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 gi
rls with an older brother. In opposite-sex dizygotic twin pairs, the u
terine 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 o
bserved playing with sex-typed toys, and mothers completed questionnai
res about the child's activities. Contrary to expectation, girls with
a boy co-twin did not spend more time playing with boys' toys than gir
ls with a girl co-twin or girls with an older brother. Although these
results might suggest that normal variations in hormones do not contri
bute to within-sex variations in childhood activities, they combine wi
th other work to suggest factors to consider in evaluating hormonal in
fluences on human behavior, including level and timing of exposure. (C
) 1997 John Wiley & Sons, Inc.