The Chemistry Between Us: Love, Sex, and the Science of Attraction

by Larry Young

The organizational hypothesis has held up remarkably well through years of refinements and additions. It holds that the default setting is on “female” during fetal development. At about the eighth week of human gestation, testicular cells will start making testosterone from cholesterol. In a typical 46,XY fetus, testes begin to form and they make more testosterone. Later, they will get a boost from the adrenal glands that also make a small amount of testosterone. This testosterone and other androgens that derive from it, like DHT, spark the construction of male genitals from the raw materials available. Testosterone, typically through conversion to other hormones like DHT (and even estrogen), also act in the brain to lay down male-typical neural circuits and permanently change the brain’s chemistry. Later in life, when androgens are secreted, they activate these male circuits giving rise to masculine behavior.