Why are some people left-handed?

No single cause explains left-handedness. Researchers think it arises from a mix of many small genetic influences and random variation in how the body and nervous system develop before birth. In Western countries, about 10 to 15 percent of people are left-handed, according to MedlinePlus Genetics, and the biological basis of hand preference is still poorly understood. [1]

Researchers in the 1970s talked about a single left-handedness gene, but Quanta Magazine reports that none exists. Large studies have instead found variants in about 40 genes that each slightly raise the chance of left-handedness, and the more of these variants a person carries, the more likely they are to be left-handed. [2] [1]

Many of these are tubulin genes, which build microtubules, the filaments that give cells structure and carry cargo inside them. A 2025 review in Trends in Genetics notes that tubulin genes take part in brain development processes such as axon growth and guidance. Some of the same genes are linked to conditions including schizophrenia, dyslexia and autism, which are more common among left-handed or mixed-handed people. [3] [2]

Heritability is modest. The Trends in Genetics review gives the most reliable twin-study estimate as around 25 percent. Consistent with this, Quanta reports that two left-handed parents have a left-handed child only 25 to 30 percent of the time, and if one identical twin is left-handed, the other is too only 20 to 30 percent of the time. [3] [2]

Ultrasound studies indicate that dominant hand is set before the brain is connected to the limbs. From about 10 weeks after conception, a fetus moves one arm more than the other, and that arm predicts later handedness with high accuracy. A 2017 eLife study found large left-right differences in gene expression in the spinal cord of fetuses aged 8 to 12 weeks, which could wire up more movement on one side. The brain becomes involved only afterward. [2]

This does not explain why the right side wins about nine times in ten. One proposal is that microtubule-related processes in spinal progenitor cells lean rightward, and that gene variants weaken this bias or let chance tip it the other way. This is a hypothesis rather than an established mechanism. [2]

Sources differ in emphasis on non-genetic factors. MedlinePlus and BBC Science Focus mention the prenatal environment, hormone levels during pregnancy, early experience with holding and throwing, and culture, and note that injury to the right hand can change hand use. [1] [4]

The Trends in Genetics review argues that measurable environmental factors play only a minor part. A UK Biobank study found that sex, breastfeeding, season of birth, multiple birth, birthweight and place and year of birth were each associated with handedness, but together they had minimal predictive value. A meta-analysis found breastfeeding linked to a 9 to 22 percent lower prevalence of not being right-handed. The review suggests most remaining variation reflects noise or individual developmental variation that is hard to detect. [3]