What causes balding?

Balding occurs when hair follicles shrink and become less productive.

Instead of the new hairs being cultivated to replace the ones that fall out naturally each day, they are allowed to remain embryonic, at the most, in a light, peach-fuzz stage.

Probably some hormonal action triggers the follicles to behave this way, but it is not known what causes that hormonal action, only that sex, age, and heredity appear to be involved.