Scientists Identify Evolutionary Origin of Human Right-Handedness

About nine out of every ten people on Earth favor their right hand. This ratio holds across continents, across centuries of archaeological evidence, and across cultures that have had no contact with one another. No other primate species comes close to this degree of population-level lateralization. For decades, scientists have proposed explanations — tool use, language, social pressure, asymmetric brain wiring — without consensus. A study published in PLOS Biology in May 2026 now offers the most comprehensive account yet: humans became overwhelmingly right-handed because we stood up on two legs and grew enormous brains [1].

The Oxford Study: Methodology and Findings

The research team — Dr. Thomas A. Püschel and Rachel M. Hurwitz of the University of Oxford's School of Anthropology and Museum Ethnography, along with Professor Chris Venditti of the University of Reading — assembled data on 2,025 individual primates across 41 anthropoid species [1]. They drew exclusively from studies that used the standardized "tube task" protocol, in which an animal must hold a tube in one hand and extract food with the other, to ensure comparability across species [2].

Using Bayesian phylogenetic comparative meta-analytical methods run across 100 phylogenetic trees, the team simultaneously tested ten competing eco-evolutionary hypotheses for the direction and strength of handedness. These included tool use, diet, habitat type, body mass, social organization, brain size, and locomotion patterns [1][2].

The central finding: when the researchers modeled primate handedness without accounting for brain size and the intermembral index — the ratio of arm length to leg length, a standard anatomical marker of bipedal locomotion — humans appeared as a stark evolutionary outlier, with a mean handedness index (MHI) of 0.76, far above any other species [2]. But when those two variables were added, the human anomaly disappeared. As Dr. Püschel put it: "This is the first study to test several major hypotheses for human handedness in a single framework" [3].

The intermembral index measures how forelimb length compares to hindlimb length. Humans have an unusually low index of about 72, reflecting legs that are much longer than arms — a signature of bipedal adaptation. Most other primates have indices near or above 100, reflecting bodies built for climbing or quadrupedal locomotion [2].

A Two-Stage Evolutionary Process

The study proposes that right-handedness intensified in two stages. First, the transition to upright walking freed the hands from the demands of locomotion, creating new selective pressure for fine, lateralized manual behavior. Second, as brains expanded and reorganized over millions of years, the rightward bias hardened into the near-universal pattern observed in modern humans [1][3].

The researchers estimated handedness indices for extinct hominins based on known brain volumes and limb proportions. The resulting gradient is striking [2]:

Source: Püschel et al. 2026, PLOS Biology

Data as of May 15, 2026CSV

Ardipithecus ramidus, which lived roughly 4.4 million years ago and retained significant arboreal adaptations, shows an estimated MHI of just 0.16 — a faint rightward lean comparable to some modern monkeys. Australopithecus afarensis, the species of the famous fossil "Lucy" (about 3.2 million years ago), reaches 0.32. The bias then accelerates with the genus Homo: H. ergaster at 0.50, H. erectus at 0.54, Neanderthals at 0.64, and modern Homo sapiens at 0.76 [2].

One telling data point: Homo floresiensis, the diminutive "Hobbit" species from the Indonesian island of Flores, had a small brain and retained partial climbing anatomy. Consistent with the model's predictions, it shows only a weak rightward bias [3].

Humans Among the Apes: A Spectrum, Not a Switch

Humans are not the only primates with population-level hand preferences, but the degree of asymmetry is unmatched. Research compiled by William Hopkins and colleagues on 777 great apes found that gorillas show roughly 75% right-handedness on coordinated bimanual tasks, chimpanzees about 67%, and bonobos around 63% [4][5]. Orangutans are the outlier in the other direction: about 66% favor the left hand for the tube task, which researchers attribute to their arboreal lifestyle, where the right hand often grips branches for support while the left manipulates objects [4].

Source: Püschel et al. 2026, PLOS Biology; Hopkins et al. 2011

Data as of May 15, 2026CSV

The Püschel team's model accounts for this spectrum. Species that spend more time on the ground and have larger brains tend toward stronger rightward bias. Species that are more arboreal and have smaller brains show weaker or reversed preferences. Humans sit at the extreme end of both variables [2].

The 90/10 Split: How Universal Is It?

