Study Suggests That Tomboys May Be Born, Not Made

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 ass