Do hairs know they've been cut?

01 August 2013


Some hair, ideally short and clean




I have a question about body hair. When we remove unwanted body hair by plucking/shaving, we know that they grow back more or less to its natural length.

When we trim them, the hair also seem to grow back to its natural length, but this has mystified me for some time. a) How does the hair know it's been cut without stress caused to the hair follicle, and therefore start growing again; and

b) How does it know when to stop growing when it reaches the intended length?

Many thanks,

Maki Aoyama


Ginny - So, the short answer is that your hair doesn't know that it's been cut. This is a really common myth. People think that if they trim their head hair when they're trying to grow it, that it'll make it grow faster and people say that if shave your legs or your face, it makes it grow back faster, but that's not actually the case. The only thing that affects how long your hair is and how fast it grows is the hair itself and your genetics. So, hair goes through 3 phases when it's growing. There's the anagen phase which is the growth phase and this is when it actually grows, when it gets longer. Now, on head hair, that can last between 2 and 6 years and how long that phase is, counts as how long your hair could grow to. So, some people seem to be really lucky and they can grow hair down to their waist whereas other people, no matter how hard they try, it will never get pass shoulder length.

Chris - Not their eyelashes though, surely.

Ginny - No, not their eyelashes. Now, that's a point because body hair has a much shorter anagen phase. It's only a couple of months and that's why it never gets as long as head hair. I think there are few exceptions of weird people who've managed to grow their underarm hair ridiculously long in the Guinness Book of Records, but with a couple of weird exceptions, it doesn't get as long as the hair on your head because this anagen phase only last a couple of months. Now after that phase, what happens is the hair is cut off from the blood supply so the follicle sort of dies and it enters a dormant stage and then a bit of time after that, it'll fall out as a new hair begins to grow. So, what you're seeing is you're trimming say, your leg hair while it's still in the anagen phase - it's going to continue growing. But it can only ever get to a certain length because that anagen phase will end and it will fall out.


Okay what if I cut my hair in the middle of an anagen phase, how does my hair follicles know to start another anagen phase and still complete it as normal?

This argument doesnt make sense. If you shave your leg hair then it will all regrow to the same max length. Assuming the argument is correct then if you shave your legs in the final weeks of the anagen cycle then the hair will stop growing well short of its max length. Experience suggests otherwise.

I don't think all the hair grows together. if so, then everyone would be bald for some time before the hair started growing back. If they don't grow simultaneously, then some hair will grow back to full size (if it was cut right in the start of the anagen phase) and some other hair will grow back to half or less the full size (if it was cut on half of the anagen phase or later), and maybe you just don't notice it.

Literally just came to see how hair didn’t grow “out of hand” and I run into an argument lol. Divine creator probably not. Low IQ I wouldn’t know, I didn’t give them an IQ test. Saying the person has a low IQ without hard proof is pretty much the same thing as what that person was saying “DIVINE CREATOR just because it’s too perfect”

Well, technically couldn't you make your hair grow longer by interacting with the anagen phase? What I mean is, say your arm hair is nearing the end of the anagen phase and you somehow know this. Then while you technically can't get hair past a certain length, you could aid it by pulling the previous hair out to keep I from having any resistance. It wouldn't make it substantially longer but it would break that thing about not being able to make it grow longer. Now, if you were able to interact with the genetic coding for the anagen phase and tell your body to grow it twice as long, that genetic mutation would allow you to grow longer hair on your body.

Wild how hair on body is programed to only grow to certain length! Divine creator!!!

Well it's cool, but is it any more divine than stomac knowing it's full, lets say? (:

Devine creator? Sigh. Such low IQs

You figure someone else has a low IQ because they believe in a divine creator, but you spelled "devine" wrong....

Its pretty ignorant to say someones stupid because of their personal beliefs.

So if I believe in Big Foot or the flat earth theory, my beliefs are just as valid and intelligent?

Maybe valid but I wouldnt say intelligent.

Is your comment that intelligent? But brilliant people believe in many kinds of different worldviews.

There is a correlation between belief in a creator and Low IQ. Although anecdotal only, It bears out in daily interactions too. Believers have minds of a lower order.

As education increases the belief in divinity decreases, this does not mean that "Believers have minds of a lower order". This suggests more of a third factor of education being the correlation rather than being naturally of lower IQ. However this still is just correlation and in fact a good 1% of the top scientist, people with a higher IQ than you or me, still believe in a higher power. for you to remark that someone must have a low IQ just because they believe in something beyond facts is rather hypocritical considering thats what you just did.

Divine creator? Lmao. How about evolution? Not very likely to reproduce with super long body hair

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