SCIENCE FILE: An exploration of issues and trends affecting science, medicine and the environment.

Q: Why do people, especially men, go bald?

A: Scientists do not fully understand the balding process, but it is clearly a combination of aging, male hormones and heredity. In people who inherit the as-yet-unknown gene for baldness, hair follicles shrink with age until they produce only very thin, nearly invisible hairs. This shrinking process requires the presence of the male hormone testosterone, which is why male-pattern baldness affects mostly males. In the absence of testosterone--as in males who have been castrated--a full head of hair remains even when there is a family history of baldness. Non-genetic loss of hair can be caused by drugs, infections, excessive amounts of oil or dandruff and hair treatments that tighten the scalp and damage hair.