in 2007, researchers discovered that specific alleles of at least one of three single-nucleotide polymorphisms upstream of the already known LRRTM1 gene, were linked to left-handedness another theory is 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 an increased risk of auto-immune disorders, learning disorders, dyslexia, and stuttering, as well as increased spatial ability. regards source is wikipedia