Approximately 10% of the world population is left-handed. What causes this anomaly?
While there are several theories that try to explain this trait -- such as the positioning of the baby during the final trimester, hormone exposure during pregnancy, or epigenetic factors -- the most consistent and hardwired explanation boils down to plain old genetics. In other words, the reason you're right-handed or left-handed is because it's written in your DNA.
Research suggests that handedness displays a complex inheritance pattern. For example, if both parents of a child are left-handed, there is a 26% chance of that child being left-handed. A large study of twins from 25,732 families also indicated that the heritability of handedness is roughly 24%.
To date, two theoretical single gene models have been proposed to explain the patterns of inheritance of handedness. However, the growing weight of evidence from linkage and genome-wide association studies suggests that genetic variance in handedness cannot be explained by a single genetic location. Rather, handedness appears to be polygenic, and is possibly determined by 40 different genes acting on this one trait!