Influence of Testosterone on Fetal Brains

Definition

• Testosterone is considered the male sex hormone, but in fact both men and women have it in their bodies. According to "Understanding Human Sexuality," in men testosterone is "secreted by the testes and is responsible for stimulating secondary sex characteristics such as beard growth, maintaining the genitals and their sperm-producing capability, and stimulating the growth of bone and muscle." In fetuses, the level of testosterone seems to affect the personality and preferences of an individual.

Testosterone in Development

• Chemical mixes vary as fetuses go through stages of development. Although both boys and girls have testosterone, levels vary greatly between genders, especially during specific stages of growth. Melissa Hines of the Department of Psychology at City University in London, UK, found that in humans, there appears to be a dramatic difference between hormone levels of girls and boys between weeks eight to 24 in the womb. In weeks 34 to 41, boys typically have 10 times more of the masculine hormone than girls. Hines concludes that these differences appear to have long-term effects in gender differentiation.

Effects

• Scientists Dick F. Swaab, MD, PhD, and Alicia Garcia-Falgueras, PhD, at the Netherlands Institute for Neuroscience in Amsterdam think that fetal peaks and valleys of testosterone and other chemicals fix the structures and circuits in the brain. In other words, the organization and programming of a boy or girl's brain seems to be affected by these gender-related differences.

Case Studies

• In a case study on rats called "Sexual Differentiation of the Brain," R.W. Goy and Bruce McEwen showed that after injections of testosterone, female rats showed an increase in rough-and-tumble play, behavior that's more typical to males. The same trends are seen with humans. Girls with congenital adrenal hyperplasia (CAH), a disorder that causes an increase in masculine hormones, also show play more common in boys. Hines noticed girls with CAH had "preferences for toys that are usually chosen by boys, such as vehicles and weapons, increased preferences for boys as playmates and increased interest in rough-and-tumble play.

Conclusions

• Over time, these affects of testosterone levels in the fetuses are thought to explain the generalized differences between the sexes, including the way people feel masculine or feminine, how they speak and their levels of aggression. Hines concludes that "just as testosterone levels affect the formation of external genitalia, so they affect the behavior of the sexes."