Tomboys May Be Born, Not Made

Testosterone Levels During Pregnancy Linked to Behavior

From the WebMD Archives

Nov. 12, 2002 -- The difference between a girl who enjoys playing ball or playing house may lie in how much testosterone she was exposed to in the womb, according to a new study. Researchers say they've found a link between testosterone levels during pregnancy and gender-role behavior of pre-school girls.

The study found that the more testosterone a girl was exposed to in the womb, the more likely she was at a young age to engage in tomboy-like behavior, such as playing with toys that boys typically prefer.

The results appear in the November-December issue of Child Development. Researchers say previous studies in animals have found a similar relationship between maternal testosterone levels and behavior.

Using data from the Avon Longitudinal Study of Parents and Children (a long-term study of pregnancy and child health), researchers looked at 679 children born during an 18-month period in the early 1990s. Blood samples were taken from the mothers during pregnancy and analyzed for levels of testosterone (the male sex hormone).

Once the child reached age 3 ½, a caregiver completed a questionnaire about the child that assessed the child's involvement in various gender-role behaviors, such as playing with sex-typical toys, games, and activities.

Researchers found a link between high testosterone levels among the mothers during pregnancy and higher "masculine" scores of their daughters, but no relationship was found between testosterone levels and boys' gender-related behavior.

Researcher Melissa Hines, PhD, of the City University in London, and colleagues say other factors such as maternal education, older brothers and sisters in the home, a male adult in the home, and parental adherence to traditional sex roles may also affect the development of tomboy-like behavior. But they say those factors still did not account for the association found between testosterone and behavior found in the study.

Even so, the researchers say that the relationship between testosterone and child behavior is small, and accounted for only 2% of the variance in the gender-role behavior found in pre-school girls in this study, which leaves plenty of room for the other factors to play a role.

Researchers say girls may be especially susceptible to the effects of testosterone because boys are ordinarily exposed to higher levels of prenatal testosterone, but social factors may also have an impact.

"Compared to girls, boys are more strongly encouraged to behave in sex-typical ways and are more strongly discouraged from engaging in cross-gendered behavior," write the researchers. "Thus, girls may be more likely than boys to manifest hormone-related predispositions to gender role behaviors more characteristic of the other sex, because these predispositions are less likely to be counteracted by other influences."

SOURCE: Child Development, November-December 2002

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