Left-handed life: challenges, myths and curiosities

The world we live in is more designed for right-handed people, i.e. people who preferably use the right hand, while left-handed people are more often faced with uncomfortable inconveniences: from ergonomic scissors to conference chairs with folding tables, from can openers to keyboard shortcuts located to the right. For left-handed people, there are more difficulties related to daily life.

Since 1992, the "Lefthanders International" Club has established the international left-handed day which falls annually on August 13th: the purpose of the anniversary is to raise public awareness on the implications of left-handedness, including its difficulties and strengths.

Are left-handed people born this way? From a neurological point of view, are there any differences compared to right-handed people? Is it true that many great artists were left-handed? Does forcing children to use their right hand affect the adult brain?

Is a person born a left-handed? Why?

The process that leads to manual laterality (LM), i.e. the preferential use of one hand over the other (and therefore also of the left hand over the right), seems to be influenced by various factors: genetic, prenatal, cerebral and environmental. A deterministic imprinting at the basis of manual laterality is suggested by several evidences.

Beyond the fact that there is a higher correlation between the manual laterality of biological parents and children than that of adoptive parents and children, twin studies have offered an important contribution to the inheritability of manual laterality. What emerges from these studies, which typically compare twins sharing the same genome and heterozygous twins, is that about 25% of the phenotypic variance of manual laterality has a genetic component.

Just to make a comparison, the heritability of a trait such as height is around 80%. This means that manual laterality does have a genetic basis, but that other factors certainly play a role in the development of manual preference. Furthermore, as associative studies conducted on the whole genome (GWAS) indicate, manual laterality appears to be a complex phenotype influenced by multiple genetic variants rather than by a single gene.

If a question like "Is a person born a left-handed?" is difficult to liquidate with a binary answer (yes/no), a sufficiently consolidated fact is that the majority of individuals show a preference for the use of the right hand.

According to a recent meta-analysis, the preferential use of the right hand would characterize about 90% of the Western population. It is questionable why, in this regard, if left-handers represent a much smaller share of the general population, the left hand preference is maintained from an evolutionary point of view. An interesting fact is that, more generally, where a lateralization bias is present in the behavior of most vertebrates, the direction of that bias (left vs right) is almost always represented unequally at the population level.

Some theories have proposed that the maintenance of such a balance is regulated by group selective pressures. In particular, cooperation between individuals would favor the predictability of behavior, and therefore the most represented form of lateralization; conversely, the competitive interaction between individuals would favor the unpredictability of behavior, and therefore the form of minority lateralization. Maintaining unequal proportions of each form of lateralization at the population level would therefore favor the interaction between functionally asymmetric individuals.

From a neurological point of view, are there any differences compared to right-handed people?

The question is complex and involves different levels of description. It is complex first of all because the phenomenon of hemispheric lateralization, a fundamental principle of the brain organization of our species, remains obscure at times. From a functional point of view, compared to the left manual preference, the right manual preference seems to be associated with a more marked hemispheric lateralization for typically lateralized cognitive functions, such as language and spatial attention. This means that, in the face of tasks that involve these functions, in left-handers there is a greater share of atypical lateralization, and more distributed and bilateral involvement of brain areas.

In this regard, it has been hypothesized, also on the basis of neuroanatomical data, that in left-handed subjects the two hemispheres operate in a more integrated way than in right-handed subjects, with more efficient levels of interhemispheric communication. On the other hand, a less marked lateralization in left-handed subjects could derive, at least in part, from the pressure exerted by an environmental architecture suitable for right-handed people. The fact that left-handed subjects are sometimes induced to use their right hand to perform a certain task, thus accumulating practice with that hand, is probably due to a conflict between the intrinsic dynamics of these subjects and external environmental needs. This could result in a reduced asymmetry of motor behavior, with repercussions at the level of hemispheric dominance.

It is said that most of the most imaginative artists (painters, actors, musicians) were left-handed: how much is that true?

The preferential use of the left hand has been associated with everything and the opposite of everything: from cognitive advantages to learning disorders, from sexual orientation to the predisposition to develop psychiatric disorders such as schizophrenia and autism. An alleged association among the best known is that between manual laterality and intelligence, with respect to which particularly contrasting results have emerged.

Some studies in the past have reported a positive association between some forms of intelligence and preferential use of the left hand. Others have reported cognitive advantages associated with the preferential use of the right hand. A recent meta-analysis shows, on the other hand, that the differences between right-handed and left-handed subjects in terms of general cognitive ability are in fact negligible. It is therefore difficult, at the moment, to argue that there is a relationship of any sign between manual laterality and cognitive ability.

The same can be said for another rather elusive construct often associated with the preferential use of the left hand: creativity. Indeed, it seems that some great artistic personalities of the past, from Michelangelo to van Gogh, were left-handed. One could hypothesize, in this regard, that in a world suitable for right-handed people, the fact of being left-handed stimulates particularly ingenious forms of adaptation. Then one wonders how many other "creative" personalities have been in the past and are currently right-handed, without this noticing precisely because the preferential use of the right hand is largely prevalent in the general population.

It should be emphasized that, where scientific evidence exists for the associations just mentioned, it is likely that the conflicting results derive, at least in part, from the way in which the constructs involved are conceptualized and measured. Manual laterality itself has been treated in the literature as a dichotomous variable, categorical or continuous, and measured with different tools.

It seems that the Dutch painter van Gogh (1853-1890) was left-handed. Vincent van Gogh, Self-portrait (detail), 1889. © Musée d'Orsay, dist.RMN-Grand Palais / Patrice Schmidt

Historically, left-handed children were forced to use their right hand; does this forcing affect the adult brain?

Until a few decades ago, the preferential use of the left hand was often considered socially execrable, and therefore to be corrected during childhood. This type of attitude was based among other things on the belief that manual laterality was learned as you learn the multiplication tables, and completely modeled by environmental factors. In fact, the emergence of postural and motor asymmetries, including manual preference, is documented as early as the first trimester of gestation. Longitudinal data indicate that the preferential use of single upper limbs in the womb is a good predictor of postnatal manual laterality.

Among other things, even when social pressures translate into a "converted" left-handedness for example for writing, left-handed subjects usually maintain the left hand preference for many other manual activities. It has been asked whether this "converted" left-handedness, in fact a conflict between manual preference and social pressures or educational standards during childhood, impacts the structure and function of the adult brain. Apparently, it seems like it does.

Using positron emission tomography (PET) imaging, pioneering work by Siebner and colleagues has shown that “converted” and persistent left-handedness has an impact on brain motor control of writing. In particular, while in right-handed subjects an experimental handwriting task was associated with an activation of parietal and premotor areas of the left hemisphere, in "converted" left-handed subjects the same task activated more bilaterally distributed patterns, with selective recruitment of the parietal cortex , premotor and right temporal. The involvement of the right hemisphere in "converted" left-handed people would reflect persistent "masked" left-handedness and/or the suppression of unwanted left hand movements during the writing task. Traces of this "conversion" have also been identified at the level of the cerebral macrostructure.

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