Male pattern baldness, also known by its clinical name androgenetic alopecia, is caused by chemistry. It starts with testosterone, the male sex hormone. Testosterone is harmless until it gets converted to dihydrotestosterone (DHT) while in contact with oil glands in the hair follicles. These glands hold the enzyme Type II 5-alpha reductase, which changes the male hormone.

DHT, in turn, gets in follicles and shrinks them. While simultanously making the protective membrane of the scalp thicken, restricting blood flow to the capillaries. This makes future hair growth impossible. The hairs fall out at a natural rate, but aren't replaced with new hairs.