It is well established that the distribution of the combinations of handedness (LL, LR, and RR) in twin pairs is roughly binomial regardless of zygosity or sex. This fact has challenged current genetic models of handedness. We show here that: (a) there must be at least one environmental determinant of handedness; and (b) the binomial distribution does not in itself invalidate genetic models of handedness. In singletons, left-handedness is strongly associated with low birth weight. In twins, this association is weaker (apparently as a consequence of a stronger association with birth order--i.e., with being first-born--within the pair). We speculate that the hazards associated with being first-born in twin pairs (e.g., trauma) are more closely associated with left-handedness than are the hazards associated with being second-born (e.g., hypoxia).