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

Prenatal exposure to testosterone is proposed to 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.