leftists 2 million years
The earliest evidence of right-handedness among humans traces back to roughly 1.8 million years ago, a finding attributed to researchers at the University of Kansas. The clue comes from the fossil record of our ancient ancestors, especially the wear patterns on teeth that hint at skilled, deliberate use of one hand. While it is possible that left-handedness emerged around that period, the scarcity of remains from ancient left-handed individuals makes a definitive conclusion elusive.
Today, estimates place left-handed individuals at about 15 percent of the global population. In contrast, among other primates such as bonobos and chimpanzees, the distribution between left and right-handed behavior tends to be roughly balanced. Early humans did not show a clear, fixed handedness at the dawn of the species. After the development of spoken language, a noticeable shift toward handedness aligned with brain lateralization became more pronounced, with the left hemisphere taking charge of language tasks and the hands following this organization in a complementary pattern.
Researchers from the University of Kansas explain that lateralization in the brain distributes functions across the two hemispheres. The left side, central to language, influences the right side of the body, while the right hemisphere engages the left side. This arrangement helps explain why speech centers are predominantly in the left hemisphere for most right-handed people, and why left-handed individuals exhibit a different, albeit consistent, brain organization. A clinician from Sechenov University notes that about 90 to 95 percent of right-handed people have their language centers in the left hemisphere, while this pattern appears in roughly two-thirds of left-handed individuals.
The origins of handedness remain largely a mystery. Many researchers think it arises very early, possibly during embryonic development. Some point to genetics, others to hormonal influences or environmental factors, and still others to the complex interplay between these drivers.
embryonic theory
Italian researchers have proposed that by the eighteenth week of fetal life, the hand a fetus favors can be detected with high accuracy using ultrasound. In a study of more than twenty fetuses observed at 14, 18, and 22 weeks, scientists tracked subtle hand movements such as moving toward the eyes or mouth. Nine years later, they followed the children to learn which became left- or right-handed. They found that the hand used most actively and precisely on ultrasound tended to be the dominant one later on.
This work supported the idea that hand dominance is an inborn trait. Further investigation at Ruhr University suggested that conditions for right- or left-handedness begin during embryonic spinal cord development, even before direct connections to the motor cortex form. Observations show that infants begin sucking one thumb around the thirteenth week, and by the fifteenth week the motor cortex-spinal cord linkage develops, indicating spinal activity may influence later hand preference. Nevertheless, this theory has not been fully confirmed, argues Nasonova. She cautions that large, long-term studies with continuous monitoring would be required to distinguish genuine patterns from chance observations. She notes that even repeated checks over several days cannot guarantee a future preference, as a child might switch which finger is used most during a given period.
“There are not enough extensive studies to decisively confirm or refute this theory,” the neurologist adds. The discussion remains open, with researchers acknowledging the complexity of early brain and spinal cord interactions in shaping handedness.
high testosterone
Another line of inquiry from American neurologists links prenatal exposure to high levels of testosterone with a greater likelihood of left-handedness. In this view, testosterone shapes the prenatal growth pace of the brain’s hemispheres and may create subtle differences in structure between sexes, with left-handedness appearing more frequently among men. The hypothesis posits that excessive prenatal testosterone could slow growth in the left hemisphere, giving the right hemisphere an edge in early development and leading to left-handed dominance in some offspring.
Environmental and lifestyle factors can influence hormone exposure. Factors such as body weight, chronic stress, heavy physical labor, insufficient nutrition, smoking, and sun exposure are considered to have some impact on hormone levels during development, potentially affecting handedness patterns later in life.
Blaming genetics?
Genetic studies carried out at prestigious centers, including Oxford University, have identified several gene regions linked to left-handedness. Researchers found that three of the four implicated regions involve proteins important for brain development and the organization of neural networks. These genetic variations also correspond with differences in language-related white matter pathways, which may help explain why some left-handed individuals exhibit stronger or more complex language skills.
Statistical patterns lend support to a genetic component: the chance of a left-handed child from two right-handed parents is roughly 2 percent, but this risk rises to about 17 percent if one parent is left-handed and to around 46 percent if both are left-handed. Yet genetics does not tell the whole story, as some left-handedness appears after birth and is influenced by other factors.
Congenital right-handed but left-handed
Left-handedness can sometimes signal atypical development. Neurological specialists describe both nonpathological and pathological forms. In many cases, left-handedness is a natural variation and not a sign of illness. However, some situations involve adaptive changes after brain injury or disease that shift dominance toward the left or right hand. For example, injuries or strokes affecting one hemisphere can lead to compensatory use of the other hand. In such cases, a child or adult may appear left-handed due to changes in brain control, not because handedness is innate from birth.
Research indicates associations between left-handedness and certain neurodevelopmental conditions, including ADHD and autism, though conclusions vary. At the University of Athens, studies note a higher prevalence of left-handedness among individuals with autism and ADHD compared with the general population. Experts emphasize supporting a child’s natural handedness rather than forcing a switch, since retraining can drain energy, hinder school adaptation, and affect comfort at home. Practical steps involve arranging living spaces to accommodate the child’s dominant hand, creating an environment that reduces fatigue and stress, and allowing natural hand preference to guide daily activities.