# How does arm hair know to stop growing? **URL:** \ **Category:** Factual Questions\ **Created:** [November 21, 2001, 4:55pm UTC](https://boards.straightdope.com/t/how-does-arm-hair-know-to-stop-growing/94610 "2001-11-21T16:55:44Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [November 21, 2001, 4:55pm UTC](https://boards.straightdope.com/t/how-does-arm-hair-know-to-stop-growing/94610/1 "2001-11-21T16:55:44Z")
The hair on your head will keep growing but hair elsewhere on your body generally knows when to stop so we luckily don’t have to trim our arm hair once a month. However, if you cut some it knows to grow back. How can hair know to do this? ---
**Author:** ![KneadToKnow](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kneadtoknow/32/3999_2.png) [@KneadToKnow](https://boards.straightdope.com/u/KneadToKnow)\ **Post date:** [November 21, 2001, 5:02pm UTC](https://boards.straightdope.com/t/how-does-arm-hair-know-to-stop-growing/94610/2 "2001-11-21T17:02:16Z")
Here’s [one possible answer](http://www.straightdope.com/classics/a1_097b.html). Hope this helps. ---
**Author:** ![Chez\_Guevara](https://avatars.discourse-cdn.com/v4/letter/c/aeb1de/32.png) [@Chez\_Guevara](https://boards.straightdope.com/u/Chez_Guevara)\ **Post date:** [November 21, 2001, 5:18pm UTC](https://boards.straightdope.com/t/how-does-arm-hair-know-to-stop-growing/94610/3 "2001-11-21T17:18:01Z")
Arm hair, of itself, does not possess sufficient growth awareness to the extent that it knows when to increase in length or thickness commensurate with consistency of this growth. I am not an arm©hair expert on this subject, or any other, but I would hazard that arm hair growth trends are controlled by the Brain. The reduction of Arm Hair Mass by the user sends a message to the Brain which says something like, ‘_Armpit to Brain. Armpit to Brain. Are you receiving me? Over._’ The Brain acknowledges the message with a handshake using a Transparent Protocol. The Armpit then says, ‘_Please send more Hair urgently. Over._’ The Brain notes the problem and says, ‘_OK Armpit. Over and Out._’ More hair then arrives when the Brain has time and opportunity to grow it, bearing in mind its other responsibilities, like thinking about Sex all the time. ---
**Author:** ![Podkayne](https://avatars.discourse-cdn.com/v4/letter/p/edb3f5/32.png) [@Podkayne](https://boards.straightdope.com/u/Podkayne)\ **Post date:** [November 21, 2001, 7:09pm UTC](https://boards.straightdope.com/t/how-does-arm-hair-know-to-stop-growing/94610/4 "2001-11-21T19:09:21Z")
Folicles in different parts of the body are “programmed” to grow for a particular length of time. After they’re done growing, they shut off, and the hair is eventually ejected and a new hair grows in its place. Now, this is basically covered the article **Knead** linked to, though, far be it from me to contradict the master, but I had heard that head hair works the same way, just that the follicles grow longer, whereas Cecil puts forth that it grows continuously. Dunno about that. The length of time that the follicle grows and the thickness of the hair must be controlled somehow by hormones, right? But I doubt that there’s any kind of feedback loop with the brain. (I’m pretty sure **Nostradaus’** response was light-hearted, but not taking any chances. : )) ---
**Author:** ![NotMrKnowItAll](https://avatars.discourse-cdn.com/v4/letter/n/6bbea6/32.png) [@NotMrKnowItAll](https://boards.straightdope.com/u/NotMrKnowItAll)\ **Post date:** [November 21, 2001, 7:16pm UTC](https://boards.straightdope.com/t/how-does-arm-hair-know-to-stop-growing/94610/5 "2001-11-21T19:16:41Z")
Boy have I been ignorant all these years. I always heard (from who? who knows) that the length of arm hair was dictated by the rate of growth vs. the rate of wear. Hairs come to little points so I thought that was evidence of wear. Go figure. ---
**Author:** ![kniz](https://avatars.discourse-cdn.com/v4/letter/k/c37758/32.png) [@kniz](https://boards.straightdope.com/u/kniz)\ **Post date:** [November 21, 2001, 7:20pm UTC](https://boards.straightdope.com/t/how-does-arm-hair-know-to-stop-growing/94610/6 "2001-11-21T19:20:34Z")
[list][list][list][list] **GENES** …:)… ---
**Author:** ![Cat\_Fight](https://avatars.discourse-cdn.com/v4/letter/c/90ced4/32.png) [@Cat\_Fight](https://boards.straightdope.com/u/Cat_Fight)\ **Post date:** [November 21, 2001, 7:54pm UTC](https://boards.straightdope.com/t/how-does-arm-hair-know-to-stop-growing/94610/7 "2001-11-21T19:54:06Z")
Of course, there are exceptions to the rule. Lots of people have a random hair on their arm or elsewhere that just doesn’t know when to stop (and must be plucked).