There are some theories about
The most accepted theory is that each hemisphere of the brain specializes in one type of movement – it is called cross-dominance. The organ is divided into two sides, right and left, which are not symmetrical. Each of them controls the members on the opposite side. Thus, if the hemisphere that has the largest primary motor cortex (area linked to manual ability) is the right hemisphere, the person is left-handed. If it’s the left, it’s right-handed.
But when the subject is cross dominance, there is no consensus among experts. There are theories that say one hemisphere specializes in routine movements, such as opening doors, and the other in responding to contingencies, such as reacting to a ball thrown toward you. Others argue that one hemisphere deals with delicate movements (writing), and another of the crudest (kicking) movements. Therefore, people with cross-dominance differ from ambidextrous ones, who have the most symmetrical brain areas and use all members in the same way.
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The reasons that make you use this or that hand
In 2007, researchers at the University of Oxford (UK) discovered that the LRRTM1 gene is linked to left-handed. But science still can not tell how this relationship works
In contrast to the theory of cross-dominance, a German research shows that it is actually the spinal cord that determines the handedness. Although the motor cortex and marrow are connected, they develop in parallel. In addition, dominance manifests first in the marrow and then passed to the cortex
The American neurologist Norman Geschwind (1926-1984) argued that hormones in pregnancy (produced in women by the ovaries) can affect the fetus. A high testosterone level, for example, would influence the formation of the right hemisphere of the brain. This would increase the chances of the baby being left-handed
Until the middle of the 20th century, schools taught children to write only with their right hand. Whoever was left-handed had to turn around to learn to use the other hand, not to receive serious punishment. So many right-handed adults are, in fact, left-handed
CONSULTING Marcio Balthazar, an expert in cognitive neurology and the Neurology Department of the behavior of Unicamp
SOURCE Article Epigenetic regulation of gene expression fetal spinal lateralized underlies hemispheric asymmetries (various authors), journal Molecular Psychiatry and sites The Globe and Science Daily