Meta-analyses covering more than two million participants across dozens of countries consistently find that 85–90% of humans are right-handed and 9–12% are left-handed, with a small fraction showing no strong preference [6][7]. But that global average masks significant regional variation.

Source: Multiple sources; Papadatou-Pastou et al. 2020

Data as of Jan 1, 2026CSV

The Netherlands reports one of the highest left-handedness rates at 13.2%, while China reports just 3.5% [6]. These differences are widely attributed to cultural pressure rather than biology. In parts of East Asia, the Middle East, and sub-Saharan Africa, the left hand is traditionally associated with uncleanliness, and children who show a natural left-hand preference are actively discouraged or forced to switch [6][7]. When researchers control for cultural suppression, the underlying biological rate appears to be similar worldwide — roughly 10% [7].

The new Oxford study does not directly explain why left-handedness persists at this stable minority frequency. The authors acknowledge this as an open question [2]. The leading hypothesis comes from evolutionary biology: negative frequency-dependent selection. The "fighting hypothesis," first articulated by Michel Raymond and colleagues in 1996, proposes that left-handers hold a combat advantage because right-handed opponents are less practiced at defending against left-handed attacks [8]. This advantage is strongest when left-handers are rare; as their frequency rises, the surprise effect diminishes, creating an equilibrium. Supporting evidence includes the overrepresentation of left-handers in combat sports and a positive correlation between a society's homicide rate and its proportion of left-handers [8][9].

A modified version of this hypothesis, published in Laterality in 2026 by Rodway and colleagues, argues that right-handers also held a direct combat advantage: because two-thirds of the heart sits in the left chest, right-handed combatants wielding sharp weapons were more likely to strike their opponent's vulnerable left side [10]. Crime data show the right upper body is stabbed 2.4 times more frequently than the left, consistent with the predominance of right-handed attackers [10].

What This Finding Displaces

For decades, four main hypotheses competed to explain human right-handedness:

Tool use. The idea that making and using stone tools drove hand specialization. Archaeological evidence of right-handed flaking patterns in early Homo supported this view [11]. But the Oxford study found that tool use did not predict handedness direction or strength across primates when tested in their phylogenetic framework [2].

Language lateralization. Because speech is predominantly controlled by the left hemisphere — which also controls the right hand — researchers long suspected the two were linked by a shared neural substrate [12]. Genome-wide association studies have identified some overlapping genetic loci between handedness and language lateralization, but the Oxford team's comparative approach found brain size, not language per se, to be the operative variable [2][12].

Social conformity. The proposal that cultural pressure to use the right hand amplifies a mild biological bias. Cross-cultural variation in left-handedness rates supports this as a modulating factor, but it cannot explain why the biological bias exists in the first place [6][7].

Asymmetric brain development. Research has shown that handedness correlates with structural asymmetries in the motor cortex and that genes involved in left-right body patterning (particularly microtubule-related genes like NME7) influence both hand preference and cerebral cortical asymmetry [12][13]. The Oxford study does not contradict this — it operates at a different level of explanation. Brain asymmetry may be the proximate mechanism; bipedalism and encephalization are the evolutionary conditions that selected for it [2].

No single hypothesis was clearly corroborated when the researchers excluded humans from the dataset, suggesting that many existing frameworks were, in the authors' words, "overly anthropocentric" — built to explain humans rather than capturing genuine primate-wide patterns [2].

The Case for Caution

Several methodological limitations warrant scrutiny.

Cross-species comparability. The authors themselves flag that human handedness data typically come from self-report questionnaires measuring culturally reinforced adult preferences, while primate data come from experimental protocols like the tube task. These measure different things. A human who reports being "right-handed" on a survey is describing a lifetime of reinforced behavior; a chimpanzee reaching into a tube is performing a single controlled task [2].

Skeletal proxies for extinct species. The handedness estimates for Ardipithecus, Australopithecus, and early Homo are inferred from brain volume and limb proportions, not from direct behavioral observation. The relationship between these anatomical features and actual hand preference in living species is statistical, not deterministic [2][14].

Sample constraints. The study's 2,025 individuals span 41 species, but coverage is uneven. Some species are represented by dozens of individuals; others by far fewer. The statistical power to detect effects varies accordingly [2].

