Handedness describes an individual’s preference for using one hand over the other for tasks like writing or throwing. The dominant hand is stronger, faster, or more dexterous, while the non-dominant hand is less so. Most people are right-handed, but about 10-15% of the global population is left-handed. A smaller percentage exhibits mixed-handedness or ambidexterity, preferring different hands for different tasks or performing equally well with both. This characteristic, usually clear in early childhood, has long been explored to understand its origins.
The Genetic Clues
Evidence suggesting a genetic influence on handedness comes from observations within families and studies involving twins. Handedness often runs in families; children of left-handed parents, for instance, have an increased chance of being left-handed themselves.
Twin studies further support this idea, showing that identical (monozygotic) twins are more likely to share the same handedness than non-identical (dizygotic) twins or other siblings. Even so, about 19.3% of identical twin pairs can be discordant for handedness. This indicates that while genetics play a role, handedness is not determined by a single gene with simple inheritance.
Researchers have identified several chromosomal regions and specific genes potentially linked to handedness, such as the LRRTM1 gene on chromosome 2p12 and the androgen receptor (AR) gene on the X chromosome. The LRRTM1 gene has been associated with both relative hand skill and brain asymmetry, and its influence is thought to modify brain development. Although these genes have been implicated, no single gene is considered the sole determinant. Handedness is a complex trait influenced by multiple genes, possibly up to 40, each contributing a small effect. These genes contribute to the body’s right-left asymmetry during early development.
Beyond Genes: Environmental and Developmental Factors
Beyond genetic predispositions, various non-genetic influences contribute to the development of handedness. Prenatal developmental factors, such as in-utero positioning, might reinforce an early hand preference.
Hormonal influences during prenatal development, such as exposure to testosterone, have also been investigated. Some research suggests high levels of prenatal testosterone in females are associated with weaker lateralization of hand skill. However, the exact influence of hormones is still being clarified, as results are not entirely consistent across studies.
Social and cultural factors can also shape handedness. In some societies, pressure to conform to right-handedness has influenced the prevalence of left-handedness, leading some individuals to develop proficiency with their right hand. Early motor experiences and consistent use of one hand can also contribute to the development of handedness, as greater practice with a preferred hand leads to enhanced skill performance.
The Interplay of Nature and Nurture
Handedness is a multifactorial trait, arising from a complex interplay between genetic predispositions and environmental or developmental factors. While genes provide a foundation, they do not solely dictate an individual’s hand preference. The heritability of handedness, accounting for approximately 24% of the variation, suggests that environmental factors contribute significantly to the remaining differences.
This interaction means that even with a genetic inclination towards a certain handedness, factors like prenatal conditions, hormonal exposure, and societal influences can modify its expression. While a single “handedness gene” has not been identified, scientific consensus points to handedness as a trait where various influences combine to determine an individual’s dominant hand.