True ambidexterity, the ability to use both hands with equal skill, is rare. Only about 1% of people have no clear hand preference, while roughly 90% favor their right hand and 10% favor their left. What produces that small sliver of the population involves a mix of genetics, prenatal hormone exposure, and brain structure that researchers are still piecing together.
Handedness Is a Spectrum, Not a Switch
Most people aren’t purely right-handed or left-handed. Researchers measure hand preference on a sliding scale. The most widely used tool, the Edinburgh Handedness Inventory, scores people from negative 100 (purely left-handed) to positive 100 (purely right-handed), with zero representing no preference at all. Someone who writes with their right hand but throws with their left, or who switches hands depending on the task, falls somewhere in the middle. This is called mixed-handedness or cross-dominance, and it’s far more common than true ambidexterity.
In a large UK study tracking over 500,000 people, about 1.7% reported using both hands equally. But when researchers followed up with the same individuals at different time points, the self-described ambidextrous group gave inconsistent answers, suggesting that many of them actually had a subtle preference they didn’t notice. Genuine equal-handedness, where a person performs comparably with either hand across a range of tasks, is likely even rarer than that 1% to 2% figure suggests.
Genetics Play a Role, but No Single Gene Decides
Handedness runs in families, but the inheritance pattern is complicated. There is no single “handedness gene.” Instead, multiple genes each nudge the odds slightly in one direction. One of the most studied is a gene called LRRTM1 on chromosome 2, which is involved in brain development. A specific version of this gene shifts people toward left-handedness, but only when inherited from the father. When the same variant comes from the mother, it has no measurable effect. This parent-of-origin pattern, known as genomic imprinting, means the mother’s copy of the gene is essentially silenced at certain points during fetal brain development.
The paternal version of this gene variant, carried by about 9% of the population studied, shifted hand preference roughly one standard deviation toward left-handedness. That’s a meaningful push, but it’s far from deterministic. Most carriers still end up right-handed. Ambidexterity likely emerges when several of these small genetic influences line up without tipping clearly in one direction, combined with environmental factors during development.
Prenatal Hormones and Brain Development
One of the most debated theories about handedness involves testosterone exposure before birth. The original hypothesis, proposed by Geschwind, Behan, and Galaburda in the 1980s, suggested that high prenatal testosterone slows development of the left hemisphere and strengthens the right, pushing a person toward left-handedness or reduced hand preference. Since the left hemisphere typically controls the right hand, anything that disrupts its dominance could, in theory, produce weaker lateralization.
The evidence, however, has been mixed. A competing theory argues the opposite: that lower testosterone leads to less pruning of brain connections, producing a larger bridge between the hemispheres (the corpus callosum) and less pronounced hand dominance. Some direct measurements of testosterone in amniotic fluid have found that higher prenatal testosterone is actually associated with right-handedness, contradicting the original hypothesis. Studies of opposite-sex twins, where a female twin is exposed to slightly more testosterone from her male twin, have supported this reversal as well. The short answer is that prenatal hormones clearly matter for brain lateralization, but scientists haven’t settled on exactly how.
How Ambidextrous Brains Differ
The brain’s two hemispheres are not mirror images. In most right-handed people, language processing is strongly concentrated in the left hemisphere. Left-handers and ambidextrous individuals tend to have a more distributed pattern, with both hemispheres sharing the workload more evenly.
One structural difference involves the corpus callosum, the thick band of nerve fibers connecting the two hemispheres. Imaging studies have found that non-right-handed individuals tend to have a larger corpus callosum, particularly in the region connecting the frontal lobes. A larger corpus callosum allows more communication between hemispheres. In a study of 74 people, those with a larger corpus callosum showed stronger left-hemisphere language activation, driven partly by greater information transfer from the right hemisphere and partly by the left hemisphere suppressing duplicate activity on the right side. The relationship held regardless of handedness, gender, or age, though right-handers still showed the strongest leftward language bias overall.
For ambidextrous people, this more balanced hemispheric setup may be why neither hand clearly dominates. The brain hasn’t committed as strongly to routing fine motor control through one side.
Links to Neurodevelopmental Conditions
Atypical handedness, including ambidexterity, shows up more frequently in certain neurodevelopmental conditions. Among people on the autism spectrum, 30 to 40% display inconsistent hand preference, and 15 to 20% are left-handed, roughly double the rate in the general population. Researchers have proposed that this reflects atypical brain organization, particularly reduced left-hemisphere specialization. Children with autism who lack a consistent hand preference tend to score lower on cognitive tasks compared to autistic children with a clear dominant hand.
Similar patterns have been observed with ADHD and schizophrenia, where rates of non-right-handedness are elevated. One theory suggests that prenatal testosterone exposure during critical windows of brain development leads to unequal hemisphere maturation, favoring the right hemisphere at the expense of left-hemisphere functions like language. This doesn’t mean ambidexterity causes these conditions or that being ambidextrous is a warning sign. It means the same underlying variation in brain organization that produces mixed handedness can also contribute to different cognitive profiles.
Cognitive Tradeoffs
Ambidextrous individuals don’t simply get the best of both hands. Large-scale studies have found small but consistent cognitive differences. In a British study of 11-year-olds, ambidextrous children scored lower on verbal, nonverbal, reading, and math assessments compared to both right-handers and left-handers. Follow-up research in adults confirmed the pattern, with ambidextrous people performing less well on measures of arithmetic, memory, and reasoning.
The likely explanation ties back to brain lateralization. Specializing one hemisphere for a task, like concentrating language in the left hemisphere, appears to be efficient. When the brain distributes functions more evenly across both sides, it may process certain types of information less efficiently. This doesn’t mean ambidextrous people are less intelligent in any meaningful everyday sense, but on average, strong lateralization seems to offer a slight processing advantage on timed cognitive tests.
Why Ambidexterity Persists
If strong handedness confers cognitive benefits, why hasn’t evolution eliminated ambidexterity entirely? The same question applies to left-handedness, which has remained at roughly 10% across cultures and throughout recorded history. Archaeological evidence from tool use and dental wear suggests this ratio has held for thousands of years, pointing to some form of balancing selection keeping the trait around.
One leading explanation involves fighting. In combat, a left-handed or ambidextrous fighter has an element of surprise against opponents accustomed to right-handed attacks. This advantage increases when left-handedness is rare, creating what evolutionary biologists call negative frequency-dependent selection: the trait becomes more valuable precisely because it’s uncommon. Studies of traditional societies with higher rates of interpersonal violence do show higher rates of left-handedness, consistent with this hypothesis. The same advantage extends to modern competitive sports, where left-handed athletes are overrepresented in interactive sports like boxing, fencing, and tennis.
Another theory connects handedness to the evolution of language. Since speech and gesture are deeply linked in the brain, the strong left-hemisphere dominance that produces right-handedness may have been selected for as human language became more complex. Ambidexterity may simply represent the natural variation that any biological trait produces, persisting because the selection pressure favoring strong lateralization isn’t absolute.