What Causes Left-Handedness?
There are many factors that contribute to handedness, and they are not completely understood. The influencers are both genetic and environmental. This is proven by studies of identical twins: the idential twin of a left-handed person has a 76% chance of being left handed. This probability is greatly elevated compared to the overall chance of being left-handed, indicating that genetics is involved, but if only genes were involved, the probability would be 100% because identical twins have identical genes. Furthermore, in 2007, scientists discovered a gene that appears to be linked to left-handedness.1 The gene is thought to affect brain symmetry and to be inherited by people from their fathers.
Other hypothesized contributors to left-handedness include exposure to high levels of testosterone before birth2 and ultrasonic scans on unborn children. These suggestions, however, are not well supported.