1. How Many People in a Class Are Left-Handed?
Imagine a class of 30 students. How many might be left-handed? A major review of 200 studies involving about 2.4 million people found that the best estimate for left-handedness is 10.6%. If you apply this to a class of 30, that is about 3 students [1].
However, the number changes depending on how "left-handed" is defined. Using the strictest definition, the rate is 9.3%. Using the loosest definition (everyone who is not right-handed), it is 18.1%. Age, how it is measured, gender, and ancestry also change the numbers [1]. Left-handed people are a minority, but they are common in every class. This raises a question: is this difference passed down from parents?
2. How Much of Handedness Comes From Parents' Genes?
A large study in Australia and other places looked at 25,732 families with twins and siblings, totaling 54,270 people. It found that about one-quarter (23.64%) of the variation in handedness can be explained by genetics. The rest is due to environmental factors that are not shared by siblings [2].
It is important to understand what this 24% means. It does not mean that 24% of one person’s handedness is genetic and 76% is environmental. Instead, it describes the variation across the whole group [2]. The study also found no special effects unique to twins, such as hormone exchanges in the womb or mirror-image relationships. The rate of left-handedness was the same for twins as for non-twin siblings [2]. So, genetics explains about a quarter of the difference. But when scientists looked at the DNA itself, they found many involved areas.
3. Why There Is No Single Left-Handed Gene: 48 Genetic Areas
By combining data from the UK Biobank, 23andMe, and an international consortium, scientists analyzed the genomes of about 1.77 million people. They found 41 genetic areas linked to left-handedness and 7 linked to being ambidextrous (using both hands equally), for a total of 48 areas [3].
Many of these genes work in the central nervous system (the brain and spinal cord). Each area has a small effect. This suggests that the idea of one single "left-handed gene" is incorrect [3]. Think of it like a tug-of-war. One person pulling is weak, but many people pulling slightly can change the direction of the rope. Handedness is thought to move because of the combined effect of many small influences. However, the real mechanism is more complex than a simple tug-of-war. So, what do these genes actually do in the body?
4. How Genes Influence the Brain and Body Structure Linked to Handedness
An analysis of about 400,000 people in the UK Biobank (38,322 left-handed and 356,567 right-handed) found four related areas. Three of these were near genes involved in brain development and shape, such as MAP2 and MAPT. The study also found that left-handed people have stronger connections between the left and right networks in the brain that handle language [4].
Another interesting example involves the gene PCSK6. This gene is known to help set up the left-right axis of the body. In a study of 1,113 adults, variations in this gene were linked not to whether someone was right- or left-handed, but to how strongly they favored one hand over the other. This finding has not yet been widely repeated in other groups [5]. Genes may not directly decide "left or right." Instead, they may slightly shift the bias by influencing how the brain and body are built.
5. Why Arm Movement Differences Appear Before Birth
Unborn babies’ arm movements can be seen with ultrasound. In records of 72 fetuses at 10 weeks, 85% moved their right arm more than their left arm. This is close to the adult rate of right-handedness (about 89%) [6]. Note: This figure comes from a commentary article, not the original research paper.
Another study looked at 29 babies at 14, 18, and 22 weeks of pregnancy and compared this to their handedness at age 9. The speed of arm movements after 18 weeks could predict handedness with 89–100% accuracy. However, this was a small study of only 29 people, and it showed prediction, not certainty [7].
Furthermore, a study of the spinal cords of 5 fetuses at 8–12 weeks found that gene activity differed between the left and right sides depending on the stage of development. This was most noticeable in areas that move arms and legs. Because arm movement was biased before the brain connected to the spinal cord, scientists suggest that spinal differences might be the molecular starting point for handedness. With only 5 results, we cannot yet say this is the cause [8]. Genetics explains about a quarter of the variation, while the rest is linked to non-shared environments or chance. Whether fetal bias is part of that is still unknown.
6. How Can You Test Hand Use Yourself?
If you want to investigate, try this today. Stack erasers or button a shirt using your left and right hands for one minute each. Count how many you can do. Do not look for which hand is "better." Instead, notice how ease of movement changes between hands.
To compare with family, ask who uses which hand for throwing a ball, cutting with scissors, or brushing teeth. Make a table. You may find that some people prefer one side, while others switch depending on the task. You can also look for books about brain and body development at your library.