Genes play a role in left-handedness
Have you ever wondered why some people are left-handed while most people are right-handed?
The science behind this simple difference is more complicated than you might think!
About 10.6 percent of people are left-handed (Papadatou-Pastou and co-workers, 2020). It has been known for a long time that left-handedness runs in families. Two left-handed parents have a higher chance of having a left-handed child than two right-handed parents. Therefore, genes likely play a role in determining whether someone is born left-handed or right-handed. For a long time, scientists believed that there was just one handedness gene, but recent research has proven that this idea is wrong. Instead, we now know that more than 40 genes play a role in determining whether someone is left-handed or right-handed (Cuellar-Partida and co-workers, 2021).
A new model of left-handedness genetics
A major problem in research on the genetics of left-handedness is that many of the old theoretical models that used to explain left-handedness no longer fit the research data from modern genetic studies, mostly because they assumed that only one gene plays a role in left-handedness. In contrast, a genetic model of left-handedness needs to consider that it is polygenic; many genes play a role (Ocklenburg and co-workers, 2025). This is, by the way, the case for pretty much every psychological variable that is investigated in genetic psychology; only very few things are determined by single genes!
An important recent development in left-handedness research is the publication of the liability-threshold polygenic model of left-handedness by scientist Silvia Paracchini, a genetics researcher from the School of Medicine at the University of St Andrews in Scotland (Paracchini, 2024; Ocklenburg and co-workers, 2025, for an overview).
As pointed out above, more than 40 genes play a role in left-handedness. Functionally, these genes mostly play a role in the early development of various aspects of the brain, such as the growth and guidance of axons (projections of nerve cells that transport neural information) or the cytoskeleton of brain cells. For each of these genes, a person can have different variants, that is, a version of the gene that shows minimal differences from the same gene in other people. For each gene, there is a specific variant that makes left-handedness more likely and other variants that make right-handedness more likely. The liability-threshold polygenic model of left-handedness now assumes that it is not one specific gene that determines handedness. Instead, it assumes that someone becomes left-handed if they accumulate many of the variants that favor left-handedness. If a person has enough of these left-handedness variants in their genome, they pass a developmental threshold that puts their brain on a developmental trajectory towards left-handedness. If a person has only a few or none of the left-handedness variants, they become right-handed. Therefore, it does not matter at all which specific left-handedness variants somebody has, but how many. This also implies that two left-handers can have completely different gene variants causing their left-handedness, which obviously makes research on the genetics of left-handedness very complicated!
Takeaway: A major step towards understanding the genetics of left-handedness
The liability-threshold polygenic model of left-handedness by Paracchini solves a major problem of older models of handedness genetics by integrating the findings that handedness is determined by many genes! Now, handedness researchers need to put it to the test in experimental studies.