(ORDO NEWS) — 13 August was celebrated as International Left-Handed Day What science knows about them.
Who are lefties?
The answer is not as obvious as it sounds. Usually left-handers and right-handers are judged by the leading hand. To determine it, use questionnaires. The most famous is Edinburgh, but they are all similar: a person is asked how he prefers to perform different actions. Most revealing is in which hand he holds the hammer, although outside laboratories and doctor’s offices they often judge by which hand a person writes.
There is also a leading leg (and even an eye!). This is noticeable in football: left-handers are highly valued because it is more convenient for them to play on the left half of the field and it is difficult for right-handed opponents with them. In 2016, scientists calculated that in the junior teams of the Netherlands, 31% of players had the leading left leg. Outside the field, left-handers are three times less common: they are slightly more than 10%, and another 9% of people prefer one or the other hand (and, for that matter, a leg).
But regardless of preference, people can still use the other hand. How well it works is assessed by three indicators: grip strength, speed and agility. It would seem that skill directly depends on preferences, but experiments say that this is not always the case. Someone’s second hand listens better than others, in addition, the specific task matters.
It may be more correct not to divide people into two clear groups, but to place them on a scale where there are pronounced left-handers and right-handers at the edges, but there are also intermediate cases. Geneticist and neurologist Daniel Geschwind once said that left-handers are more accurately called “wrong-handed” because they often have good control of both hands.
Why are some left-handed and others right-handed?
Already at the 14th week of pregnancy, the human embryo usually turns its head to the right and sucks on the finger of its right hand (interestingly, when kissing, only two-thirds of people tilt their head to the right – another example for the scale and against dividing into two groups). This usually indicates whether the child will be right-handed or left-handed – it can be assumed that this trait is to some extent innate.
According to one hypothesis, in left-handed people in the womb, the brain is formed in a special way due to testosterone. Therefore, there are about a quarter more left-handers among men than among women. And the level and susceptibility to testosterone can depend on the genes of the mother and child. Heredity is also indicated by the fact that left-handed parents often have the same children, and if one of the twins is left-handed, then the second will turn out to be more likely than the average. True, it happens in another way.
Scientists have tried to find a gene that determines this trait. Several candidates were under suspicion. When DNA decryption became cheaper, it was possible to find dozens of small differences, which individually slightly affect whether a person is left-handed or right-handed. But, firstly, it is not clear exactly how they act, and secondly, even all together they do not allow you to say with confidence who is standing in front of you.
Analysis of data on half a million Britons showed that there are more left-handers among those who were born with low body weight, twins and women born in the summer, but if we take all seasons, then there are still more left-handed men. They are more often those who have not been breastfed. Finally, the proportion of left-handers increased until 1970, and among those born outside the UK there are almost a third fewer. The last two patterns are easier to explain by prejudice and upbringing, rather than nature.
What prejudices are there about lefties?
In many cultures, the left side is bad. Christians associate the left hand with the devil, Muslims consider it unclean. When someone is out of sorts, we ask: “Did you get up with your left foot?” The words “right” and “right” are similar for a reason. And the English word sinister, which means “ominous”, “evil”, comes from the Latin for the left hand. In English, left-handedness is still called sinistrality. That is why from time immemorial left-handers were treated with caution.
In the USSR and other countries, they were retrained, and sometimes they still do it. It’s not so much about superstition or the fact that left-handedness is considered a pathology – it’s just that many things, for example, computer mice, make them asymmetrical, comfortable for right-handers. But in the second half of the 20th century, when this practice was gradually becoming a thing of the past, there were suggestions that left-handedness – at least in some cases – is still a deviation in brain development.
So left-handedness is a pathology or a variant of the norm?
Although left-handedness is a well-known but mysterious phenomenon, it was not really studied until the 1970s. Since then, scientists have hypothesized that birth injuries are left-handed, or that sex hormones (especially testosterone) can interfere with brain development during pregnancy. These guesses were supported by statistics: supposedly, left-handed people are more likely to have some neurological, mental disorders and even autoimmune diseases, and they live on average seven years less than right-handers.
But left-handers and right-handers are not only among people: they are pigeons, bees, zebrafish and other animals, which are separated by millions of years of evolution. True, such a large disproportion as in humans – nine right-handers for one left-hander – is rare. For example, in great primates, according to some estimates, this ratio is 65/35.
These differences come down to the work of the nervous system, primarily the brain. The cerebral hemispheres are responsible for different functions. Why this is so is not fully understood. The main hypothesis is that duplicating functions would be ineffective. The brain accounts for only 2% of body weight, but it consumes 20% of energy, and a large head in babies complicates childbirth. Specialization, on the other hand, allows faster processing of information and even parallel computation.
It was once thought that right-handed people were dominated by the left hemisphere, while left-handed people were dominated by the right. In fact, not much is known about this. A recent experiment showed that right-handed people tend to suppress one hemisphere of the other, while left-handed people tend to work in concert. True, the participants performed only one simple task (pressed a button), and the difference in brain function was manifested only in the fact that left-handers, on average, coped better with their right hand than right-handers with their left. In other experiments, verbal abilities and orientation in space were tested: according to the first indicator, left-handers and right-handers do not surpass each other, but orient themselves a little better than right-handers.
Lefties are likely different from right-handers, but there is a lack of good research on this, and the difference does not appear to be as big as stereotypes say. Not all right-handers listen only to reason, just as not all left-handers have exceptional intuition and creativity. Dyslexia, schizophrenia and other disorders are indeed more common in left-handers, but this does not mean that left-handedness is a pathology.
Left-handers and right-handers began to be studied closely just a few decades ago, and technologies that allow you to look into the brain appeared even later. But this is precisely why there is hope that scientists will soon be able to better understand why we are different and what that means.
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