17 | AGO | 2019
What causes people to be left-handed?
International Left Handers Day is celebrated on August 13th – Photo: File photo/EL UNIVERSAL

What causes people to be left-handed?

13/08/2019
21:10
Mexico City
El Botiquín.mx
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Social factors are also related to left-handedness

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Daniel Abrams, co-author of a study published in the magazine Life, revealed that the most important factor for a society to be more efficient is a high level of cooperation, which in humans has generated a majority of right-handed individuals.

Throughout history, left-handed people have not always been treated with dignity. For many years they were chased for being wicked, or even, as witches, although they represent 10% of the population.

Actually, the word “sinister” comes from “left,” and despite it being a long time since the persecution of the left-handed, there are still hypotheses of why they write with that hand.

Some theories have tried to explain why some people are left-handed, including the obsolete idea of being related to the stress of the mother during pregnancy.

Now, a study published in the magazine Life found that the answer could be on the spinal cord. The research, developed by specialists from Germany, Holland, and South Africa, discovered that the activity of the genes in the spinal cord was asymmetric in the uterus and could be what makes a person to be left or right-handed.

The movements of the arm and the hand start in the brain in an area called motor cortex, which sends a signal to the spinal cord that translates in movement.

Researchers found that while the fetus is growing, up to 15 weeks, the motor cortex and the spinal cord are not connected yet, but the determination of left or right-handedness has taken place.

In other words, the fetus can start with movements and chooses its favorite hand before the brain starts to control the body. To study this, researchers analyzed the gene expression in the spinal cord between the eighth and twelfth week of pregnancy.

They found a significant difference in the left and right segments of the spinal cord, which control the movement of arms and legs and concluded that the asymmetric nature of the spinal cord could be because of epigenetics, or how organisms are affected by the expression of their genes instead of by the genes themselves.

These changes are usually caused by environmental influences and can affect the growth of the baby. These differences can affect the right and left sides of the spinal cord differently, which would cause left-handed and right-handed individuals.

For a long time, scientists have tried to find out why left-handed people are so rare. In 2012, researchers from Northwestern University developed a mathematical model to show that the percentage of left-handed people was the result of human evolution, specifically, of a balance in cooperation and competence.

In other words, they thought that, although the base for left or right can be genetic, there could be a social factor that explains why the proportion is so high.

The more social the animal, in which cooperation is highly valued, the more its population will be inclined to a certain side, said Daniel Abrams to Life Science.

The most important factor for an efficient society is a high level of cooperation, he added. In humans, this has resulted in a majority of right-handed people.

That is, it is possible that, for some reason, we have evolved to favor our dexterity, for which any person that deviated from this could have been conditioned to use that hand mainly, despite the genetic predisposition.

As a matter of fact, Judith Schmitz, one of the authors of the new study, said to Business Insider that the studies on twins have shown that the contribution of genetics to laterality is of around 25%.

The new study could not explain the right-handed majority, but Schmitz said how research on birds can show how genetics and the environment could be the cause.

In chickens and doves, the genetic factor determines the position in the egg before hatching: the embryo is curved in a way that the right eye turns toward the translucid eggshell, while the left eye is covered by the embryo itself, she said, and added: Thus, the right eye is stimulated by light before hatching, while the left eye does not receive light.

This combination induces visual asymmetry. If eggs are hatched in the dark, the asymmetry is prevented. But why exactly are there left-handed people? It is still a mystery, in part because left-handed people are usually excluded from research, according to experts, and it is hard to predict if a child will be right-handed or left-handed.

However, one thing we do know is that neurological differences between right-handed and left-handed people are small, and the supposed distinctions in behavior or in the psyche have been greatly discredited.
 

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