The New Neuroscience of Left-Handedness
August 13 is International Left-Handers Day—here’s what you need to know.
Posted Aug 11, 2019
Since 1976, left-handers all around the globe have been celebrating International Left-Handers Day on August 13. While the reasons for left-handedness still largely remain a scientific mystery, neuroscientists work hard to understand this fascinating phenomenon.
Here are some recent scientific findings about left-handedness you might not have heard of, yet.
1. There are approximately 708 million left-handers in the world.
In a recent statistical integration of 200 studies on left-handedness with an overall sample size of more than 2.3 million individuals, we found that the percentage of left-handedness is about 9.2 percent (Papadatou-Pastou et al., 2019). As there are about 7.7 billion people living in the world in 2019, it can be estimated that there are about 708,400,000 left-handers.
2. Men are more likely to be left-handers than women.
An analysis of 144 studies on left-handedness (total sample size: 1,787,629 individual participants) showed a 2 percent increase of left-handedness in men compared to women (Papadatou-Pastou et al., 2008). The reasons for this are not well understood, but hormonal or societal factors might play a role.
3. Cats and dogs can be left-handers, too.
Did you know that humans are not the only species that shows left-handedness? In a recent meta-analysis (a statistical integration of many different scientific studies) of paw preferences in cats and dogs, we could show that about 36 percent to 46 percent of cats were left-pawed (Ocklenburg et al., 2019). In dogs, 31-53 percent were lefties.
This shows that the development of left-handedness is not something that is typical for humans, but can be observed in other species, too. However, the low percentage of left-handers (about 10 percent) in humans seem to be specific for us, as this rate is typically higher in non-human animals.
4. Left-handers often win at sports, since their actions are more surprising to their opponents.
Did you know that left-handers are often more successful at sports than right-handers? This is especially true for interactive ball sports and interactive combat sports. For example, a Basque study from 2019 investigated the effects of left-handedness on performance in world-class water polo players (Barrenetxea-Garcia et al., 2019). The result? Left-handed male players performed more shots overall, more shots per minute, and most importantly, scored more goals than right-handed players.
It is thought that this effect is due to the fact that opponents expect a throw or a hit with the right hand, and are surprised if someone uses their left. Similar effects have also been shown in boxing and fencing.
5. Genes play a role for left-handedness.
In 2019, scientists from the Max Planck Institute for Psycholinguistics in Nijmegen, the Netherlands, conducted the biggest study on the genetics of left-handedness so far (de Kovel & Francks, 2019). They found that a small number of genes played a role for the development of left-handedness, among them MAP2 (microtubule-associated protein 2), a gene that is crucial for the development of nerve cells in the brain. If this topic interests you, read my blog post on the genetics of left-handedness here.
6. Different early life factors influence left-handedness.
But genes are not the whole story. Another Dutch study from 2019 showed that different early life factors also influence whether we become a left-hander or a right-hander (de Kovel et al., 2019). These factors include the year and location of birth, suggesting that cultural effects might play a role (e.g., societies changed their attitudes towards left-handers).
Moreover, left-handedness was influenced by birthweight, being part of a multiple birth, season of birth, breastfeeding, and sex. If you want to know more about why these factors influence left-handedness, read my blog post on this study here.
7. At 3 years of age, handedness is determined in most, but not all children.
A recent study using the Home Handedness Questionnaire (HHQ) in pre-schoolers (3 years of age) found that most of the children were right-handed (Nelson et al., 2019). However, about 25 percent of the children showed different handedness for tasks that are performed with one or with both hands. This indicates that in these children, at 3 years of age, hand use patterns might still undergo development.
8. There is also left-footedness.
Using our hands is not the only form of motor asymmetry that humans show. There is also footedness, i.e., the preference to use the left or right foot for kicking a ball or grabbing something with your toes. This is an especially important feature in soccer players (DeLang et al., 2019).
Footedness and handedness are related in most people, but not every left-hander is also a left-footer. Moreover, there are also left-kissers (read this blog post if you want to know more) and left-huggers (read this blog post if you want to know more).
Barrenetxea-Garcia J, Torres-Unda J, Esain I, Gil SM. (2019). Relative age effect and left-handedness in world class water polo male and female players. Laterality, 24, 259-273.
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.
de Kovel CGF, Francks C. (2019) The molecular genetics of hand preference revisited. Sci Rep, 9, 5986.
DeLang MD, Rouissi M, Bragazzi NL, Chamari K, Salamh PA. (2019). Soccer Footedness and Between-Limbs Muscle Strength: Systematic Review and Meta-Analysis. Int J Sports Physiol Perform, 14, 551-562.
Nelson EL, Gonzalez SL, El-Asmar JM, Ziade MF, Abu-Rustum RS. (2019). The home handedness questionnaire: pilot data from preschoolers. Laterality, 24, 482-503.
Ocklenburg S, Isparta S, Peterburs J & Papadatou-Pastou M. (2019). Paw preferences in cats and dogs: Meta-analysis. Laterality, in press.
Papadatou-Pastou M, Martin M, Munafò MR, Jones GV. (2008). Sex differences in left-handedness: a meta-analysis of 144 studies. Psychol Bull, 134, 677-699.
Papadatou-Pastou M, Martin M, Munafo MR, Ntolka E, Ocklenburg S, Paracchini S. (2019). The prevalence of left-handedness: Five meta-analyses of 200 studies totaling 2,396,170 individuals. PsyArXiv Preprints.