* 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.[10]

* Long-term impairment of the right hand: Studies show that 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 may include from 6 months or more.

* Testosterone: According to one theory, exposure to higher rates of testosterone before birth can lead to a left-handed child[11]. 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: A popular theory is that 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 [12] [13]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”

Clare Porac, a professor of psychology at Pennsylvania State University who studies handedness, explains:

The genetic proposal to explain hand preference states that there are two alleles, or two manifestations of a gene at the same genetic location, that are associated with handedness. One of these alleles is a D gene (for dextral, meaning ¿right¿) and the other allele is a C gene (for ¿chance¿). The D gene is more frequent in the population and is more likely to occur as part of the genetic heritage of an individual. It is the D gene that promotes right-hand preference in the majority of humans. The C gene is less likely to occur within the gene pool, but when it is present, the hand preference of the individual with the C gene is determined randomly. Individuals with the C gene will have a 50 percent chance of being right-handed and a 50 percent chance of being left-handed.

so, they tend to be a left-handed person. . .