Many of us think of hormones as the gender-specific molecules we learned about in middle school health class—the chemical messengers that arrive during puberty to govern our reproductive development. But sex steroids like testosterone and estrogen also play a critical role in brain development even before adolescence: shaping, activating, and fueling sexually dimorphic brain circuits. These circuits are not limited to those involved with romantic and sexual entanglements. Many have been implicated in complex behaviors including the stress response, learning, and memory—as well as linked to the development of several psychiatric disorders.
Hormones also help guide brain development. That guidance begins in the womb, starting with genes present on the X and Y chromosomes. As you may know, an individual’s sex is determined by these chromosomes. Females are given two X chromosomes, while males have one X and one Y. A variety of genes on these chromosomes help to provide a blueprint for the brain’s architecture.
For example, the Y chromosome contains a gene called SRY, which triggers a flood of androgens, the group of hormones that includes testosterone, in the womb during the first trimester. This flood not only directs the development of the penis and testes, but also drives changes to the brain’s architecture, setting the stage for “male” brain circuitry.
