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About 10% of the population is left-handed.
There have been several theories over the years about why some people favour their left hand.
A study from last year has shown that it might be nothing to do with your brain.
Rather, it could be determined by your spinal cord while you are in the womb.
Left-handed people haven't always been treated very well throughout history. Lefties have been persecuted for their disposition in the past, being labeled as evil, or even as witches, despite making up about 10% of the population. In fact, the word "sinister" derives from "left" or "left-hand."
There have been a few theories about where left-handedness comes from over the decades, including an outdated idea that it's something to do with mothers being stressed while pregnant.
It's down to the spinal cord — not the brain
Research since the 1980s has shown that our preference for being left or right-handed is probably determined in the womb before we are born, as early as the eighth week of pregnancy, according to ultrasound scans. From week 13 in the womb, babies tend to suck either their right or their left thumb.
It was previously thought that the genetic differences in the left and right hemisphere of the brain would determine whether someone was born left or right handed. But a study from 2017, published in the journal eLife, has found that the answer could lie in the spinal cord.
The research was led by Dr Sebastian Ocklenburg, Judith Schmitz, and Prof Dr H. C. Onur Güntürkün from Ruhr-Universität Bochum, along with other colleagues from the Netherlands and South Africa. They found that gene activity in the spinal cord is already asymmetrical in the womb, and this could be what causes left or right-handedness.
Arm and hand movements start in the brain, in an area called the motor cortex, which sends a signal to the spinal cord. This is where the signal is translated into a motion. While the baby is growing in the womb, up until about 15 weeks, the motor cortex and the spinal cord are not yet connected, but right or left-handedness is already decided.
In other words, the baby can already make movements, and has chosen its favourite hand before the brain starts controlling the body. This led the researchers to believe the spinal cord is the decider for the hand preference.
To study this, the team analysed the gene expression in the spinal cord during the eighth to 12th week of pregnancy. They found significant differences between left and right-handedness in the segments of the spinal cord that control arm and leg movement.
They concluded that the asymmetrical nature of the spinal cord could be down to something called epigenetics — which is how organisms are affected by changes in their genes' expressions, rather than in the genes themselves. These changes are often brought about by environmental influences, and can affect how the baby grows.
These gene expression differences could affect the right and left spinal cord differently, resulting in lefties and righties.
So why are lefties so rare?
Why are left-handed people so rare, with right-handed people making up 90% of the population? Some scientists have tried to answer this question, such as in 2012, when researchers at Northwestern University developed a mathematical model to show how the percentage of left-handed people is a result of human evolution.
They hypothesised that a balance of cooperation and competition in our evolution has led to this divide. In other words, although the basis may be genetic, there could be a social factor to it too, hence why the ratio is so high.
"The more social the animal — where cooperation is highly valued — the more the general population will trend toward one side," Daniel M. Abrams, the author of the model and an assistant professor of engineering sciences and applied mathematics at the McCormick School of Engineering and Applied Science, told LiveScience.
"The most important factor for an efficient society is a high degree of cooperation. In humans, this has resulted in a right-handed majority."
In other words, for some reason we have evolved to favour right-handedness, and so anyone deviating from this may have been conditioned to grow up this way, despite their genetic predisposition.
It's still a bit of a mystery, and it's very hard to predict whether a child will become left or right-handed once they are born. This is partly because more left-handed people need to be included in scientific research.
One thing we do know is that the neurological differences between left and right-handed people are very small, and supposed behavioural and psychological distinctions have often been debunked.
As for whether lefties are in fact evil, the jury is still out.