October 11, 2011 7:17 PM   Subscribe

Why does hair "know" when to stop growing?

So hair on you head and face (for men) grows quite long, but hair elsewhere stops after growing a specific length. My armpit hair is longer than my arm hair, which in turn is shorter than my leg hair. COOL.

What is the mechanism that "tells" hair when to stop growing? If I were to shave my arm, the hair would "know" and grow back to normal arm hair length, but no longer. Does head hair have the same "off-switch" at some point? HOW DOES IT KNOW?!
posted by arveale to Grab Bag (11 answers total) 3 users marked this as a favorite
From what I understood, it doesn't stop growing, but it falls out after a certain time. So hair that is shorter (arm hair, pubes, facial hair) just falls out more regularly. I guess then the question is how does it know how often to fall out?
posted by lollusc at 7:18 PM on October 11, 2011 [6 favorites]

From MY understanding all hair has a genetic code that gives it a predetermined set point for how long it can grow.
posted by zephyr_words at 7:20 PM on October 11, 2011

posted by DarkForest at 7:25 PM on October 11, 2011

Ugh, I was hoping the answer wasn't "genes/no one knows."
posted by arveale at 7:32 PM on October 11, 2011

How on earth wouldn't the answer be genes?
posted by RustyBrooks at 7:50 PM on October 11, 2011 [19 favorites]

Genes control pretty much all protein expression; proteins control pretty much everything else. It nearly always comes down to what your DNA codes for.
posted by ellF at 8:14 PM on October 11, 2011

Hair goes through periods of growth with periodic pauses. The hair shaft breaks when the weak spot (from during the slow period) is no longer supported by the skin. So it doesn't grow to length X and stop, it grows for however long and then falls out and then grows for however long again.

There may be a gene that directly governs this, but that's not really the way to bet. (Blah blah emergent properties blah blah.) Also there are all kinds of environmental factors that probably play into it.
posted by Kid Charlemagne at 8:14 PM on October 11, 2011 [3 favorites]

Having gone through several periods of refusing to have a haircut for over a decade, I can assure you that head hair also reaches a maximum length. Over any patch of skin, this happens when the total volume of the hairs that fall out on any given day matches the total volume of new hair grown on that day.

The longer any given hair is, the more likely it is to get caught on something and yoinked out of its follicle. That means that stronger follicles can grow longer hairs before they lose them and have to start over.

You could probably set up an experiment with small rubber bands and superglue to convince you that the average force required to remove a hair from its follicle is lower in those regions of your skin where shorter hairs persist.
posted by flabdablet at 8:35 PM on October 11, 2011

Kid Charlemagne, you mostly have it, but I think -- and I'm not an expert! -- your answer is not quite accurate. Quoting from your link: "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 maximum length -- as with the shorter lengths of other hair locations -- is genetically determined in that the length of time spent in a growth phase ("anagen"). It isn't that it breaks at some point, it's that it goes from anagen to telogen, at which point a new hair begins growing.
posted by ellF at 8:39 PM on October 11, 2011

Straight Dope column.
posted by Chrysostom at 5:51 AM on October 12, 2011

Thanks everyone! Just to clarify, my comment was not that I didn't expect genetics to be involved, I just was hoping that the end of the inquiry wasn't "Well, genes tell the hair when to grow and when to stop growing, but the mechanics are unknown." The periods or growth and dormancy makes a lot more sense now. Thanks again!
posted by arveale at 10:56 AM on October 12, 2011

« Older Lost in Translation   |   Typical 20th century typefaces? Newer »
This thread is closed to new comments.