Left-Handedness and Hormones: Is There a Link?

Past research on left-handedness has indicated that it is more common in men than in women (Papadatou-Pastou et al., 2008). Similarly, a recent large-scale study found that in the U.K., 8.6% of women and 10.6% of men were left-handed (de Kovel et al., 2019).

This, of course, leaves open the question of why male left-handers are more common than female left-handers. One idea that many scientists have discussed is that prenatal sex hormones may play a role. For example, higher levels of prenatal testosterone might lead to an increased chance of left-handedness. There is, however, one major problem with testing this idea. For most people, handedness is determined before they are born (my recent post explains the research on this fascinating topic). Thus, in order to assess the impact of prenatal testosterone on left-handedness, scientists would actually need to measure testosterone levels before birth and relate them to left-handedness much later in life.

This is exactly what psychologist Gareth Richards from Newcastle University in the U.K. and his team did in a recently published research paper (Richards et al., 2021a). In the study, the scientists measured levels of the male sex hormone testosterone and the female sex hormone estradiol in the amniotic fluid during the second trimester of pregnancy. The amniotic fluid is the liquid that surrounds the growing baby in the womb and naturally contains hormones. Hormone levels were determined for 30 boys and 30 girls born in the year 2000. The researchers then assessed the handedness of the participants when they were 15 years old. They assessed hand preference with a questionnaire and hand skill with a peg-moving task.

The result showed that natural hormone exposure before birth was not associated with left-handedness in either males or females. However, in females, high testosterone levels were associated with a weaker asymmetry in the hand skill test at age 15 and high levels of estradiol were associated with a weaker hand preference. This suggests that sex hormones do not affect the direction of handedness (whether we are left-handed or right-handed), but in women might affect handedness strength (whether we tend to use one hand in all situations or are flexible). Interestingly, in males, higher levels of prenatal testosterone were associated with slower hand speed (for both the right and left hands) at age 15, an effect that has never previously been reported.

The scientists pointed out that the sample size in this study (60 people) was rather small and might not allow for a firm conclusion on the topic. Therefore, they conducted a second study in a larger sample, using an entirely different approach.

In this study (Richards et al., 2021b), the researchers conducted a meta-analysis on left-handedness and the 2D:4D digit ratio. 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.

What is the 2D:4D ratio, you might ask? The 2D:4D ratio refers to the ratio of length between the second and fourth fingers of a hand. It is important in research on the relationship between hormones and left-handedness as it has been suggested to indicate individual differences in exposure to prenatal sex hormones. Typically, males (who have higher testosterone exposure before birth) have a lower 2D:4D ratio than females, since their ring fingers are on average relatively longer.

In the meta-analysis, Richards et al. (2021b) analyzed data from 60 studies, including data from more than 200,000 participants. The results were somewhat puzzling. For the right hand, a male-typical low 2D:4D ratio was associated with left-handedness, which is in line with the idea that males are more likely to be left-handed because high testosterone levels increase the chances of becoming a left-hander. However, for the left hand, the opposite pattern was found. Here, a female-typical high 2D:4D ratio was associated with left-handedness, which was clearly a conflicting result. When the mean 2D:4D ratio of both hands was used, no association with left-handedness was found.

Based on the results of the two studies, it is somewhat unlikely that the sex difference in left-handedness can be explained by prenatal exposure to testosterone alone. There are many other hormones that could potentially affect left-handedness. Future research should focus on getting a more complete picture of their effects, interactions, and critical influence periods when it comes to left-handedness.