Handedness begins taking shape long before a child picks up a crayon. Ultrasound studies have detected arm-movement preferences as early as the first trimester of pregnancy, and by 18 weeks of gestation, the way a fetus reaches toward its own face can predict which hand it will favor a decade later. But “begins” is not the same as “settles.” The journey from those first asymmetric movements in the womb to a firmly established hand preference stretches across years of childhood, with surprising twists along the way.
It Starts in the Womb
The earliest signs of handedness appear before a baby is born. As far back as the early 1990s, researchers using ultrasound found that most fetuses preferentially move one arm more than the other from around 10 weeks of gestation, and that this preference holds throughout pregnancy. The bias also shows up in the fetal head-position preference after birth.1Neuropsychologia. Handedness in the human fetus A landmark follow-up study tracked 75 fetuses who were observed sucking their thumbs in utero. Of the 60 who preferred the right thumb, every single one turned out to be right-handed when tested at age 10 to 12. Of the 15 who preferred the left thumb, 10 were left-handed later, though five switched to right-handedness.2PubMed. Prenatal thumb sucking is related to postnatal handedness
More recent work has sharpened the picture further. A 2017 study using detailed kinematic analysis of fetal hand movements found that from 18 weeks of gestation, the speed difference between a fetus’s two hands when reaching toward its own eyes and mouth could classify its postnatal handedness with 95 to 100 percent accuracy.3Scientific Reports. The origin of human handedness and its role in pre-birth motor control That is a remarkably strong signal coming from movements that happen months before a baby first wraps its fingers around a rattle.
Why the Brain May Not Be Running the Show at First
You might assume the brain orchestrates all of this from the start, but the timeline does not support that. At 8 to 10 weeks of gestation, when arm-movement asymmetries first appear, the motor cortex is not yet functionally connected to the spinal cord. The nerve fibers that carry voluntary movement commands from the brain do not reach the relevant part of the spinal cord until roughly 15 weeks. Research published in eLife found that gene-expression asymmetries in the fetal spinal cord itself, driven by epigenetic regulation, are the more likely origin of these early movement biases.4eLife. Epigenetic regulation of lateralized fetal spinal gene expression underlies hemispheric asymmetries In other words, the initial push toward one hand or the other seems to come from below the brain, not from it. The brain’s role grows later, as its connections to the body mature.
The Newborn Period and the Head-Turn Connection
Once a baby is born, an easily observable asymmetry takes center stage: head-turning preference. About 65 percent of newborns prefer to lie with their head turned to the right, while roughly 15 percent consistently turn left. This preference sticks around for at least two months and predicts which hand a baby will favor when reaching for objects at four and five months of age.5PubMed. Right-handedness: a consequence of infant supine head-orientation preference?
The mechanism is intuitive once you think about it. A baby lying with its head turned right sees its right hand more, moves it more, and gets more practice using it. Over the first several weeks, this creates a cascade of lopsided visual experience and motor practice that reinforces the original tilt. A longitudinal study that tracked babies with right and left head-turn preferences from birth through about 18 months found that 18 out of 20 infants maintained a stable hand-use preference consistent with their neonatal head orientation throughout that entire period.6PubMed. Postural and lateral asymmetries in the ontogeny of handedness during infancy The neonatal head-turn bias does not guarantee handedness on its own, but it appears to amplify whatever underlying asymmetry already exists.
The Fluctuating Middle Months
If you watch a six-month-old play, you might notice them reaching for a toy with the right hand one day and the left the next. This is normal and well-documented. The development of hand preference during the first year is not a smooth march toward one side; it is marked by several lateral fluctuations, with periods of right-hand dominance, left-hand dominance, and no clear preference alternating in many infants.7Developmental Psychobiology. Lateral biases and fluctuations in infants’ spontaneous arm movements and reaching
Age-related shifts add to the complexity. One study tracking reaching and grasping of a moving object found that six-month-olds used whichever hand was closer to the object, eight-month-olds showed a strong right-hand bias for both reaching and grasping, and ten-month-olds diversified their strategies again, including more bimanual (two-handed) approaches.8PubMed Central. Reaching and grasping a moving object in 6-, 8-, and 10-month-old infants: laterality and performance This kind of back-and-forth can confuse parents who are trying to figure out whether their child is left- or right-handed, but it reflects the normal process of the motor system testing and refining its organization.
