Left-handedness, as a relatively uncommon phenomenon, never fails to fascinate people. There is a common perception that left-handed people are more talented and artistic. To what extent these assumptions are correct, and what your preferred use of right or left hand can tell you about your brain structure?

Handedness represents the better performance or preference of using one hand, i.e., the dominant hand. Right-handedness is the most common type observed in 70–95% of the world population, followed by left-handedness, and then a very rare type of mixed handedness and ambidexterity. Although this is an important physiological feature in humans, it seems that the origins of handedness are not well understood.

While many scientists assume that genetics is the main determinant of handedness, others disagree and believe that other factors also play an important role. They believe that variations in handedness are related to some behavioral and anatomical measures. For instance, although just 10% of humans are left-handed, these individuals tend to be over-represented in artistic professions, have better mathematical abilities, and have a lower predisposition to diseases such as arthritis and ulcers. On the other hand, there is an increased prevalence of some health issues, including cardiovascular disease, dyslexia, asthma, multiple sclerosis, and others.

In addition to strict (constant) handedness, there is something called mixed, i.e., inconsistent handedness. Some scientists believe that mixed-handed individuals are of poorer mental and physical health, with lower cognitive parameters and higher rates of dyslexia and attention deficit hyperactivity disorder (ADHD). Mixed-handedness (a change in hand preference depending on the task) has been associated with greater atrophy of the hippocampus and amygdala, brain structures that are strongly associated with dementia and cognitive aging. Also, non-right handers (mixed or left-handed) are at higher risk of neurodevelopmental disorders, including autism, epilepsy, and schizophrenia.

Handedness reflects the structure of our brain, more specifically its asymmetry. The functional differences in the right and left brain hemispheres are believed to underline the phenomenon of hand dominance. Handedness is probably the most obvious manifestation of the fact that our brain functions in an asymmetric manner. While the left hemisphere controls right-handedness, i.e., the dominant right hand, the right hemisphere controls dominant left-handedness. The left hemisphere is also specialized for language and logic in most people, while right hemisphere specialization is related to intuition and creativity. Asymmetry of the brain and handedness become detectable very early, even during fetal development. Ultrasound examinations have revealed that even at the 10^th week of gestation, most fetuses move their right arm more often than the left one, while from the 15^th week the majority of fetuses suck the right thumb. This is believed to be predictive of future handedness. In line with this is the leftward enlargement of brain structure (seen in the first trimester of pregnancy), which plays an important role in neurological development.

Studies have linked handedness with differences observed in language lateralization. More precisely, right-handed individuals are characterized by left hemisphere control of language, while left-handers have shown right hemisphere dominance in language or bilateral speech representation. One interesting study questioned whether early childhood handedness can influence language development. The authors assessed the handedness of infants aged 6 to 18 months at each month and then again when they were toddlers (from 18 to 24 months). They found that constant use of the right hand during infancy was associated with having superior (advanced) language skills at 24 months old. On the other side, children who were not lateralized in infancy and became right or left handed as toddlers had the average expected language scores for their age.

There are also differences in the lateralization of visual areas of the brain between right and left-handed individuals. In right-handers, there is much higher activation of the right fusiform face area (the area responsible for face visualization) and the extrastriate body area (responsible for body visualization). Meanwhile, in left-handers, these areas are equally activated across both brain hemispheres.

Some researchers believe that brain volume may correlate with handedness, although the data on this subject remains controversial. One group of researchers reported that left-handed individuals had a larger brain, while another study found no difference in brain size between the right- and left-handers. As some findings indicate, left-handers are more prone to nighttime awakenings due to sleep disorders caused by periodic limb movements. It seems that left-handed individuals are more likely to experience limb movements while asleep compared to right-handers.

Since handedness has been associated with prenatal hormonal exposure, it could influence the risk of carcinogenesis later in life. Scientists investigated the impact of handedness on brain tumors, both malignant and benign ones. One study examined the associations between glioma, meningioma, and acoustic neuroma with self-reported handedness. Left-handers or ambidextrous (with equal use of both hands) individuals were at reduced risk of glioma (the most common malignant brain tumor) when compared with the right-handers. This relationship was similar for both genders. However, another very recent study found no such association. This large case-control study (which included more than 1000 glioma cases and healthy controls) reported no association between handedness and glioma risk after adjustment for age, gender, and race.

Although the brains of left-handers and right-handers differ in their structures, the available literature shows no noteworthy differences in intelligence as measured by IQ score. Nevertheless, these brain structure differences seem to reflect the more diverse and creative processing of language and emotions by left-handers than by right-handed individuals. This may explain why a greater proportion of left-handers are professional musicians, even in those cases when the musical instruments are designed for right-handers (for example, violins). Similarly, the gift for mathematics seems to be more common in the left-handed populace.

It is obvious that right and left-handers differ not only in hand preference but also in brain structure. This further reflects the ability to perform different tasks and achieve success in different professions. Although there is a clear link between non-right handedness and developmental disorders, there is no association between brain carcinogenesis and the dominance of one hand. It seems that handedness can be predicted in early childhood, even during fetal development, but further investigations are needed to elucidate the origins of our preference to use one hand or the other.

References

Cherbuin, N., Sachdev, P.S., Anstey, K.J. (2011). Mixed-handedness is associated with greater age-related decline in volumes of the hippocampus and amygdala: the PATH through life study. Brain and Behavior. 1(2): 125-134. doi:10.1002/brb3.24

Corballis, M.C. (2014). Left brain, right brain: facts and fantasies. PLoS Biology. 12(1):e1001767. doi:10.1371/journal.pbio.1001767

Nelson, E.L., Campbell, J.M., Michel, G.F. (2014). Early handedness in infancy predicts language ability in toddlers. Developmental Psychology. 50(3): 809-814. doi:10.1037/a0033803

Li, M., Wang, J., Liu, F., Chen, H., Lu, F., Wu, G., Yu, C., Chen, H. (2015). Handedness- and brain size-related efficiency differences in small-world brain networks: a resting-state functional magnetic resonance imaging study. Brain Connectivity. 5(4): 259-265. doi:10.1089/brain.2014.0291

Josse, G., Hervé, P.Y., Crivello, F., Mazoyer, B., Tzourio-Mazoyer, N. (2006). Hemispheric specialization for language: Brain volume matters. Brain Research. 1068(1): 184-193. doi:10.1016/j.brainres.2005.11.037

Inskip, P.D., Tarone, R.E., Brenner, A.V., Fine, H.A., Black, P.M., Shapiro, W.R., et al. (2003). Handedness and risk of brain tumors in adults. Cancer Epidemiology, Biomarkers and Prevention. 12(3): 223-225. PMID:12646512

Miller, B., Peeri, N.C., Nabors, L.B., Creed, J.H., Thompson, Z.J., Rozmeski, C.M., et al. (2018). Handedness and the risk of glioma. Journal of Neuro-oncology. E-published: doi:10.1007/s11060-018-2759-y

Image via sasint/Pixabay.