Standing Up and Scaling Up: New Study Links Brain Expansion to Human Handedness
One of the most persistent questions in human evolution is why nearly 90 percent of people consistently prefer their right hand. While other primates may show individual hand preferences, no known species displays the same strong population-wide pattern seen in humans. A new study published in PLOS Biology examines whether the answer may lie in two defining features of human evolution: walking upright and developing a larger brain.
Note: This article is intended for general information and educational purposes. It summarizes scientific research in accessible language for a broad audience and is not an official scientific press release.
A new study published in PLOS Biology on April 27, 2026, investigates why humans developed an unusually strong right-hand preference compared to other primates and whether this pattern is associated with two major evolutionary changes in the human lineage: bipedalism and brain expansion. Researchers Thomas A. Püschel and Rachel M. Hurwitz from the Institute of Human Sciences, School of Anthropology and Museum Ethnography at the University of Oxford, together with Chris Venditti from the School of Biological Sciences at the University of Reading, used phylogenetic comparative modeling to examine how locomotion, brain size, and other biological factors relate to handedness across primates.
Humans possess a striking and near-universal right-hand preference, an evolutionary trait unmatched by any other primate species. While individual animals across several species may favor one hand, the consistent right-handed bias observed in humans represents what the authors of the study describe as an evolutionary “singularity.”
What the Researchers Investigated
The study explored several eco-evolutionary hypotheses related to the origins of manual asymmetry in primates. The researchers focused on two main measurements:
- Direction (Mean Handedness Index, MHI): This reflects whether a species tends to favor the left or right hand at the population level.
- Strength (Mean Absolute Handedness Index, MABSHI): This measures how strongly individuals prefer one hand, regardless of whether the preference is left or right.
The researchers examined whether Homo sapiens represents an evolutionary outlier among primates and whether factors such as endocranial volume, locomotion, body proportions, tool use, and social systems may help explain the unusually strong right-handed bias observed in humans.
The paper also investigated how handedness may have changed during hominin evolution by generating predictions for extinct species, including Ardipithecus ramidus, Australopithecus afarensis, Homo erectus, and Homo neanderthalensis.
How the Study Was Conducted
To carry out the analysis, the researchers combined data from previous large-scale handedness datasets and applied Bayesian phylogenetic comparative meta-analytical methods. The final dataset included 2,025 individuals across 41 anthropoid species.
To improve comparability across species, the study only included data obtained through the “tube task,” a standardized behavioral test commonly used in primate handedness research. In this task, primates hold a cylinder with one hand while extracting food with the other.
The researchers incorporated several biological and ecological variables into their models, including:
- body mass,
- endocranial volume,
- diet,
- locomotion patterns,
- intermembral index (IMI),
- substrate preference,
- social systems,
- sexual dimorphism,
- and tool use.
The intermembral index was particularly important because it reflects limb proportions. Humans have a relatively low IMI, meaning the legs are significantly longer than the arms – a characteristic associated with bipedal locomotion.
The researchers also used their models to estimate probable handedness patterns in extinct hominin species based on phylogenetic relationships and anatomical variables.
What Makes This Study New
According to the authors, this is the first study to combine large-scale primate handedness data with phylogenetic comparative analysis and meta-analysis. This approach allowed the researchers to compare species while also accounting for evolutionary relationships and differences between datasets.
Compared to earlier studies that found inconsistent links between handedness and brain size, the present analysis identified clearer associations once evolutionary history and statistical weighting were included in the models.
The study also suggests that strong hand preference appeared relatively early in hominin evolution, while the unusually strong right-handed bias seen in modern humans became more pronounced later within the genus Homo.
A Brain-Focused Perspective on Handedness
The study places strong emphasis on the connection between handedness and brain evolution. According to the authors, hand preference is linked to specialized brain regions and differences between the brain’s two hemispheres that are associated with complex cognitive functions.
One of the clearest patterns identified in the study involved brain size. The researchers found that endocranial volume – a measurement related to brain size – was strongly associated with the direction of handedness in humans. When brain size and locomotion-related variables were included in the statistical models, humans no longer appeared evolutionarily unusual compared to other primates.
The paper suggests that as the brains of early hominins became larger over time, this may have contributed to greater specialization between the left and right hemispheres of the brain, reinforcing population-level right-handedness.
The authors also note that handedness likely develops through a combination of genetic, developmental, and behavioral influences linked to specialized neural structures.