Even by 14 months, the majority of infants do not show a hand preference for one-handed object manipulation that is statistically distinguishable from chance, according to a study that tested 90 infants monthly from 6 to 14 months.9PubMed Central. The influence of a hand preference for acquiring objects on the development of a hand preference for unimanual manipulation from 6 to 14 months The seeds of handedness are clearly planted well before this age, as the prenatal and newborn evidence shows. But what a researcher can detect in fetal kinematics and what a parent can see in their toddler’s daily play are two different things.
When Preference Finally Stabilizes
The clearest answer to “when does handedness develop” depends on what you mean by “develop.” If you mean when can we first detect a bias, the answer is the first trimester. If you mean when does a child consistently use one hand for most tasks, the answer is closer to age two or three, with full stabilization sometimes not arriving until school age.
Research tracking infants through the first two years finds that roughly half show a consistent early preference for picking up objects, while the other half follow a more variable path. By the end of the second year, though, most children display a consistent preference for tasks that require the two hands to play different roles, like holding a jar with one hand while unscrewing the lid with the other.10PubMed Central. Infant Hand Preference and the Development of Cognitive Abilities These “role-differentiated bimanual” tasks are particularly revealing: they demand more sophisticated motor planning than simply picking something up, and they tend to pull out a child’s true hand preference more reliably than single-handed reaching does.11Developmental Science. Unimanual and bimanual tasks and the assessment of handedness in toddlers
That said, handedness continues to sharpen throughout childhood. Brain imaging during adolescence reveals ongoing maturation in the corticospinal tract, the bundle of nerve fibers connecting the motor cortex to the spinal cord. This maturation shows a left-greater-than-right asymmetry that is, on average, less pronounced in left-handed individuals, and it differs between sexes, likely influenced by hormonal changes during puberty.12PubMed Central. Handedness, motor skills and maturation of the corticospinal tract in the adolescent brain So even into the teenage years, the brain’s wiring for handedness is still being fine-tuned.
What Determines Which Hand a Child Will Favor
Genetics plays a real but modest role. A genome-wide study with over 1.7 million participants identified 41 genetic regions associated with left-handedness and another 7 associated with ambidexterity, confirming that handedness is highly polygenic, meaning it is shaped by many genes, each with a small effect, rather than a single “handedness gene.”13PubMed. Genome-wide association study identifies 48 common genetic variants associated with handedness Some of those same genetic variants are also linked to structural asymmetries in the cerebral cortex, which fits with the idea that the genes influencing handedness do so partly by shaping how the brain’s left and right hemispheres develop differently.14PubMed Central. Handedness and its genetic influences are associated with structural asymmetries of the cerebral cortex in 31,864 individuals
But genes are not destiny here. Two right-handed parents can have a left-handed child, and identical twins, who share all their DNA, do not always share the same handedness. Environmental factors fill in the gaps. Birth stress is one well-documented influence: a study of nearly 1,400 subjects confirmed an association between birth complications and left-handedness.15PubMed. Family patterns in handedness: evidence for indirect inheritance mediated by birth stress Another found that infants who required resuscitation after delivery were about twice as likely to show a left-hand preference at age five.16American Journal of Epidemiology. Infant Resuscitation Is Associated with an Increased Risk of Left-handedness Cultural pressure matters too. A comparison of French and Tunisian children found lower rates of left-handedness among Tunisian children, particularly at age five, suggesting that social expectations can nudge children whose handedness might otherwise go either way toward using the right hand.17PubMed. Cultural influences on the development of lateral preferences: a comparison between French and Tunisian children
How to Tell Which Hand Your Child Prefers
Parents often want a definitive answer sooner than the biology is ready to give one. Handedness questionnaires are the most common tool used in research, but they rely on self-report or parent report, which can be unreliable for young children. Watching which hand a child reaches with during play has proven to be a more effective way to track hand preference as it emerges.18PubMed Central. Hand preference, performance abilities, and hand selection in children
For preschoolers specifically, researchers have developed tools like the Home Handedness Questionnaire, which asks parents to observe their child during everyday activities at home. It distinguishes between one-handed tasks (like holding a toothbrush) and two-handed tasks where the hands play different roles (like unscrewing a lid). In a proof-of-concept study with three-year-olds, the questionnaire was able to detect right preference, left preference, and no preference on both types of tasks.19PubMed Central. The home handedness questionnaire: pilot data from preschoolers Two-handed tasks with distinct roles for each hand tend to reveal preference more clearly than simple one-handed reaching, so if you are trying to get a read on your toddler, watching them open containers or pour from a cup may tell you more than watching them grab a block.
