text: No one knows for certain why the human population is right-handed-dominant, but a number of theories have been proposed.\n\nBrain hemisphere division of labor is the most commonly accepted theory of handedness. The premise of this theory is that since both speaking and handiwork require fine motor skills, having one hemisphere of the brain do both would be more efficient than having it divided up. And since in most people, the left side of the brain controls speaking, right-handedness would prevail. It also predicts that left-handed people would have a reversed brain division of labor. Lastly, since other primates do not have a spoken language (at least of the type we have) there would be no stimulus for right-handed preference among them, and that is true.\n\nObjections to this theory claim that it does not explain why the left hemisphere would always be the one controlling language. Why not 50% of the population left and 50% right? While 95% of right-handers do indeed use the left side of the brain for speaking, it is more variable for left-handers. Some do use the right for linguistic skills, some use the left hemisphere, and others use both. On the balance, it appears that this theory could well explain some left-handedness, but it has too many gaps to explain all left-handedness.\n\nThere are more than genetics involved in determining if a person is right- or left-handed Even when both parents are left-handed, there is only a 26% chance of their child being left-handed. Thus, it is clear that genetics is not the only cause. Handedness must also be influenced by some other causes.\n\nBirth stress is theory that states disturbs certain left-handed people, because its basic premise is that left-handedness is due to brain damage during the birth process. Nevertheless, some statistics do back this theory up.\n\nParental pressure theory explains right-handed dominance by claiming that since the parents who raised us are mostly right-handed, we came to be mostly right-handed and so on.