Handedness in opposite and same-sex dizygotic twins: testing the testosterone hypothesis

promote development of the right hemisphere and increase the incidence of sinistrality. This proposition was tested by comparing the hand preference of 59 opposite-sex and 61 same-sex dizygotic twins. Because testosterone is thought to pass between twins in utero, it was predicted that females with a male twin would show a high incidence of sinistrality compared to females with a female twin. Similarly, it was predicted that males with a male twin would be more likely to be sinistral than males with a female twin. Measures of the strength of hand preference and the incidence of sinistrality revealed no difference between the opposite and same-sex twins for either sex. The data also failed to confirm reports that first-born twins are more likely to be sinistral than second-born twins. These data add to a growing body of research which is critical of the testosterone hypothesis. (C) 1999 Lippincott Williams & Wilkins.