Handedness is shaped by a combination of genetics, prenatal development, and environmental factors, with no single cause explaining why roughly 10% of people favor their left hand. What makes this trait especially fascinating is that hand preference begins forming far earlier than most people realize, likely in the spinal cord before the brain’s motor system is even online.
It Starts in the Spinal Cord, Not the Brain
For decades, scientists assumed handedness originated in the brain’s motor cortex, the region that controls voluntary movement. That assumption turned out to be wrong, or at least incomplete. Human fetuses show clear asymmetries in arm movement as early as eight weeks after conception. At that point, the motor cortex isn’t yet connected to the spinal cord. The nerve fibers linking the brain to the spine don’t reach the relevant part of the spinal cord until around 15 weeks. This means the earliest hand preference has to come from the spinal cord itself.
A 2017 study published in eLife found striking differences in gene expression between the left and right sides of the fetal spinal cord at just eight weeks. Nearly 1,700 gene transcripts showed left-right differences, with the vast majority showing stronger expression on the right side. This asymmetry was linked to epigenetic patterns: chemical modifications on DNA that turn genes on or off without changing the genetic code itself. At eight weeks, the left spinal cord showed significantly more of a specific type of gene silencing (called CpG island methylation), which suppressed gene activity on that side. The net effect was greater right-sided spinal cord activity, which would favor right-hand movement.
One signaling pathway stood out in particular. All five of the small regulatory molecules (called microRNAs) that showed significant asymmetry were involved in the same growth-signaling pathway. This suggests a coordinated molecular program, not random variation, is driving early lateralization.
Hand Preference Is Visible Before Birth
By the second trimester, fetuses show a clear preference for which thumb they suck, and this preference is remarkably stable. A study that followed 75 fetuses observed sucking their thumb in utero found that all 60 who preferred their right thumb were right-handed when tested at ages 10 to 12. Of the 15 who favored their left thumb, 10 were left-handed as children. The prediction was stronger for right-handers than left-handers, and boys who sucked their left thumb were more likely to switch to right-handedness than girls were.
This tells us two things. First, the biological bias toward handedness is already locked in well before birth for most people. Second, left-handedness appears to be a somewhat less “fixed” trait than right-handedness, with more room for developmental factors to nudge a child one way or another after birth.
Genetics Plays a Substantial but Incomplete Role
Heritability estimates for handedness range from about 25% to 65% across different studies. That means genes account for a significant chunk of the variation in hand preference, but they’re far from the whole story. No single “handedness gene” has been found. Instead, multiple genes each make a small contribution.
Two genes have received the most attention. One, called PCSK6, produces an enzyme involved in establishing the body’s left-right axis during early development. It does this by activating a signaling molecule called NODAL, which helps the embryo distinguish its left side from its right. Variations in this gene have been linked to handedness in at least three separate genome-wide studies. The other gene, LRRTM1, is involved in how neurons form connections. Both genes point to the same basic idea: handedness is tied to the broader biological machinery that makes the two sides of our bodies and brains slightly different from each other.
The fact that identical twins, who share 100% of their DNA, don’t always share the same hand preference is strong evidence that genes alone can’t explain handedness. Environmental factors in the womb, including positioning, blood flow, and possibly hormone exposure, fill in the gaps.
The Prenatal Hormone Theory
One long-standing hypothesis, first proposed in 1982, suggests that exposure to higher levels of testosterone in the womb could influence brain development in ways that affect handedness. The theory predicted a causal link between non-right-handedness, immune disorders, and learning differences like dyslexia, all mediated by prenatal testosterone’s effect on the developing brain. Some large epidemiological studies have found statistical associations among these traits. Others have not. The theory remains debated, and the evidence is mixed enough that prenatal hormones are considered a possible contributor rather than a confirmed cause.
What’s Different in Left-Handed Brains
The brains of left-handed and right-handed people do show measurable differences, though they’re subtler than you might expect. In right-handed people, the motor areas controlling the dominant right hand (located in the left hemisphere) show higher functional connectivity. Left-handers show the reverse pattern: increased connectivity in left-hand motor areas and decreased connectivity in right-hand motor areas.
There’s also an interesting difference in how the two groups handle tasks involving both hands. When right-handed people move their non-dominant hand, the brain’s motor cortex on the same side actively quiets down, a process that helps with coordination. Left-handers don’t show this same suppression as strongly, suggesting their motor control may be organized in a less rigidly lateralized way.
Language processing also differs. Most right-handed people process language predominantly in the left hemisphere. Left-handers are more likely to use both hemispheres for language or even to show right-hemisphere language dominance. That said, the majority of left-handers still process language on the left side, just like right-handers. The difference is one of degree, not a wholesale reversal.
Why Left-Handedness Persists
If right-handedness is the biological default for about 90% of people, why hasn’t left-handedness disappeared entirely? Evolutionary biologists have proposed that left-handedness is maintained through a process called negative frequency-dependent selection. The idea is simple: in physical combat or competitive sports, being a lefty is an advantage precisely because it’s rare. Your opponent has far less experience facing someone who moves and strikes from the opposite side. This advantage grows as left-handedness becomes less common and shrinks if it becomes more common, creating a natural equilibrium that keeps the trait in the population at a stable minority frequency.
Supporting this, left-handers are consistently overrepresented in interactive sports like boxing, fencing, tennis, and baseball. The advantage disappears in non-interactive activities like gymnastics or swimming, where there’s no opponent to surprise. However, the fact that left-handedness stays at only about 10% suggests there are also evolutionary costs that prevent it from becoming more common, though what those costs are remains debated.
Handedness and Cognitive Differences
The relationship between handedness and cognitive ability is real but complicated, and far from the simple “left-handers are more creative” narrative you’ll sometimes hear. Large population studies have found that left-handed and mixed-handed children tend to score slightly lower on most developmental measures compared to right-handed children, with the gap being larger for boys. At the same time, left-handers appear to be overrepresented at the extreme high end of mathematical ability. One study found an unusually large proportion of left-handers among the top 1% of scorers on the SAT.
This pattern fits with what researchers call a wider distribution of outcomes. Left-handedness doesn’t make someone smarter or less capable on average, but it may be associated with greater variability in cognitive development. Claims about enhanced spatial ability or divergent thinking in left-handers have produced mixed results across studies, with some finding advantages and others finding none.
The association between mixed-handedness (not having a strong preference for either hand) and learning differences like dyslexia has a longer and more consistent track record in the research literature, dating back to the 1930s. Mixed-handedness may reflect a less strongly lateralized brain, which could create subtle challenges for processes like reading that benefit from efficient left-hemisphere specialization.
Handedness Is a Spectrum
Researchers don’t treat handedness as a simple binary. The most widely used measurement tool, the Edinburgh Handedness Inventory, asks people which hand they prefer for 10 different tasks: writing, drawing, throwing, using scissors, brushing teeth, using a knife, using a spoon, sweeping with a broom, striking a match, and opening a lid. The result is a score on a continuum from strongly left-handed to strongly right-handed, with many people falling somewhere in between. Strong right-handers might use their right hand for every task, while others use their right hand for writing but their left for throwing.
This spectrum reflects the underlying biology. Handedness isn’t controlled by a single on-off switch. It emerges from the interplay of dozens of genes, asymmetric gene expression in the fetal spinal cord, epigenetic modifications, prenatal environment, and postnatal experience. For most people, these factors align to produce right-handedness. For a minority, some combination tips the balance the other way.