The practical advice: if your child is younger than two and seems to switch hands constantly, there is no cause for concern. If they are between two and four and still do not have a clear preference, that is also within the range of normal, though it becomes worth mentioning to a pediatrician if it persists well past school entry, because very late or absent establishment of hand preference can sometimes accompany developmental differences.
Handedness and Developmental Conditions
Atypical handedness, which includes left-handedness and mixed-handedness, shows up at higher rates in several neurodevelopmental conditions. Children with autism spectrum disorder are more likely to be non-right-handed compared to children with intellectual disability alone.20PubMed Central. Handedness in autism spectrum disorders and intellectually disabled children and adolescents – Contrasting caregivers’ reports with assessments of hand preference Reduced hemispheric brain asymmetry, and the atypical handedness that goes with it, has also been linked to conditions like schizophrenia and stuttering.21PubMed Central. Hand Preference in Stuttering: Meta-Analyses
This does not mean that left-handedness is a warning sign. The vast majority of left-handed children are developing typically. The connection is statistical, not diagnostic: across large groups, certain conditions skew the distribution of handedness slightly compared to the general population. What is more clinically relevant is the pattern of handedness development. A child who establishes clear hand preference within the normal timeframe and then loses it, or a child who shows very strong early hand dominance before six months (when most babies still switch freely), could warrant evaluation, because those patterns sometimes reflect neurological issues like hemiplegia rather than the normal unfolding of laterality.
Foot Preference and Other Lateral Biases
Handedness is the most studied aspect of lateral preference, but it is not the only one. Children also develop preferences for which foot kicks a ball, which eye they peer through a keyhole with, and which ear they hold a phone to. These preferences do not always line up. In a study of three- to five-year-olds, about 39 percent were mixed-footed compared to only 17 percent who lacked a clear hand preference, meaning foot preference is considerably less consistent than hand preference at that age. The most common combination was right-handed and right-footed, accounting for about 52 percent of children, while another 23 percent were right-handed but mixed-footed.22ScienceDirect (Cortex). Associations Between Hand and Foot Preference in 3- to 5-Year-Olds Right-handers were more consistently right-footed than left-handers were left-footed, a pattern that holds across age groups.
The birth-stress studies tell a similar cross-lateral story. The same research that found resuscitation after delivery was linked to left-handedness also found it was linked to left-footedness, suggesting that whatever mechanism shifts hand preference can shift foot preference too.16American Journal of Epidemiology. Infant Resuscitation Is Associated with an Increased Risk of Left-handedness
Why Humans Are So Lopsided Compared to Other Primates
About 90 percent of humans are right-handed, a ratio that has remained roughly stable for thousands of years. Analysis of “negative hand” stencils in European cave paintings from the Upper Paleolithic (roughly 10,000 to 35,000 years ago) found a proportion of left-handers that was statistically indistinguishable from that of modern French university students performing the same task.23PubMed Central. Handedness frequency over more than ten thousand years Archaeological evidence of right-side biases extends back even further, with clear group-level right-handedness established from the Neanderthals onward and patchy evidence suggesting a right-handed majority in even older hominin populations.24PubMed. The prehistory of handedness: archaeological data and comparative ethology
This kind of overwhelming population-level bias is not found in other primates. Individual chimpanzees, gorillas, and monkeys do develop hand preferences, sometimes quite strong ones. In a meta-analysis of bimanual tasks, about 82 to 90 percent of non-human primates tested showed an individual preference for one hand or the other.25PubMed. Hand preferences in coordinated bimanual tasks in non-human primates: A systematic review and meta-analysis But at the population level, those individual preferences split roughly evenly between left and right, producing no species-wide bias. A comprehensive analysis across anthropoid primates confirmed that human right-handedness represents an unparalleled extreme and that human lateralization patterns do not align with trends seen in other primates, pointing to unique selective pressures in our evolutionary history.26eLife. The evolution and biological correlates of hand preferences in anthropoid primates The leading candidates for those selective pressures involve the co-evolution of handedness with tool use, language, and the dramatic hemispheric specialization that characterizes the human brain, though pinning down exactly which came first remains an open question.