Left-Handedness and Neurodiversity: A Surprising Link

Across people, brain functions such as social skills, mood, and attention are highly diverse. For example, some of us are very social and enjoy spending time with others 24/7. Others are less social and prefer spending most of their time by themselves or with a few close family members or friends.

Similarly, some people are almost always in a good mood while others show more negative emotions. Or think of attention: Some people can concentrate for hours on very complicated tasks while others tend to lose attention after working on something for only a few minutes.

The concept of neurodiversity was coined to reframe certain forms of diversity in brain functions in a non-pathological way. For example, autism is classified as a neurodevelopmental disorder, a term that implies that it is an inherently pathological condition.

Supporters of the neurodiversity movement, by contrast, argue that the autism spectrum is part of the diversity of brain functions in the general population. Therefore, according to them, people on the autism spectrum should not be treated as "sick" and do not need to be “cured” but instead should be accepted and supported by society as they are.

One commonly observed form of diversity in brain function is handedness. While about 90 percent of people are right-handed, about 10 percent are left-handed. Recently, I was asked by a reader whether there is a connection between left-handedness and neurodiversity. Let's have a look at the scientific literature on this interesting question!

The Autism Spectrum and Left-Handedness

The neurodiversity movement was created by people on the autism spectrum. Therefore, let us first have a look at the data on left-handedness and the autism spectrum.

Since so many scientific studies on this topic have been published over the years, it was possible to conduct a so-called meta-analysis on handedness in individuals on the autism spectrum (Markou et al., 2017). A meta-analysis is a form of statistical analysis in which the data from several published studies are combined. This way, methodological problems that can occur in a single study do not affect the data as much and the results are more robust. In short, meta-analyses provide very trustworthy results.

In the meta-analysis on handedness and the autism spectrum, the scientists analyzed data from 723 people that were on the autism spectrum and 476 people that were not (Markou et al., 2017). Three different analyses were calculated, one for left-handedness, one for mixed-handedness, and one for left-handedness and mixed-handedness combined.

The researchers found that individuals on the autism spectrum were 2.49 times more likely to be left-handed than people without autism. Altogether, about 28 percent of individuals on the autism spectrum were left-handed as compared to about 10 percent in the general population. Moreover, individuals on the autism spectrum were 2.34 times more likely to be mixed-handed than the general population and 3.48 times more likely to be either left-handed or mixed-handed than the general population.

So yes, people on the autism spectrum do appear to have a higher chance to be left-handed than the general population.

ADHD and Left-Handedness

The autism spectrum is not the only neurodevelopmental condition that is represented within the neurodiversity movement. Another prominent condition within the movement is attention deficit hyperactivity disorder (ADHD).

As for the autism spectrum, a meta-analysis on handedness has been performed for people with ADHD (Nastou et al., 2022). Here the results were less clear-cut.

For left-handedness and mixed-handedness, there were no statistically significant results, but strong trends toward higher percentages in ADHD. However, when both were combined it was shown that people with ADHD had a statistically significant increase in left-handedness and mixed-handedness combined compared to the general population.

Overall, individuals with ADHD had a 27.3 percent chance of being either left-handed or mixed-handed compared to 18.1 percent in the general population. So the results suggest that the effects are smaller than for the autism spectrum, but generally go in the same direction.

Why Are Left-Handedness and Mixed-Handedness More Common in Neurodiverse Than Neurotypical People?

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The question of why left-handedness and mixed-handedness are more common in neurodiverse than in neurotypical individuals is not fully answered, yet, but there are some clues.

One important fact to consider is that left-handedness has not much to do with the hands. Typically, it is impossible to distinguish left-handers and right-handers by just looking at their hands.

Instead, handedness is determined by the brain. For left-handers, the right motor cortex is dominant for complicated motor tasks like writing. For right-handers, the left motor cortex is dominant for fine motor tasks. Since handedness is determined by the brain, genes that are relevant for whether someone is becoming a left-hander or a right-hander often are relevant for brain development, and some relevant genetic pathways have also been shown to be involved in neurodevelopmental conditions.