Very interesting one about Left-handedness. A week ago, happened to see a documentary in NGC on the same. Few points are here for you.
Left-handedness is the preference for the left hand over the right for everyday activities such as writing. Most left-handed people exhibit some degree of ambidexterity. Left-handedness is relatively uncommon; seven to ten percent of the adult population is left-handed.
Causes of left-handedness
• Hand orientation is developed in fetuses, most commonly determined by observing which hand is predominantly held close to the mouth
• Long-term impairment of the right hand: People with long-term impairment of the right hand are more likely to become left-handed, even after their right hand heals. Such long-term impairment is defined as eight months or more.
• Testosterone: Exposure to higher rates of testosterone before birth can lead to a left-handed child. This is the Geschwind theory, named after the neurologist who proposed it, Norman Geschwind. It suggests that variations in levels of testosterone during pregnancy shape the development of the fetal brain. Testosterone suppresses the growth of the left cerebral hemisphere and so more neurons migrate to the right hemisphere. The highly developed right hemisphere is now better suited to function as the center of language and handedness. The fetus is more likely to become left-handed, since the right hemisphere controls the left half of the body. The theory goes on to tie the exposure to higher levels of testosterone and the resultant right-hemisphere dominance to auto-immune disorders, learning disorders, dyslexia, and stuttering, as well as increased spatial ability.