The Mystery of Left-Handedness

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Until recently, left-handedness was a matter of great prejudice, and in many cultures, it was common to force left-handed children to write with their right hand. Throughout the world, the prevalence of left-handedness is highly variable, ranging from approximately 5% to 25% and – for unknown reasons – is more common in men than in women. So what defines our handedness? And why is being left-handed less common?

This over-representation of right-handers is regarded as an indication of a genetic component defining handedness. Twin studies have shown a higher handedness concordance rate in identical twins than in fraternal (non-identical) twins. Moreover, it has been estimated that the probability of a child being left-handed is 8% when both parents are right-handed, 22% when only one parent is left-handed, and 36% when both parents are left-handed. Interestingly, when only one parent is left-handed, there is a higher prevalence of left-handedness in children with a left-handed mother than in children with a left-handed father, indicating a possible maternal transmission.

These facts have stimulated further research on the genetics of left-handedness. Due to the complexity of this behavioral trait, it is most likely that it may be associated not only with a single gene, but with multiple genetic and environmental factors. In fact, genome-wide association studies have failed to find a single gene that is significantly linked to left-handedness. A great difficulty in such studies is that handedness, by being a complex behavior, is not directly controlled by genes; genes may instead have an indirect effect by, for example, influencing brain development or function.

The human brain is an asymmetrical organ with different functional specializations of the left and right hemispheres. The brain is organized such that the left hemisphere controls the motor functions of the right side of the body and the right hemisphere controls those of the left side. Many brain functions are lateralized, i.e. they are processed in one of the hemispheres, which allows for a “division of labor” between the two hemispheres. This is thought to be an important evolutionary adaptation that has boosted the brain’s efficiency. Handedness is an example of a cerebral lateralized function (as well as motor speech), being dominantly processed in the left hemisphere. For most people, this results in right-handedness.

Interestingly, there are indications that left-handedness may be associated with a smaller lateralization of cognitive processing: 97% of right-handers have their motor speech area located exclusively in the left hemisphere. On the other hand (literally), motor speech processing is located exclusively in the left hemisphere only in 60% of left-handers, while 30% have bihemispheric processing and 10% have right hemisphere processing.

Studies have shown that the corpus callosum, the largest structure connecting the left and right hemispheres (commissure), of left-handers tends to be larger. Therefore, this may be a sign of a greater connectivity between hemispheres and may be associated with certain cognitive skills. In fact, a greater interhemispheric connectivity could be associated with the reported observations that left-handers can have better mathematical skills or higher creativity, for example.

Although handedness seems to be a matter of natural variation, it is intriguing why left-handedness is less common. Since neither left- nor right-handedness confers any obvious evolutionary advantage, a similar prevalence in the population would be expected. Also, if one were less advantageous, it would be expected to eventually disappear. But that is not the case.

Therefore, some researchers defend that there must be some disadvantage to being left-handed, which has led to a search for associations of this trait with other features or even diseases. Consequently, left-handedness has been linked to all types of disorders, although not always accurately.

But there is growing evidence from neuroimaging studies that the smaller cerebral lateralization associated with left-handedness can also be associated with neurodevelopmental disorders such as autism, specific language impairments, epilepsy, or schizophrenia, for example. However, it is hard to determine whether weak cerebral laterality causes the disorder or vice versa. Also, it is possible that genetic influences that determine weak laterality may also induce neurodevelopmental disorders without a cause-effect relation. The mystery persists.

References

Brandler WM, & Paracchini S (2014). The genetic relationship between handedness and neurodevelopmental disorders. Trends in molecular medicine, 20 (2), 83-90 PMID: 24275328

Forrester GS, Quaresmini C, Leavens DA, Mareschal D, & Thomas MS (2013). Human handedness: an inherited evolutionary trait. Behavioural brain research, 237, 200-6 PMID: 23022751

Gutwinski S, Löscher A, Mahler L, Kalbitzer J, Heinz A, & Bermpohl F (2011). Understanding left-handedness. Deutsches Arzteblatt international, 108 (50), 849-53 PMID: 22259638

Ocklenburg S, Beste C, & Güntürkün O (2013). Handedness: a neurogenetic shift of perspective. Neuroscience and biobehavioral reviews, 37 (10 Pt 2), 2788-93 PMID: 24091023

Uomini NT. (2009). The prehistory of handedness: archaeological data and comparative ethology. J Hum Evol. 57(4):411-9. PMID: <a hrefUomini NT (2009). The prehistory of handedness: archaeological data and comparative ethology. Journal of human evolution, 57 (4), 411-9 PMID: 19758680=”http://ift.tt/1vJ0HAz;>19758680

Image via Schankz / Shutterstock.

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