Why most people are right-handed

2024 Author: Malcolm Clapton | [email protected]. Last modified: 2023-12-17 03:44

What will you answer if a child asks why he is right-handed? This is a very interesting question to which there is no definite answer. But we will try to figure it out.

The answer, it would seem, is simple: because more than half of humanity is right-handed. Excluding small differences by region and time period, about 90% of people were and still are right-handed.

Most of the time, when it comes to the dominant hand, the focus is on the left-hander. There are fewer of them and this is considered unusual. In fact, we do not know what is considered common and what is not, since the reasons for the dominance of the left or right hand have been little studied. Scientists used to think that the differences between left-handers and right-handers are in heredity, but in reality this is not the case.

Asymmetry

Humans are very asymmetrical creatures, and this applies not only to how we use parts of our body, but also to how they are positioned. For example, the heart is usually displaced to the left, and the liver is usually displaced to the right. In addition to asymmetry, there is also a relationship between them, and not always direct.

The left hemisphere of the brain is responsible for the work of the right limbs, and the right hemisphere is responsible for the work of the left.

This connection can be traced using a special technique invented in the 1950s by the Japanese physician Juhn Atsushi Wada. The test of the same name is used to prepare brain operations and allows you to identify the belonging of certain functions to a particular hemisphere. During the test, an anesthetic is injected into one of the carotid arteries of a person, which literally turns off the corresponding hemisphere of the brain for several minutes. The results are very interesting.

So, 90% of right-handers, after turning off the left hemisphere, cease to control the right hand and lose the ability to speak. But the odd thing is that 70% of left-handed people are also speechless when the left side of their brain is paralyzed. For the rest, the ability to process speech by the right hemisphere or both is approximately equally distributed. No one really knows why most right-handers are asymmetrical and most left-handers are not.

Genes

Previously, dominance of one of the hands was considered a feature that was easy to explain. One of the most popular genetic models was proposed in 1985 by psychologist Chris McManus and suggested that a single gene is responsible for this, which comes in two varieties: dextral (D) and chance (C). The first is responsible for right-handedness, and the second adds randomness in the literal sense of the word. The combination of the parents' genes determines whether the child will be right-handed or left-handed: DD is right-handed, CC is left-handed with a 50 percent probability, and CD is left-handed with a 25 percent probability.

However, in 2013, a study was published in the journal Heredity showing that it's not just inheritance that affects right-handedness. After analyzing the genomes of 3,940 twins, the scientists found that the probability of the same dominant hand in identical twins with completely identical genes practically does not differ from that of fraternal twins, the genetic similarity of which is no more than that of ordinary brothers and sisters. Moreover, they did not even manage to find a single common gene in unrelated people who had the same leading hand.

This suggests that simple genetic models are already outdated, but scientists are in no hurry to write them off. In their opinion, heredity still affects right-handedness, but not completely, as previously assumed, but only by 25%. And this is very unusual.

For comparison, blood groups that affect transfusion compatibility are based on simple heredity and are almost 100% dependent on parental genes. Human growth is more complex, influenced by over 300 different genes. The strongest of these adds only about 4 millimeters of height. In different studies, the numbers differ slightly, but scientists agree that 60-80% of a person's height depends on heredity. Body mass index, color of skin, hair and eyes, as well as other physical traits, have very high heritability rates of 60–70% or more.

Society

So, if our right-handedness does not depend entirely on genes, then what is the cause of it? It is known that the prevalence of right-handers or left-handers is influenced by social influence. In most Western cultures at the beginning of the 20th century, left-handers were forcibly retrained as right-handers, sometimes even by violent methods. Whereas in Australia, according to a 1981 study, the number of left-handed people in the population increased from 2% to 13.2% between 1880 and 1969. Scientists explain this by the fact that it was more convenient to be left-handed and people became left-handed.

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

As for the leading hand of our closest relatives - monkeys, here scientists disagree. Some researchers who have studied chimpanzees and other monkeys believe that the probability of right-handedness in individual individuals is about 50 to 50. Others disagree, citing studies of more specific animal behavior, such as using a tool. Their results indicate that most monkeys are right-handed. The ratio of right-handers to left-handers among monkeys is much lower than among humans, at 2: 1 versus 9: 1. However, the genetic factor in monkeys is more pronounced, so they can presumably inherit the right-handedness or left-handedness of their ancestors.

Output

It is very difficult to collect all this evidence together and unambiguously answer the question of whether right-handedness is congenital or acquired. According to scientists, it turns out that heritability does not always depend on genes alone.

We are right-handed because of our genes, our culture and other factors that affect us before and after we are born.

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

One way or another, right-handedness is a prime example of how our view of genetics has changed. We used to think that everything depends on a single magic gene, but in reality everything turned out to be much more complicated.