Left Handedness Research- Developments in the Last Half Century

A fact that has puzzled scientists for several decades is that only 10% of the human population is left-handed, while a 50:50 right-left split is observed in other animals. From the analysis of archaeological data, right-handedness has probably been the norm for several million years. Several theories have been proposed to explain handedness, many of which have no support of scientific evidence. Some of them have endured strong evidence against them and has attained the status of a myth.

One of the early ideas that have developed in this field of research is that handedness is a genetically determined trait. Some researchers went further by suggesting that left-handedness is a Mendelian recessive trait. But this reasoning was countered by studies that proved the existence of handedness discordance in identical twins. On the other hand, there was another line of reasoning that environmental influence is the cause of handedness.

Handedness is closely entangled with the idea of brain lateralisation. As measuring brain lateralisation directly is a difficult task, handedness can indirectly measure brain lateralisation. Asymmetry in the human brain is just one among many examples of asymmetry across the biological and physical world. In his doctoral thesis of 1848, Louis Pasteur pointed out the existence of asymmetry in the biochemical level with the discovery of the optical isomers of tartrate. He later linked this optical asymmetry with the asymmetry of the crystals derived from each of the optical isomers.

Paul Satz’s Pathological left-handedness is another crucial concept that developed in the field of handedness research. If random symmetric processes damage an asymmetric brain, then the outcome would be asymmetric. This phenomenon would result in the manifestation of left-handedness in naturally right-handed people. Pathological right-handedness is also possible, but the lower frequency of natural left-handedness in the population constrains its rate of occurrence.

The theory of pathological left-handedness, although true, was extrapolated to an extreme extent. Bakan and others even claimed that left-handedness is one of the various neuropathological conditions caused by birth stress. But on analysing the huge dataset of the National Child Development Study (NCDS), which contained midwife recorded birth details for 11,000 births along with a record of handedness determined by professionals at ages 7 and 11; Chris Mcmanus was able to demonstrate that there was probably no correlation between left-handedness and birth trauma.

One of the most popular myths regarding lateralisation is ‘brainedness’- most people predominantly use one of the two hemispheres for thinking, left if they are scientific and right if they are artistic. Left-handers are supposed to either use the right hemisphere more or use both hemispheres. The two hemispheres indeed specialise in different kinds of tasks. But there is no scientific evidence that shows that one hemisphere of the brain dominates over the other.

This idea primarily emerged from the study of ‘split-brain patients’- in whom the corpus callosum is severed, thereby dividing all cortical links between the two hemispheres. The corpus callosum is the bundle of nerve fibres that connect the two halves of the brain. So this process called commissurotomy isolates the cortex of the two hemispheres. This procedure was used as a treatment for certain forms of epilepsy as it helped to restrain the seizure responsible for the condition to one of the hemispheres.

Astonishingly, the patients experienced no significant difficulties in daily life, even after undergoing such a major surgery. But more intense examinations proved that they suffered from specific disruptions in perception and cognition. This discovery helped the scientific world study the differences between the two hemispheres and their cooperation.

Recent studies with the help of brain imaging technology have not been able to find any evidence that supports the theory of brainedness. Many unanswered questions in the field of lateralisation research would probably be answered with the help of large databases which collects genetic, neuroimaging and social data. The UK Biobank has made significant progress in this field.

For instance, Clyde Francks and others conducted a genome-wide association analysis of hand preference in the large, population-based UK Biobank cohort. In their study, they could not find any evidence for genes involved in visceral laterality playing a role in handedness. But they were able to associate the microtubule-associated gene MAP2 with handedness. The interesting leads provided by such studies would be explored in more detail by researchers in the coming decades.