Researchers have posed various theories about the causes of left-handedness. Various statistics, such as identical twins with different handedness, have blown most genetic theories. Some theorized that differences in development in the womb was a factor. This NY Times article describes Dr. Amar J. S. Klar’s hypothesis that there is a specific dominant gene that causes right-handedness. If that gene is not present, a person has a 50-50 chance of being right or left-handed. William H. Calvin has a wonderful essay that presents the natural selection-based theory that females carrying infants next to their hearts and throwing stones at small animals led to right-handedness predominance.