News Release

Study suggests that tomboys may be born, not made

Peer-Reviewed Publication

Center for Advancing Health

Levels of testosterone during pregnancy appear to influence the gender-role behavior of preschool girls, according to a new study.

Researchers measured pregnant women's levels of testosterone, then evaluated the behavior of their children at age 3 1/2. The greater the maternal testosterone level, the more likely girls were to engage in "masculine-typical" gender-role behavior, such as playing with toys typically preferred by boys. No correlation was found for boys' behavior, however.

The researchers based their hypotheses on animal studies that have shown a correlation between maternal levels of testosterone and behavior in female offspring.

"Because hormones influence basic processes of brain development, they also exert permanent influences on behavior," says lead author Melissa Hines, Ph.D., of City University in London "In both rats and rhesus monkeys, genetic female animals treated with testosterone during critical periods of prenatal or early postnatal life show increased levels of … male-typical play behavior as juveniles."

Hines and her co-authors note that girls with congenital adrenal hyperplasia (CAH), a genetic disorder involving prenatal exposure to high levels of male hormones, tend to prefer masculine-typical toys and activities.

The study results appear in the November-December issue of Child Development.

Participants were part of the Avon Longitudinal Study of Parents and Children, a long-term study of biological, environmental and social factors associated with pregnancy outcomes and child health. A total of 13,998 pregnant women -- who represented 90 percent of all pregnancies occurring in the Avon, England, area during an 18-month period in the early 1990s -- enrolled in the study. Data from 679 offspring of the 14,138 children born during the study were analyzed.

The researchers obtained blood samples from the pregnant women during routine prenatal medical care; 55 percent of the women had blood taken between weeks 8 and 24 of the pregnancy; a quarter of the women had the samples taken between weeks 5 and 7, and the remainder after week 25. The samples were analyzed for levels of testosterone and a hormone that limits the ability of testosterone to act, called sex hormone binding globulin.

Once each child reached age 3 1/2, a primary caregiver completed the Pre-School Activities Inventory (PSAI), which assesses the child's engagement in various sex-typed behaviors, such as play with certain toys, games and activities. Higher scores indicate more masculine-typical behavior. The questionnaire was completed again when the child was 3 1/2.

The authors found a link between testosterone level in mothers and girls' scores on the PSAI, with high testosterone levels related to high "masculine" scores. No relationship was found between testosterone levels and boys' gender-role behavior, however.

The researchers note that "sex differences in childhood gender-role behavior, including toy, playmate and activity preferences, develop as a consequence of numerous influences. … For instance, parents, teachers and peers provide more positive reinforcement for sex-congruent play than for play that is not sex-congruent."

They therefore identified background variables from the data that may be linked to gender-role behavior, such as maternal education, the presence of older brothers or sisters in the home, the presence of a male adult living in the home, and parental adherence to traditional sex roles.

"These background variables cannot account for the observed relation between testosterone and PSAI scores in pre-school age girls," writes Hines.

The authors suggest that the effects were not seen in boys because boys ordinarily are exposed to higher levels of prenatal testosterone.

"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," they write. "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."


The study was supported by the Wellcome Trust. The Medical Research Council, Department of Health, the Department of the Environment, British Gas and other companies also support the Avon Longitudinal Study of Parents and Children. The U.S. Public Health Service (HD 24542) also supports Dr. Hines' research.

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