Sex-typed play in opposite-sex twins

To examine possible prenatal hormonal influences on sex-typed play, we compared girls with a boy co-twin to girls with a girl co-twin and girls with an older brother. In opposite-sex dizygotic twin pairs, the uterine environment may allow transfer of testosterone from the male to the female fetus. Singletons with an older brother provide a control for shared social environment. Girls aged 3 to 8 years (N = 91) were observed playing with sex-typed toys, and mothers completed questionnaires about the child's activities. Contrary to expectation, girls with a boy co-twin did not spend more time playing with boys' toys than girls with a girl co-twin or girls with an older brother. Although these results might suggest that normal variations in hormones do not contribute to within-sex variations in childhood activities, they combine with other work to suggest factors to consider in evaluating hormonal influences on human behavior, including level and timing of exposure.