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Left-Handed
A person who is left-handed uses his or her left hand more than the right hand. Writing is not as precise an indicator of handedness as it might seem. more...
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A better determination of left-handedness results when the differentiation between an individual's major motor activities versus fine or small muscle motor activities is employed. Thus the applications of one's left-hand for eating, writing and similar fine motor activities would be differentiated from major motor activities such as throwing a ball or swinging a bat, etc. While there are numerous individuals who use their left hand for fine motor activities, there is a much smaller group of people who are "total lefties" i.e. do both fine and major activities with their left hand.
Demographics
In 1998, a study suggested that approximately 7 to 10 percent of the adult population was left-handed. Studies indicate that left-handedness is more common in males than females. Left-handedness, in comparison to the general population, also appears to occur more frequently in identical twins, and several groups of neurologically disordered individuals (such as people suffering from epilepsy, Down's Syndrome, autism, mental retardation and dyslexia). Statistically, the identical twin of a left-handed person has a 76 percent chance of being left-handed, identifying the cause(s) as partly genetic and partly environmental.
Causes of left-handedness
- See main article at handedness.
Hand orientation is developed in unborn children, most commonly determined by observing which hand is predominantly licked or held close to the mouth. Current genetic research suggests there is a genetic factor involved.;
In 2007, researchers discovered LRRTM1, the first gene linked to increased odds of being left-handed. The researchers also claim that possessing this gene slightly raises the risk of psychotic mental illnesses such as schizophrenia.;
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 6 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 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.;
Ultrasound theory: Ultrasound scans may affect the brain of unborn children, causing higher rates of left-handedness in mothers who have ultrasound scans compared to those who do not. This is probably based on a few studies where this relation is studied. In one of these the authors claim that "...we found a possible association between routine ultrasonography in utero and subsequent non-right handedness among children in primary school." However later in the same article the authors state that "Thus the association ... may be due to chance" and "The result was not significant, suggesting that the study had insufficient statistical power to resolve the relationship between ultrasonography and subsequent left handedness in the child";
Read more at Wikipedia.org
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