In 1985, a hand preference survey was completed by 973 dizygotic and 1158 monozygotic male twin pairs, all veterans of World War II. This is the largest single twin study of handedness on record. As in state-of-the-art animal research, the laterality criterion was sensitive to both direction and consistency (degree, strong or weak) of handedness. Significant pairwise concordance was shown for the total group, and for consistency and directional factors separately. However, no zygosity differences were demonstrated.