Why are some of us left-handed?

June 24, 2015 11:26 pm | Updated 11:26 pm IST

A possibility: Genetic factors might offer an explanation for the asymmetry, and have been studied. photo: Reuters

A possibility: Genetic factors might offer an explanation for the asymmetry, and have been studied. photo: Reuters

Why are some of us left-handed while others use the right hand for most activities? This question is yet to be answered in a convincing way. Estimates are that anywhere between 9-20% of us humans are lefties. Since this is a minority, lefties face a variety of inconveniences in daily life, be it in opening doors by turning knobs or opening locks, using can openers, or in many others minor but necessary matters and tasks. And more often than not, children who are naturally left-handed are weaned away from it and encouraged (read ‘forced’) to use their right hand. This occasionally leads to awkwardness, and hence derision by the righties- majority. Recall the words like gaucherie (clumsiness, derived from the French gauche for left) and sinister, as opposed to dexterous (from the Latin dexter meaning right and sinister forleft).

An eminently readable review on why some people are left-handed has appeared six years ago, written by Llaurens, Raymond and Faurie ( Phil. Trans. Royal Soc. B , 2009, 364, 881-894, freely downloadable, highly recommended). They analyze several factors that appear involved. Anthropology shows that this asymmetry is an age-old feature. Neanderthals (35000 years ago) and even earlier homo (a million years ago) were predominantly ‘righties’ and some lefties. We thus have a historical (even prehistoric) legacy, or what evolutionary biologists call as selection pressure with a preponderance of right-handers.

It varies based on geography and social factors. Analysis of writing habits in people across 17 countries in Europe, Africa, Asia and Australia revealed lefties to be anywhere between 2.5 to 12.8%. And in most populations, the proportion of left-handers among women was lower than in men. Is this because of genetics (x-chromosome which women bear) or discrimination is not clear.

Genetic factors might offer a possibility, and has been studied. Left-handedness seems to run in families. My wife Shakti is a leftie, so was her mother, so are some of her nephews. Our daughter, Katyayani, displayed some left-handedness in her infancy (but is now a rightie) while her daughter Kimaya is a left-hander. Several readers will surely cite similar family-runs of left handedness. Whether this is a genetic or learning level phenomenon is unclear. One way of looking at this a little sharper has been to study twins (particularly identical twins). One such study suggests that the tendency for hand preference was more in identical twins that in non-identical. While more studies are warranted, this result suggests a possibility of heritability, thus allowing the role of natural selection.

As of now, however there is no “handedness gene” that has been discovered, although the Oxford psychiatrist Tim Crow has suggested that mutations in the gene PCDH11X might be responsible for the evolution of handedness, brain asymmetry, language, and might even have led to the events that created us humans. And Dr Francks, also of Oxford, has suggested the role of the gene LRRTM1 in chromosome 2 might be involved. But there may not be one, but many even if somebody shows that a region or a block in a chromosome (such as X), the claim would be challenged since family/society/cultural biases will influence hand usage.

That we may have an evolutionary history for handedness is getting increasingly implicated. That the great apes too show handedness-preference and asymmetry is well known. A recent surprising study showed that >Australian kangaroos are left handed (see >The Hindu , 20-06-15, and Giljov et al., Current Biology , 25, 1-7, 2015)! This study suggests that such preference may be common in bipedal animals, while tetrapods (four-legged animals) do not display any such preference. Four years ago, Brown and Magat reported that not just bipedal mammals, but even parrots show individual preference of one limb over the other to explore the environment or manipulate objects ( Biol. Lett . 2011, 7, 496-498). This interesting result follows similar studies on chicken and pigeons.

Dr. Lesley Rogers, who studied chickens, suggests a connection between the regions of the brain (the hemispheres) and handedness. Her study and the more recent parrot study suggest a connection between the eye and handedness. If a parrot, for example focused on food (say a fruit) with its right eye, then it apparently would tend to use its right foot to grasp and move the food around. And one which focuses using the left eye will use its left. This theory thus connects the hemisphere (right or left) of the brain with the handedness of the animal, that is: whichever hemisphere dominates should determine the handedness. To put it blandly: “right eye leads to and right handedness, and left eye to left”. This theory too needs further validation before it can be accepted.

I believe we have a good opportunity to test this ‘eye-handedness’ connection in our eye institutions. One way of doing so is to study newborn children who are born with both eyes blind, specifically due to cataract. We want newborns, because they have not been weaned away from their inherent, natural, handedness, and cataract because their vision can be successfully restored after surgery soon enough. If we were to mount a multi-centre project to study each of these infants for their handedness before and after vision restoration, we might have an answer to the eye-handedness connection theory. We propose discussing this with other eye centres (and neonatal clinics) and take on this project.


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