Why do most people - right-handers

The answer seems to be simple: because more than half of humanity - the right-hander. Excluding small differences depending on the region and time period, about 90% of the people were and are right-handed.

Most often, when it comes to the dominant hand, all the attention is paid to left-handers. They are smaller, and it is considered unusual. In fact, we do not know what is considered usual and what is not, because the causes of the dominance of the left or right hand, have been little studied. Previously, scientists believed that the differences between left- and right handed in heredity, but in reality it is not.

Asymmetry

People - very asymmetric creation, and this applies not only to the way we use parts of his body, but also to how they are arranged. For example, the heart is usually shifted to the left, and the liver - right. In addition asymmetry, there is also a relationship between them, and not always a straight line.

For work right limb meets the left brain, and left for work - right.

This relationship can be tracked using a special technique, devised in 1950 by the Japanese doctor Chon vada (Juhn Atsushi Wada). The self-titled test is used for the processing operations on the brain and identifies affiliation of certain functions of a specific hemisphere. During the test in one of the carotid arteries of human anesthetic is administered, which literally disables corresponding brain hemisphere for a few minutes. The obtained results are very interesting.

Thus, 90% of right-handed people the left hemisphere after disconnection no longer control the right hand and lose the ability to speak. But the strange thing is that 70% of left-handers also deprived of speech when paralyzed the left side of their brain. The rest of the faculty of speech processing the right hemisphere or both shared about equally. No one knows why the majority of right-handers are asymmetrical, and the majority of left-handers - no.

genes

Previously, the dominance of one of the arms is considered especially that easy to explain. One of the most popular genetic models was proposed in 1985 by psychologist Chris McManus (Chris McManus) and it suggests that it is responsible for a single gene, which comes in two varieties: dextral (D) and the chance (FROM). The first is responsible for right-handedness, and the second adds a chance in the truest sense of the word. The combination of the genes of parents determines the child is right-handed or left-handed: DD - the right-hander, SS - left-hander with a 50 percent chance, and the CD - left-hander with a 25 percent probability.

However, in 2013 in the Journal of Heredity it was published a study showing that right-handedness affects not only heredity. Having analyzed the genomes of 3,940 twins, the researchers found that the probability of the same dominant hand in identical twins with a fully identical genes is virtually identical to that of fraternal, the genetic similarity of which no more than ordinary brothers and sisters. Moreover, they even could not find a single gene in a common way of unrelated people having the same dominant hand.

This suggests that the simple genetic models have become obsolete, but scientists are in no hurry to write them off. According to them, heredity still affects the right-handedness, but not completely, as previously assumed, but only by 25%. And this is very unusual.

For comparison, blood group, affecting compatibility with the transfusion, based on a simple heredity and almost 100% dependent on the parental genes. Man's height - a more complicated thing, which affects more than 300 different genes. The strongest of them adds only about 4 millimeters in height. In various studies, the figures are slightly different, but scientists agree that 60-80% of the human growth depends on heredity. In the body mass index, skin color, hair and eyes, as well as other physical traits heritability figures are very high and make up 60-70% and higher.

Society

So, if our right-handedness does not depend entirely on the genes, what is its cause? It is known that the prevalence of right-handed or left-handed affects social influence. In most Western cultures at the beginning of the XX century lefties forcibly retrained in the right-handed, sometimes by violent means. While in Australia, according to held in 1981 to research, the number of left-handers in the population between 1880 and 1969 increased from 2 to 13.2%. Scientists attribute this to the fact that being left-handed was easier and more people become left-handed.

However, cultural differences do not fully explain the right-handedness.

As for the leading hand our closest relatives - the apes, there scientists disagree. Some researchers studying chimpanzees and other apes, believe that the probability of right-handedness of individual species - about 50 to 50. Others do not agree with them, referring to the study of more specific behavior of animals, such as the use of tools. Their results indicate that the majority of the monkeys - right-handers. Value for righties and lefties among monkeys is much lower than among men, and is about 2: 1 to 9: 1. However, the genetic factor in monkeys is expressed more clearly, so they may be expected to inherit the right-handedness or left-handedness of their ancestors.

Output

Collect all the evidence together and clearly answer the question whether the right-handedness of congenital or acquired, is very difficult. According to scientists, it turns out that heritability is not always dependent on genes alone.

We right-handed due to genes, because of our culture, and other factors that affect us both before and after birth.

Higher level testosterone in the womb increases the likelihood that the child will be left-handed. After the birth of the children copy the behavior of their parents, which also affects the determination of the dominant hand. Much depends on the environment in which a child grows and develops.

Anyway, right-handedness - a vivid example of how to change our view of genetics. Previously, we thought that everything depends on a single gene magic, but it is actually much more complicated.