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International Left-Handers Day: What is the science behind being left or right-handed?

Thursday 13 August marks International Left-Handers Day, an annual observance to raise awareness of and celebrate people who are left-handed

Sabrina Barr
Thursday 13 August 2020 14:01 BST
(Getty Images/iStockphoto)

Nature versus nurture is a powerful conundrum. To what extent are certain attributes inherited from our parents and ancestors, and to what extent are our mannerisms learnt during our developmental years?

There are myriad theories concerning handedness – whether a person is right or left-handed. While certain facts we know – that there are far more people (around 90 per cent) whose right hand is more dominant than their left – other aspects of the topic are shrouded in debate.

For centuries, the notion of being left-handed has been surrounded by stigma. According to a Time article published in 1969, in the Middle Ages people who were left-handed were at risk of being accused of witchcraft, while there are many accounts of left-handed children being made to use their right hand instead. Not to mention, the various idioms that point to right-handedness being favourable (such as having “two left feet” or giving a “left-handed compliment”).

In 1976, Dean R Campbell, founder of the Lefthanders International Inc, established International Left Handers Day as an annual observance on 13 August, to raise awareness of and celebrate people who are left handed. As the day is celebrated around the world, the question remains – what is the science behind being left or right-handed?

Is it in your genes?

As a child, if you can remember naturally picking up a pen with a certain hand, this could lead you to believe that your dominant side was likely determined by your genes. According to an article published in the Smithsonian Magazine in 2013, “scientists have speculated for years that a single gene” could be the key to understanding left or right-handedness.

A study published in the scientific journal Plos Genetics in the same year went a step further, proposing that “handedness is a polygenic trait”, i.e. a trait that is determined by a combination of genes, rather than just one gene.

According to another study published in 2013 called “The Evolution of Human Handedness” by researchers at University College London and the University of Nottingham, 40 loci (the position of genetic markers on chromosomes) could be involved in the determination of handedness.

Clare Porac, a professor of psychology at Pennsylvania State University in the US, began studying handedness after conducting research into eye dominance. In an article written by Professor Porac for Scientific American in 2004, she explained that some researchers argue that “evolutionary natural selection produced a majority of individuals with speech and language control in the left hemisphere of the brain”, the side of the brain that “controls the movements of the right hand – and notably the movements needed to produce written language”.

The professor continued, stating that the “genetic proposal to explain hand preference states that there are two alleles, or two manifestations of a gene at the same genetic location, that are associated with handedness”.

One of these genes, Porac explained, means that a person has a 50:50 chance of being right or left-handed, while the other “promotes right-hand preference in the majority of humans”.

Despite the plethora of research into the impact genetics can have on handedness, a study of twins published in 2006 found that just 25 per cent of the variance of handedness was down to genetic factors, with 75 per cent of variance coming down to environmental factors that were “unique to the individual”.

Do environmental factors play a part?

Going back to the two alleles described by Porac, according to the professor, the hand preference of people with the first gene (with a 50:50 chance of being right or left-handed) “can be influenced by external cultural and societal pressures, a phenomenon researchers have documented”. She explained that people with this gene could show dominance in one hand depending on “pressures of familial training” and other environmental factors. Thus outlining the fact that handedness may not be solely down to genetics.

In 1997, a team of researchers conducted a study called “Environmental Influences in Hand Preference”, assessing students from Ivory Coast and Sudan. They concluded that certain “cultural and environmental factors could change ‘natural’ hand preference”, such as a person changing the hand they use to do a certain activity, such as eating.

In a more recent study, carried out in 2019 by researchers from the Max Planck Institute for Psycholinguistics in The Netherlands, they investigated whether early life factors may have a significant influence on a person being left or right-handed. Using the UK biobank, which included the data of approximately 500,000 people, they found that the likelihood of a person being left-handed “was affected by the year and location of birth, likely due to cultural effects”.

Other factors also affected hand preference, the researchers stated, including the time of year a person was born and whether they were breastfed. They concluded that while “on average” people who were left or right-handed “differed for a number of early life factors”, overall, “these factors had only a minimal predictive value for individual hand preference”.

It is apparent that this topic is one that will fascinate scientists and members of the public for years to come. But whether being left or right-handed is down to genetics, environmental factors or a combination of the two, it is clear that being left-handed is still somewhat of a rarity.

Of course this means left-handed people have to deal with issues like household objects (scissors and notebooks) being designed to suit right-handed consumers. But they should remember that left-handed athletes famously have an advantage in sports, as right-handed players in disciplines such as tennis and cricket may find it harder to adapt to left-handed opponents. Just look at Rafael Nadal – with 19 singles Grand Slams titles under his belt, his left-handed tennis technique is one of the most revered in the sport.

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