The number of left-handers is estimated at 10% of the human population against 90% of right-handers, this numerical superiority makes that today we live in a world made essentially for the right-handers. This disproportion may be due to several criteria, notably the genetic factor and the manual preference that we will analyze in this article.
The innate and the acquired
When one of the parents is left-handed, there is a 10 to 17% chance that the child is left-handed, and if both parents are left-handed, 46% of the children are left-handed. Moreover, two Anglo-Saxon studies concerning the frequency of left-handed children according to sex and the manual preference of the parents also show that for the transmission to be hereditary, the mother (for example) must be left-handed Even if both parents are.
A study conducted by Professor Chris McManus in 2003 showed that two left-handed parents have an average of 26% chance of having a left-handed child. Another study on twins was carried out in 2006 and emphasized that the heritability of manual handwriting preference is 24%. These two studies thus show that the probability of being left-handed is little or not linked to the genetic factors and that it is essentially of the acquis.
Other observations, on the other hand, assume that being left-handed is innate or possibly that the child is born with a predisposition to being. It is possible to know if the newborn will be left-handed from the 29th week of gestation by testing his reflexes through stimuli. In September 2013, an article published in the journal PLOS Genetics highlights the fact that the PCSK6 gene plays a preponderant role in manual preference. It appears that its suppression in certain studies leads to a totally asymmetrical evolution of the embryos. The manual preference would therefore be partly genetic at the time of lateralization of the embryo.
The lateralization of the brain
The human body is lateralized, there is a predominance of the left or right part of the body; It is the same for the brain whose areas are asymmetrical in both cerebral hemispheres. In 1861, neurologist Paul Broca discovered that part of the left hemisphere is connected to language and controls the movements of the right limbs and vice versa for the right hemisphere.
The left hemisphere, responsible for the language and movements of the right limbs, allows us to pronounce words and articulate them through a precise muscular control. The subtlety of this joint affects the movements of the right hand.
The role of testosterone
Since there are more left-handers than left-handers, it would seem that there is a link between sex and manual preference and therefore between testosterone and manual preference. The development of the right hemisphere, dominant in the left hand, is favored by a high testosterone level.
As mentioned at the beginning of this article, we live in a world intended for right-handers so if they want to get out, left-handers must either become right-handed or left-handed and acquire objects made especially for them (scissors, computer mice , Keyboard etc.).
How do left-handers stand out?
Left-handers also have to deal with spatial difficulties because some do not visualize space in a "normal" way; It is the case of the directions or directions of rotation which for them are reversed: they have difficulty to locate and when they have to make a move in a clockwise direction, they have some difficulties.
It seems that left-handers use certain areas of the brain that are not exploited by right-handers, and are known for their artistic and creative side. Many celebrities recognized as geniuses were left-handers, Leonardo da Vinci, Michelangelo, Jimi Hendrix and Marie Curie. This is due to the fact that their sense of space and forms is more developed.
Formerly considered pariahs who needed to be put on the right path, left-handers were able to adapt to a world of right-handers by making their difference a force. They have created associations, forums on the internet and even created the International Left-Handed Day which takes place on 13 August.