Key Findings from the Study
According to the authors, the analyses revealed several patterns related to the evolution of handedness in primates:
- Humans showed the strongest right-handed tendency. Compared to other primates, humans displayed by far the clearest population-level preference for the right hand. The study reported an MHI value of 0.76 for Homo sapiens, far above the average pattern observed across anthropoid primates.
- Brain size and upright walking were closely linked to human handedness. When the researchers included brain size and body proportions related to bipedalism in their models, humans no longer appeared evolutionarily unusual compared to other primates.
- Tree-dwelling primates often showed stronger hand preferences. According to the study, species that spend more time in trees generally showed stronger individual hand preferences than mostly ground-dwelling species.
- Walking upright may have changed how the hands were used. The authors suggest that bipedalism freed the hands from locomotion, potentially increasing the need for more specialized and coordinated hand use.
- Strong hand preference appeared early in human evolution. The study found evidence that several extinct hominin species already showed relatively strong hand preferences millions of years ago.
- Extreme right-handedness became stronger later in the genus Homo. Earlier hominins appeared to show weaker right-hand bias, while later species such as Homo erectus and Homo neanderthalensis showed progressively stronger predicted right-handedness.
Key Questions About the Study
As summarized by Neuroscience News, the study raises several broader questions about the evolution of human handedness:
Q: Why are humans the only primates that are overwhelmingly right-handed?
A: According to the study, two major evolutionary factors appear central: upright walking and large brain size. As early hominins adopted bipedal locomotion, the upper limbs were freed from locomotor functions, potentially increasing selective pressures for manual specialization. The paper further suggests that increases in brain size and cortical reorganization may have reinforced population-level lateralization patterns over time.
Q: If right-handedness is an evolutionary rule for humans, why did the “Hobbit” (Homo floresiensis) species break it?
A: The study reports that Homo floresiensis showed comparatively weaker predicted handedness directionality than other Homo species. According to the authors, this may be related to the species’ smaller brain size and anatomical features associated with both bipedalism and arboreal activity.
Q: Does this study explain why left-handed people still exist?
A: Not completely. The paper focuses primarily on explaining the emergence of strong rightward population bias in humans. The authors also note that cultural, developmental, and genetic influences likely contribute to the persistence and variation of handedness patterns across human populations.
Authors’ Conclusions
According to the authors, upright walking and brain expansion likely worked together during human evolution to shape modern handedness patterns. The paper suggests that walking on two legs freed the hands for more specialized use, while larger brains may have strengthened differences between the brain’s hemispheres and reinforced right-handedness.
The study also notes that many earlier theories about handedness may have focused too heavily on humans and may not fully explain handedness patterns across other primates.
The authors acknowledge several limitations, including differences between datasets, the fact that handedness can vary depending on context, and the possible influence of culture on human hand preference.
Finally, the paper recommends future studies involving other animals that show limb preferences, including parrots and kangaroos, to explore whether similar evolutionary patterns may have developed independently.
Understanding the Broader Context
These findings contribute to ongoing scientific research examining how locomotion, anatomy, and brain evolution may relate to behavioral asymmetries across species. The study describes how human-specific traits, including upright walking and increased brain size, coincide with unusually strong population-level handedness patterns.
The results also highlight that strong hand preference is not entirely unique to humans. Several non-human primates demonstrated high handedness strength, although humans remained distinct in the consistency and direction of right-handed bias across populations.
Conclusion
The study adds to a growing body of research examining how the human brain and body evolved together over time. By combining phylogenetic comparative analysis with meta-analysis across multiple primate species, the researchers identified strong associations between handedness, brain expansion, and locomotor adaptations.
At the same time, the paper emphasizes that the full evolutionary origins of human handedness remain complex and likely involve additional genetic, developmental, environmental, and cultural factors. Future studies using larger fossil datasets and broader cross-species comparisons may help refine current models of behavioral lateralization and human evolution.
The information in this article is provided for informational purposes only and is not medical advice. For medical advice, please consult your doctor.
References
- Püschel TA, Hurwitz RM, Venditti C (2026) Bipedalism and brain expansion explain human handedness. PLOS Biology. DOI:10.1371/journal.pbio.3003771
- Neuroscience News. Why 90% of Humans Share the Same Dominant Hand.
https://neurosciencenews.com/bipedalism-brain-handedness-30698/
