Handedness does run in families, but genes turn out to be only a modest part of the story.1PubMed Central. Genetic influences on handedness: data from 25,732 Australian and Dutch twin families Two people with the same genes can still end up favoring different hands. And the preference is set remarkably early, visible in the womb well before birth.2PubMed Central. The origin of human handedness and its role in pre-birth motor control
How Much of Handedness Comes From Genes
Twin studies put a number on it. In a study of more than 25,000 twin families, genes accounted for about 25% of the differences in handedness between people.3PubMed Central. Genetic influences on handedness: data from 25,732 Australian and Dutch twin families Combined twin studies land on almost exactly the same figure.4PubMed Central. Handedness in twins: meta-analyses About 75% came from influences unique to each person, not from genes or a shared home.5PubMed Central. Genetic influences on handedness: data from 25,732 Australian and Dutch twin families
The method works because identical twins share essentially all their genes, while fraternal twins share about half. If genes mattered a great deal, identical twins would match in handedness far more often than fraternal twins. They match only somewhat more often.
That 25% describes differences across a population, not how much of one person’s handedness is genetic. It does not mean a quarter of left-handers inherited it and the rest did not.
The “unique influences” share is everything that differs between two people and is not a gene or a shared home. It includes random variation in how each body and brain develops before birth. Part of it is also measurement noise, such as how a person happened to answer a handedness questionnaire.
These unique influences also explain why identical twins, who share all their genes, often differ in handedness. They develop in slightly different positions and conditions in the womb, and small random differences during brain wiring can tip each twin toward a different hand.
Will Left-Handed Parents Have a Left-Handed Child
Among more than 2,600 couples, both parents were right-handed in about 8 of 10 cases, and both were left-handed in fewer than 1 in 100.6PubMed. A new family handedness sample with findings consistent with X-linked transmission Couples with a left-handed mother had more left-handed children than two right-handed parents did, especially sons.7PubMed. A new family handedness sample with findings consistent with X-linked transmission Couples with a left-handed father had more left-handed daughters but not more left-handed sons.8PubMed. A new family handedness sample with findings consistent with X-linked transmission So the mother’s handedness may matter somewhat more, a pattern some researchers link to genes on the X chromosome.
Two right-handed parents can have a left-handed child, and most left-handers in fact come from families where both parents are right-handed, simply because right-handed couples are so common. Even two left-handed parents are more likely than not to have a right-handed child, since right-handedness is the default for about 9 in 10 people. No test can predict a baby’s eventual hand from the parents’ genes or handedness.
Handedness can also skip generations. Many small genetic nudges combine with chance, so a child can be left-handed when neither parent is.
There Is No Single Left-Handed Gene
A large genetic study found 48 common variants linked to handedness, 41 to left-handedness and 7 to ambidexterity.9PubMed Central. Genome-wide association study identifies 48 common genetic variants associated with handedness Each one raises the odds of being left-handed or ambidextrous only by a small amount.10PubMed Central. Genome-wide association study identifies 48 common genetic variants associated with handedness Long-popular theories that one gene sets your handedness are not supported.11PubMed Central. Genome-wide association study identifies 48 common genetic variants associated with handedness No consumer DNA test can reliably tell you whether a child will be left-handed.
Most of these variants sit outside the gene code itself and likely change how strongly genes are switched on.12PubMed Central. Exome-wide analysis implicates rare protein-altering variants in human handedness Some involve the tiny internal scaffolding of cells and are linked to left-right differences in the brain’s outer layer.13PubMed Central. Handedness and its genetic influences are associated with structural asymmetries of the cerebral cortex in 31,864 individuals One such gene, NME7, may connect brain asymmetry to the left-right layout of organs like the heart.14PubMed Central. Handedness and its genetic influences are associated with structural asymmetries of the cerebral cortex in 31,864 individuals
Genes can also affect how strongly you favor a hand rather than which one. One version of the gene PCSK6 was linked to how strongly people favor one hand, not to which hand.15PubMed Central. PCSK6 VNTR Polymorphism Is Associated with Degree of Handedness but Not Direction of Handedness
What Else Shapes Which Hand You Use
Fetuses already favor one hand, and arm movements toward the face by about 18 weeks of pregnancy predicted later handedness in one study.16PubMed Central. The origin of human handedness and its role in pre-birth motor control All 60 fetuses seen sucking a right thumb became right-handed children, while 10 of 15 left-thumb suckers became left-handed.17PubMed. Prenatal thumb sucking is related to postnatal handedness
Pregnancy and birth circumstances play a small part. Girls born before 37 weeks tended to have a weaker hand preference at age five.18PubMed Central. Is Handedness at Five Associated with Prenatal Factors? Babies born in winter or spring had a very slightly weaker preference.19PubMed Central. Is Handedness at Five Associated with Prenatal Factors? In one study, mothers over 32 and certain birth orders were linked to more left-handed daughters, possibly explaining up to about 29% of left-handed women.20PubMed. Maternal age and parity correlates of handedness: gender, but no parental handedness modulation of effects Hormones like testosterone around birth seem to play little or no role.21PubMed. A longitudinal examination of perinatal testosterone, estradiol and vitamin D as predictors of handedness outcomes in childhood and adolescence
Social pressure matters too. Children who were forced to switch hands often end up mixed-handed, using different hands for different tasks.22PubMed. Coerced training of the nondominant hand resulting in cortical reorganization: a high-field functional magnetic resonance imaging study Their brains show a distinct activity pattern, unlike natural right- or left-handers.23PubMed. Coerced training of the nondominant hand resulting in cortical reorganization: a high-field functional magnetic resonance imaging study Because forced switching can leave children mixed-handed, the sensible approach is to support the child’s natural hand, for example with left-handed scissors and a seat that leaves elbow room.
