Left-Handedness Is Influenced by Early Life Factors

Out of 100 people, about 90 are right-handers and 10 are left-handers. If you prefer the left hand for activities such as writing and drawing, you might have wondered what made you become left-handed. The answer might be more complicated than you would have imagined. Initially, scientists thought that left-handedness might be caused mostly by genetic influences, as it runs in families and is to some extent heritable. However, in 2009, a large scale twin study that analyzed handedness in twins and their families in more than 25,000 Australian and Dutch families (Medland et al., 2009) had a surprising result. Only about 25% of the individual variance in handedness can be explained by genes, while 75% are determined by environmental influences.

New research by Carolien G. F. de Kovel and colleagues from the Max Planck Institute for Psycholinguistics in Nijmegen, The Netherlands, now offers insight into which non-genetic factors influence left-handedness. The new study, published in the scientific journal Scientific Reports (de Kovel et al., 2019), used data from the UK biobank study, a large dataset with about 500,000 participants, to understand environmental factors that influence left-handedness. Left-handedness is usually determined in the first few years of life, and there is even evidence that it may already be present before birth, as hand movement preference can be observed in ultra-sound recordings of unborn children (Hepper, 2013). Thus, de Kovel and colleagues expected that events happening early (rather than later) in life would have an effect on left-handedness.

In their cohort of 501,730 individuals, 87.4% of participants were right-handed, 10.4% were left-handed, and 2.1% had no preference. The authors found that the probability of being left-handed was affected by several factors: First, year and location of birth affected handedness. For year of birth, there was an increase of left-handedness up to 1970. This likely reflects a decrease in handedness re-learning, a practice in which left-handers were forced to switch to the right hand when writing in schools that had been common in schools in the UK. Handedness re-learning reduced the number of individuals who would have been characterized as left-handers in standardized handedness tests. For location of birth, people born in England were found to have a 10.1% chance of being left-handed, while people born outside the UK only had a 6.8% chance. This likely reflects that some societies have social taboos that may include using the left hand for writing or eating, e.g., because the use of toilet paper is uncommon and left hand is used for personal hygiene.

Moreover, the probability of being left-handed was influenced by birthweight and being part of a multiple births. Having low birthweight and being part of a multiple births both increased the chance of being left-handed. In addition, not being breastfed also increased the chance of being left-handed (see my previous blog post on this topic).

Moreover, the season of birth was shown to affect left-handedness in women, with women born in summer having the highest chance of being left-handed. The reasons for this finding are not clear, but previous studies have discussed seasonally fluctuating factors such as the chance of catching virus infections or hormonal fluctuations over the year to potentially play a role. However, more research is need before any conclusions can be drawn.

The last factor that influenced left-handedness in the study was the participants’ sex. Being male was associated with a higher chance of being left-handed. This finding is in line with a meta-analysis on sex and left-handedness that included 144 studies and more than 1.7 million individuals (Papadatou-Pastou et al., 2008). Here, males showed an increase of 12% in left-handedness compared to females. In other words, if 10 out of 100 women are left-handed, about 11 out of 100 men are. This effect is assumed to be due to hormonal influences, e.g., testosterone, or different maturational trajectories.

One important outcome of the study was also that all the described effects were statistically significant, but not very big. Therefore, it is very likely that there are other, yet unknown, factors that influence left-handedness – the scientific quest to uncover the roots of left-handedness remains exciting!