Why are some people left handed?18 April 2016
It’s hard being a left-hander in a right-handed world.
Left-handers have been ostracised throughout history, associated with sin, devil-worship and witchcraft. Nowadays, lefties have to deal with the trials and tribulations of spiral notebooks, desks that fold the wrong way and impossible right-handed scissors.
This engrained discrimination is apparent in our language. The Latin word ‘sinister’ originally meant ‘left’ before taking on connotations of ‘evil’. The Anglo-Saxon ‘lyft’ means‘weak’ or ‘broken’. Contrastingly, the English word ‘right’ is a synonym for ‘correct’.
Handedness is a cultural phenomenon as much as it is a scientific one.
But why does hand preference exist at all?
Human geneticist Sylvia Paracchini says the answer is simple, “It would be a waste to have both hands work equally well… Such duplication would be unnecessary in terms of evolution”. How we define handedness has proven to be a problem. Writing alone is not a clear enough determinant of hand preference and other actions such as throwing, kicking or eating reflect hand dominance more accurately. Sometimes people write with one hand but brush their teeth with another. Some people – rare as they may be – have no preference at all.
William Brandler, a PhD student at Oxford University stands by his claim that left-handedness is inherantly rooted in genetics. In addition to asking, “Why is there a bias in the first place?” he asks, “Why aren’t there societies where you see a bias to the left?”.
Approximately 10 per cent of our population identifies as being left-handed and this value has remained fairly constant throughout history. So it seems that our dexterity is rooted in our genes.
Family studies show that handedness is at least somewhat hereditary. The British Royal family has an usually high prevalence of left-handedness – with the Queen Mother, Queen Elizabeth II, Prince Charles and Prince William all donning left handedness in succession.
The McManus family study reports that the likelihood of children being left-handed is affected by their parents’ hand preference. According to this study, children with two right-handed parents have an eight per cent likelihood of being left-handed. About 18 per cent of children with one right-handed and one left-handed parent will be left-handed, and 26 per cent of children with two left-handed parents will be lefties. This shows a significant genetic effect.
But the gene that determines our hand-preference remains elusive – that is, if it exists at all. As of now, there is no evidence for a strong genetic factor. Professor Armour, a professor of human genetics at the University of Nottingham says, “A survey that compared the whole-genome genotypes for right and left-handed people should leave such a gene nowhere to hide”.
If handedness is genetic, and right-handedness is dominant, how has this small yet persistent minority remained?
It is more likely that there are many weak genetic factors – one estimate suggests there may be as many as 40 – which interact with one another. Thus, this genetic factor will be very difficult to find and just as difficult to erase from the gene pool.
In a recent paper published in PLOS Genetics, a team of scientists from Oxford, St. Andrews and Max Plank Institute in the Netherlands claim to have found a network of genes that could be responsible for left-handedness. This gene, PCSK6, is involved in the establishment of left and right symmetries in a growing embryo.
This suggests that the left-right asymmetry of our brains could have something to do with our hand preference.
One predominant hypothesis for this bias rests upon the compartmentalisation of our brain and suggests that language ability is tied to our hand preference. Since both are generally managed by the left hemisphere, most people are right-handed (as the left hemisphere controls the right side of our body and vice versa). However, this theory has been challenged by the fact that 70 per cent of left-handers are left-brained for language too.
Whilst we may not understand what causes left-handedness, it almost certainly affects the way in which people think and behave.
Another handy explanation for this somewhat strange lingering of left-handers is an evolutionary one. On the one hand, lefties benefit from a competitive advantage in sports and in combat.
For example, 50 per cent of top players in baseball are left-handed. Because right-handers are in the majority, both left and right-handed players focus on practicing against right-handed opponents. Thus, left-handers benefit from being in the minority. However, we would expect natural selection to favour lefties until we had a 50:50 ratio and they no longer had this advantage.
But cooperative pressures, such as tool sharing, counteract competitive pressures. Since most tools are designed for right-handers, left-handers are less favoured in a cooperative world. And so the 90:10 ratio is an equilibrium of sorts.
Lefties have persevered through the ages, and it looks as though they’ll continue to do so for a very long time. In this day and age, handedness doesn’t matter much – but at least we can commend lefties for the incredulous persistence of their genes.
And yes, whilst life may be harder for lefties, what with avoiding spiral notebooks and awkward dinner settings – at least you can associate yourself with the likes of Obama, Oprah and Aristotle, if for nothing but your handedness.