As the left-handed ones know well, the world still is of the dexterous. Shears, abridores of can, corkscrews, accessories of computer, and until spoons pies for babies are made for the right hand. But why more than 90% of the people prefer to use the right hand? In the truth, why exists any preference for the use of one of the hands, whichever?
It is not necessary specular very to find an advantage for the preferential use of one of the hands. If it did not have preference, the two sides of the body, equally capable, could enter in dispute in the hour to start to walk, to reach the key of the car, to play the ball, or to catch the pencil and to write. Already it imagined to have that to lose time deciding with that foot to kick the soccer ball that falls of the sky redondinha, alone between you and the goal? “The automatic” preference for one of the sides of the body decides this problem, and thus you can concentrate yourself in more important things, as to dribble the defense if approaching.
Beyond deciding the problem of the choice, the preference for one of the sides of the body is economic: all the efforts of the training can be concentrated in one alone hand, arm, or foot. The problem is that the beneficial effect of the practical one are specific for the trained side, and therefore would not be very economic to need to train the two hands to make the same task. Already thought how much time people would pass in the school if everybody needed to learn to write well with the two hands? Or it learned to touch violão of the two sides? Or it trained a booty however with an arm, however with the other?
Everything this justifies the lateralização, and not only in the man. In fact, other animals that use the hands to eat or to manipulate objects, as birds, mammals, and even though sapos, also have preference for one of the legs. The difference is that the distribution of preferences is more democratic in them of what in the man. In the mouse, for example, 50% of the animals prefer to use the leg esquera, and 50% prefer the right. The really cabeluda question is why in the men the preference is 90% of the times in favor of the right side of the body, controlled for the left side of the brain.
What it has of so special in the left side of the brain? Pra colloquy start, has the first function whose localization in the cerebral cortex was discovered: the language. In 96% of the people, it is the left side of the brain that shelters the capacity says to produce it. It will be that the use of the right hand can be only one consequence of speaks to be dominated by the left brain?
Yes and not. Yes because, as it would be of if waiting, the incidence of control of speaks for another side of the brain is bigger in the left-handed people: 27% of them speak with the right side of the brain, against 15% of the ambidextrouses, and only 4% of the dexterous. But this cannot be everything - senão, seventy percent of the left-handed people would not have to be left-handed…
Which is then the base for the preference for the right hand? Not yet it is known to the certainty (oooooohh…), but it is certain that the explanation must take in consideraçnao a series of different factors:
1- genetics, since much above of the 8-10% of the population in general is known that it has families with high incidence of preference for the left hand. The possibility to have a left-handed child is of 10% when father and mother are dexterous, 20% when or the father or the mother is left-handed (especially she will be the mother), and 26% when both are left-handed. Famous examples in history are the English real family and the Scottish clan of the Kerr - who, also, in contrast constructed in its castles stairs spirals, the counter-clockwise direction, that gave left-handed advantage espadachins of the family.
2- hormones, as the testosterone, that in such a way influence the functional lateralização of the brain how much the difference between the sexos, since it has left-handed more (12,6% of the men) of what left-handed (9,9% of the women) in the world. Said the difference thus it can not seem much, but making the accounts, it has 30% almost more left-handed men of what left-handed women! One of the most accepted theories prayer that an increase of the effect of the testosterone in the brain during the gestation would delay the development of the left side of the brain, giving advantage to the right side.
3- the partner-cultural as the repression of the left-hand use or same influence the forced conversion;
4- mechanical factors, as the position of the embryo in the uterus, that influence the development of the functional lateralização in the brain before exactly of the birth, since to the rising the brain already has assimetrias, and already exist differences in the movement of the two hands