If there is one thing that is for certain about left-handers, it is that they are much rarer than right-handers. But how much of the world’s population is actually left-handed? Scientific studies have given widely varying answers to this question, ranging from less than 3% to more than 15%. This wide variation is likely due to two factors: cultural effects and small study samples.
Most scientific studies are only conducted in people from one country. But it has been shown that the country in which the data is collected can have a huge impact on handedness measures. For example, a recent study found that people born in England had a 10.1% chance of being left-handed, while people born outside the U.K. only had a 6.8% chance (de Kovel et al., 2019). This likely reflects that some societies have social taboos that may include using the left hand for writing or eating, perhaps (for example) because the left hand is used for personal hygiene. In these countries, lower rates of left-handedness are likely to be observed. Thus, a study on how many people are left-handed should ideally include datasets from all over the world. The other problem is that many studies only test small samples, less than a hundred people. In these small studies, chance could lead to more or fewer left-handers being included than in the whole population, making the results not very reliable.
A team of researchers led by Greek scientist Marietta Papadatou-Pastou from the National and Kapodistrian University of Athens set out to solve this problem by conducting the biggest ever global study on left-handedness, which has just been published in the journal Psychological Bulletin (Papadatou-Pastou et al., 2020).
The research team (which I have been a part of) from Athens, the University of Oxford, the University of Bristol, Ruhr University Bochum in Germany and the University of St Andrews in Scotland conducted a meta-analysis of 262 datasets from 200 different published studies on left-handedness. A meta-analysis is a form of statistical analysis that integrates the results of many different scientific studies. It has the advantage of a larger sample size, increasing statistical power and rendering the analysis less likely to be affected by characteristics of individual studies.
Overall, more than 2.3 million individuals were included in the study, making this the largest published study on left-handedness so far. They found that across studies, the prevalence of left-handedness lies between 9.3% and 18.1%, with the best overall estimate being 10.6%. So given these new results, how many people in the world are left-handed? As of March 2020, the world’s overall population is estimated to be somewhere around 7.8 billion people, and 10.6% of that would be almost 827 million people.
Getting an accurate understanding of how many people are left-handed is important for many fields in psychology and related sciences. For example, it can help in understanding human evolution. While many species show left-handedness (see my blog post on cats here and on dogs here), only humans seem to show the roughly 90:10 distribution overserved in the study. As handedness has been linked to both the capacity to make and use tools and to produce and understand language, it is an interesting feature in the context of the divergent evolution of humans from apes.
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de Kovel CGF, Carrión-Castillo A, Francks C. (2019). A large-scale population study of early life factors influencing left-handedness. Sci Rep, 9, 584.
Papadatou-Pastou M, Ntolka E, Schmitz J, Martin M, Munafò MR, Ocklenburg S, Paracchini S. (2020). Human handedness: A meta-analysis. Psychol Bull, in press.