Why does head hair grow indefinitely but other body hair doesn’t?

Dear Cecil: Why does head hair (as well as beard hair, I think) grow indefinitely, whereas hair on the rest of the human body grows to a certain length and then stops? Kim M., Washington, D.C.

Cecil replies:

What makes you think scalp hair grows indefinitely? Didn’t you ever listen to the title song from Hair (“Don’t never have to cut it ‘cuz it stops by itself”)? Each scalp and beard hair grows two to six years before stopping, attaining a typical maximum length of two to three feet. Then it becomes dormant for about three months, whereupon a new hair starts growing and pushes the old one out of the follicle from behind. That’s why even someone who’s not balding loses 70 to 100 head hairs a day. By comparison, the growth cycle for other body hair is only a few weeks.

Maximum scalp-hair length varies greatly among individuals. The all-time champ had 10-1/2 feet, although Diane Witte of Worcester, Massachusetts, is said to be closing in fast. Diane, whose hair grows at the rate of a half-inch per month, had 10-1/4 feet as of 1988 and by now has surely broken the record. Cecil’s staff contacted Ms. Witte in hopes of being there for the historic finale, but we were unable to agree on terms. No matter. I’m sure it’ll turn up eventually on “Wide World of Sports.”

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.