Conflation risk. Handedness and brain lateralization are correlated but not identical. About 95% of right-handers and 70% of left-handers show left-hemisphere language dominance. Using handedness as a proxy for broader neurological asymmetry can obscure important distinctions [12].

The eLife journal's 2022 analysis of hand preferences across anthropoid primates noted that "variation in methods and measures has resulted in past disputes over the existence of population handedness in nonhuman great apes," cautioning that discordant methods across laboratories can mask or fabricate patterns [5].

Living in a Right-Handed World

For the roughly 10% of the population that is left-handed, the practical consequences of a right-hand-dominant built environment are measurable.

Many everyday objects — scissors, can openers, computer mice, spiral notebooks — are designed for right-handed use [15]. More consequentially, industrial and surgical equipment often positions safety switches, controls, and cutting edges for right-handed operators. Power saws, drills, and lathe machines can pose genuine safety risks when operated left-handed [15][16].

In education, left-handed children in some countries still face forced switching. Studies in the UK found that one in five left-handed workers reports practical problems at work due to their handedness, affecting an estimated 852,000 people. Only a quarter of British businesses provide left-handers with specialist equipment like ergonomic mice or keyboards [16].

The economic dimension has been quantified. Harvard economist Joshua Goodman's analysis found that left-handers earn 10–12% less annually than right-handers, a gap he attributed partly to higher rates of behavioral and learning difficulties among left-handers and partly to the friction of operating in environments designed for the majority [17]. A 2006 Institute for Fiscal Studies working paper similarly documented earnings penalties for left-handed workers, though the precise mechanisms remain debated [18].

No government or international standards body has published a comprehensive accounting of the safety or economic costs imposed by right-hand-dominant design. The gap in the data is itself notable.

Downstream Implications: From Fossils to the Clinic

The Püschel study's most significant contribution may be methodological: by establishing that bipedalism and brain expansion explain the direction and strength of human handedness within a primate-wide framework, it provides a scaffolding for connecting evolutionary and clinical research.

Atypical hemispheric asymmetries are more prevalent in several psychiatric and neurodevelopmental conditions. Schizophrenia, autism spectrum disorder, and intellectual disability show the strongest associations with non-right-handedness [19][20]. Dyslexia and stuttering, both conditions with linguistic symptoms, also show elevated rates of left- and mixed-handedness [19]. A 2020 review in Cortex found that conditions manifesting early in life and involving language were most strongly linked to atypical handedness, consistent with the idea that disruptions in brain asymmetry affect both motor and language functions [19].

Whether atypical lateralization is a cause, a correlate, or a consequence of these conditions remains unclear [19]. But if the evolutionary origins of lateralization are now better understood — rooted in the biomechanical consequences of bipedalism and the neurological consequences of encephalization — researchers may be better positioned to identify which developmental pathways are disrupted in these conditions. Genome-wide studies have already identified at least 40 genetic loci associated with handedness, many involving genes that regulate left-right body asymmetry [12][13]. The gene NME7, for instance, appears to link brain lateralization to visceral organ asymmetry, suggesting that handedness is one expression of a deep body-patterning program [13].

Source: OpenAlex

Data as of Jan 1, 2026CSV

Academic interest in handedness and lateralization has grown substantially over the past fifteen years, with publications rising from 977 in 2011 to a peak of 2,762 in 2023, according to OpenAlex data. The Oxford study is likely to accelerate this trend by providing a unified evolutionary framework that connects comparative primatology, paleoanthropology, and neuroscience.

What Remains Unknown

The Oxford study answers why humans are so strongly right-handed relative to other primates. It does not answer why any individual is left-handed, nor does it explain the precise genetic and developmental mechanisms that translate bipedalism and large brains into a population-level rightward bias. The 25% heritability of handedness estimated from twin studies [12] leaves 75% of the variance to environmental and stochastic developmental factors — a reminder that evolutionary explanations and individual-level explanations operate at different scales.

The fighting hypothesis offers a plausible account of why left-handedness is maintained rather than eliminated, but the evidence remains circumstantial [8][9]. And the question of how cultural suppression interacts with biological predisposition to produce the observed global variation in left-handedness rates is still largely unresolved [6][7].

What the Püschel team has done is narrow the field. The origin of human right-handedness is no longer a mystery with ten equally plausible answers. It is a question with a strong leading candidate — and a clearer set of remaining unknowns.