Handedness, or lateralization, refers to the consistent preference and superior performance of one hand over the other for tasks requiring fine motor skills. This fundamental human asymmetry is a complex trait, and the answer to whether it is strictly genetic involves a nuanced mix of hereditary and non-hereditary factors.
The Role of Genes in Handedness
Handedness is a polygenic trait, meaning it is influenced by the cumulative effect of multiple genes working together, rather than a single gene that dictates a left or right preference. The inheritance pattern is non-Mendelian and probabilistic, where a person inherits a genetic predisposition for laterality, but not a certainty of one hand over the other. Heritability studies, particularly those involving twins, estimate that additive genetic factors account for approximately 24% to 25% of the variation in handedness across a population.
Evidence from twin studies shows that while identical twins share 100% of their genes, they are not always concordant for handedness; between 10% and 25% of identical twin pairs can be discordant, where one is right-handed and the other is left-handed. Family studies also demonstrate a clear, but limited, genetic influence. If both parents are right-handed, the chance of their child being left-handed is relatively low, but this likelihood jumps to about 26% if both parents are left-handed.
Beyond Genetics Developmental and Environmental Factors
Factors outside of inherited DNA account for the majority of handedness determination, with many of these influences occurring during prenatal development. One long-standing theory proposed that prenatal exposure to hormones, such as elevated levels of testosterone, might influence the direction of lateralization. Modern research suggests that sex hormones may not determine the direction of handedness, but rather the strength or degree of hand skill asymmetry. For example, high levels of prenatal testosterone in females have been associated with a weaker lateralization of hand skill later in life.
Other non-genetic prenatal factors, such as birth weight, multiple births, and maternal anxiety, have also been identified as potential influences on laterality. Postnatal factors, while less significant in determining the initial preference, have historically played a role in the expression of handedness. In many cultures, left-handed children were subjected to educational or social pressure, which sometimes resulted in a forced switch to using their right hand for tasks like writing.
The Brain and Spinal Cord Connection
The physical mechanism of handedness is rooted in the functional asymmetry of the central nervous system, particularly the way the brain’s hemispheres are wired. The left cerebral hemisphere typically controls the right side of the body, and it is dominant for language in the vast majority of right-handers and even in most left-handers. The right hemisphere controls the left side of the body, and it is associated with a different set of cognitive functions.
The preference for a hand, however, appears to be established much earlier than the maturation of the brain’s motor cortex. Ultrasound studies show that fetuses begin to display a clear hand preference, such as thumb-sucking, as early as the 8th to 12th week of pregnancy. At this stage, the motor cortex in the brain is not yet functionally connected to the spinal cord. This finding suggests that the initial preference is driven by asymmetric gene expression and activity within the spinal cord itself, in the segments that control arm and hand movements. The spinal cord’s inherent left-right asymmetry, which develops before the brain fully takes control, is now considered a primary starting point for the eventual establishment of handedness.
Global Prevalence and Cultural History
Globally, the prevalence of left-handedness has been remarkably consistent throughout human history, with approximately 10% of the world’s population exhibiting a left-hand preference. Archaeological evidence suggests this strong bias toward right-handedness has been present for hundreds of thousands of years.
Despite its stability, left-handedness has historically been viewed through a lens of suspicion and negative bias across many cultures. The linguistic bias is evident in the Latin word for left, sinister, which became synonymous with malevolence and ill-omen. This deep-seated cultural stigma often led to attempts to correct or suppress left-handedness in schools and social settings. The steady rise in the recorded rate of left-handedness over the last century in Western societies is likely a reflection of increased cultural tolerance, allowing more individuals to express their natural preference rather than a change in the underlying biology.