parrots split roughly 50-50. No other ape population shows a strong directional bias, which means the search for a deep evolutionary precedent for lefties keeps returning empty.
- In-utero and developmental factors show small effects in large datasets, none large enough to account for the 10% residual.
The contribution of this study is to clear the structural-anatomy explanations off the table. Whatever causes left-handedness in modern humans, it is not their brain volume or their gait.
Handedness as a Fossil Proxy for Cognition
The second-order effect of the paper is methodological. Until now, recovering handedness from a fossil meant finding stone tools or skeletal asymmetries (longer right humerus, thicker right cortical bone) and inferring from there. The new model lets a paleoanthropologist read MHI off two measurements: endocranial volume and intermembral index. Both are recoverable from a partial skeleton.
That converts handedness into a cognitive proxy. Strong rightward bias becomes a marker for the package of traits (larger brain, full bipedalism, and the lateralised language and motor circuits that ride along with both) that defines the genus Homo’s later branches. A fossil with a bias score below 0.4 reads as something other than modern-grade cognition, regardless of which species it nominally belongs to.
That is where the hobbit’s 0.28 lands its punch. Homo floresiensis was classified into the genus Homo on morphological grounds when its fossils were described in 2004. The handedness framework treats it as cognitively closer to an australopith. Anthropology has argued this both ways for two decades. Oxford’s framework gives the small-brain camp a number it can cite.
Frequently Asked Questions
Why Are Most Humans Right-Handed?
Right-handedness emerged from the combination of habitual upright walking and the expansion of the human brain. Upright posture freed the hands from locomotion and created selective pressure for fine, lateralised manual skills. Larger brains then locked in a population-level rightward bias, with the bias hardening through Homo ergaster, Homo erectus, and the Neanderthals before reaching its modern peak in Homo sapiens.
Is Left-Handedness Genetic?
Only partly. Twin and family studies suggest genetics explains roughly 25% of variance in handedness. A 2024 UK Biobank exome study put the rare-variant contribution at under 1%. Most of the cause of left-handedness in modern humans is still unexplained, and the Oxford paper has narrowed the search by ruling out species-wide anatomy.
What Was Homo Floresiensis?
A small-brained, metre-tall human species described in 2004 from fossils found in Liang Bua cave on the Indonesian island of Flores. Often called the hobbit, it had a brain volume of roughly 400 cubic centimetres and survived until about 50,000 years ago, overlapping with modern humans in Southeast Asia.
How Can Scientists Know if an Extinct Species Was Right-Handed?
Two methods now exist. The traditional approach reads flake-scar geometry on stone tools or asymmetric wear patterns on teeth and bones. The new Oxford method predicts hand preference from two skeletal measurements (brain size and the ratio of leg length to arm length), letting researchers score species that left no usable tool record.
Are Other Animals Right- or Left-Handed?
Most primate species show individual hand preferences, but no population-level bias on the human scale. Kangaroos lean left as a species. Parrots split close to 50-50. Modern humans are the only species with a near-universal directional preference, which is exactly what the Oxford model is built to explain.
What Does the Study Not Explain?
It does not explain why the left-handed 10% exists. Brain size and bipedalism are shared by all modern humans, so they cannot generate within-species variation. The paper narrows the search for the cause of left-handedness; it does not resolve it.
Where Was the Study Published?
In PLOS Biology, on 27 April 2026, by Thomas A. Püschel and Rachel M. Hurwitz of the University of Oxford and Chris Venditti of the University of Reading. The work was funded by a Leverhulme Trust Research Leadership Award.
