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We searched on "cause of left-handedness" and thumbed through the sites in Yahoo!'s Left-Handers category. We turned up plenty of opinions, but no definitive answer to your question. According to Scientific American's Ask the Experts feature, scientists have been studying "handedness" (i.e., which hand people tend to use for writing and other precise motor skills) for about 160 years, and they still haven't discovered what causes someone to prefer one hand over the other. Most people in the world
are right-handed -- only 5% to 30% are left-handed. No one's sure if left-handedness is genetic, a learned behavior, caused by prenatal or birth trauma, or if it's some combination of these factors. Studies of families and identical twins prove only an indirect genetic cause for left-handedness. Within families, one study found that if both parents are right-handed or if only the father is left-handed, a child has a 1 in 10 chance of being left-handed. If only the mother is left-handed, the odds rise to 2 in 10. If both parents are left-handed, the child has a 4 in 10 chance of being left-handed. So, while left-handedness might be passed from parent to child, the chances are still greater that a child will be right-handed.
Identical twins have identical genes and, if handedness were wholly genetic, both twins would be either right- or left-handed. But in studies of left-handed twins, only 76% of the pairs were both left-handed. Others studies suggest that men are 1.5% to 4% more likely to be left-handed than women. This difference could be linked to the male hormone testosterone. According to the alt.lefthanders Frequently Asked Questions page, some scientists speculate that exposure to excess levels of testosterone during pregnancy may cause babies to be left-handed. Adult females normally produce a small amount of testosterone, as do male fetuses -- these two influences could lead to high testosterone levels. But, much like the genetic
link, the testosterone link to left-handedness hasn't been proven convincingly. A recent Swedish study connects ultrasound scans during pregnancy with an increase in left-handed babies. The study examined men born in the 1970s and suggested a 30% increase in left-handedness among those who were exposed to ultrasound (which was less sophisticated at that time) in the womb. The study seems to indicate that prenatal trauma is one possible cause of left-handedness. In the end, no one knows for sure why some people are left-handed and others are right-handed. Maybe if you ask again in 10 years, science will have a more concrete answer.
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