On Left-Handedness, Its Causes and Costs. The New York Times organizes a discussion of the mystery of why some people are left-handed around the work of a geneticist who believes that about twenty per cent of the population lacks a specific dominance gene that makes others right-handed; people without the gene have a 50-50 chance of being right- or left-handed. Most interesting fact for me in this discussion: around 18% of identical twins have different handedness.