Mixed-handed people do some tasks with one hand and others with the other, such as writing with the right but throwing with the left. Truly ambidextrous people are equally quick and accurate with both hands, which is rare. Many people who call themselves ambidextrous are really mixed-handed. Handedness is better pictured as a spectrum from strongly right to strongly left than as two boxes.
Left-Handers Through Human History
Worldwide, about 9 in 10 people are right-handed.24PubMed. More than 500,000 years of right-handedness in Europe Scratch marks on fossil teeth show right-handed actions in Europe more than 500,000 years ago.25PubMed. More than 500,000 years of right-handedness in Europe Stone tools from 1.4 to 1.9 million years ago appear to have been made mostly by right-handers.26Journal of Human Evolution. Archaeological evidence for preferential right-handedness in the lower and middle pleistocene, and its possible implications A clear right-hand bias is well established from Neanderthals onward.27PubMed. The prehistory of handedness: archaeological data and comparative ethology
Other primates differ. Across 38 primate species, none came close to humans’ strong population-wide right bias.28eLife. The evolution and biological correlates of hand preferences in anthropoid primates Only gorillas and chimpanzees showed any group-level lean at all.29eLife. The evolution and biological correlates of hand preferences in anthropoid primates
Culture has shaped the recorded rates. Left-handed children were punished in many places, including Australia, the Soviet Bloc and former Yugoslavia, into the 1970s.30TheCollector. The History of Left-Handedness: Why Is This a Bias?31Srpski Arhiv Za Celokupno Lekarstvo. Historical aspects of left-handedness China has among the lowest rates of left-handedness, partly due to pressure to write right-handed.32TheCollector. The History of Left-Handedness: Why Is This a Bias?
How Left-Handed Brains Differ
Language sits mainly in the right half of the brain in about 4% of strong right-handers, 15% of ambidextrous people and 27% of strong left-handers.33PubMed. Handedness and hemispheric language dominance in healthy humans Left-handers also show less left-right difference across several brain functions.34PubMed Central. Left-Handers Are Less Lateralized Than Right-Handers for Both Left and Right Hemispheric Functions A language-related brain area is less lopsided in left-handers.35PubMed. Anatomical left-right asymmetry of language-related temporal cortex is different in left- and right-handers This is an average difference across groups, with lots of overlap between left- and right-handers.
The bundle of fibers connecting the brain’s two halves was about 11% larger in left- and mixed-handers in an early study of brains after death.36PubMed. The brain connection: the corpus callosum is larger in left-handers Later scans linked a thicker bridge to a weaker hand preference in either direction.37PubMed Central. When more is less: associations between corpus callosum size and handedness lateralization A newer analysis found almost no difference in size.38PubMed Central. Hand preference and the corpus callosum: Is there really no association?
Because handedness only shifts the odds, before brain surgery doctors check which side handles language rather than assuming from handedness. And because the bridge findings remain unsettled, claims that left-handers are better at multitasking go beyond the evidence.
Health, Risk and the Left-Handed Advantage
Being left- or mixed-handed is more common among people with dyslexia, ADHD, autism, schizophrenia and several other developmental conditions.39PubMed Central. Non-right-handedness and psychiatric disorders: A synthesis of epidemiological, genetic, and neurobiological evidence These conditions all begin early in development, the same period when hand preference forms.40PubMed Central. Non-right-handedness and psychiatric disorders: A synthesis of epidemiological, genetic, and neurobiological evidence For dyslexia, the clearest link is with mixed-handedness.41PubMed. Elevated levels of mixed-hand preference in dyslexia: Meta-analyses of 68 studies Some genes may raise the odds of both schizophrenia and left-handedness.42PubMed Central. The genetic relationship between handedness and neurodevelopmental disorders Higher rates in these groups do not mean a left-handed child is likely to have any of the conditions; the vast majority of left-handers have none of them.
Living in a right-handed world carries some risk. Left-handed students were nearly twice as likely to report an injury needing medical care over two years, in one Canadian survey.43PubMed Central. Left-handedness and accident-related injury risk Most machines and tools are designed for right-handers.44Journal of Occupational Health. Association between handedness and occupational accidents: a prospective cohort study Left-handed scissors, knives and can openers reduce awkward grips.
Left-handers also have an edge in some settings. Left-handed boxers and MMA fighters are overrepresented and win more often.45PubMed Central. Left-handedness is associated with greater fighting success in humans Being rare means opponents are less used to them.46PubMed Central. Why are some people left-handed? An evolutionary perspective In some interactive sports left-handers make up a much larger share of top players, but not in solo sports.47PubMed. Frequency-dependent maintenance of left handedness in humans This rarity advantage may explain why left-handers have stayed about 1 in 10 for so long.48PubMed. Frequency-dependent maintenance of left handedness in humans