What causes common balding?

People who become bald have hair follicles that are genetically programmed to miniaturize under the influence of postpubertal androgens. Probably, several genes (inherited from both mother and father) influence the severity of balding. Until very late in the balding process, the number of hairs does not decrease, but the hairs become progressively smaller until they are no longer visible to the naked eye. Except in very marked and long-standing balding, very fine, short hairs can be seen exiting from follicular orifices if a magnifying lens is used.

Randall VA: Androgens and hair growth, Dermatol Ther 21(5):314–328, 2008.