What is the role of randomness in human hair growth?

[JT Smith]

I'm curious about the nature of hair growth. From what I've been able to find from websites (e.g. Hair Growth - Wikipedia) the simple story is that each hair grows at a set rate for a set amount of time, then rests for a bit, then falls out. Then the cycle begins again. All the hairs on one's head/arm/leg/etc. are unsynchronized in their growth stages. For scalp hair the growth period is typically on the order of 3-5 years and the rest phase is a few months or less.

I found one paper that discussed an experiment where a small part of the scalps of ten men, some of whom were balding, were examined over a period of fourteen years. This paper added a twist to what I'd read elsewhere, namely that there is a significant degree of randomness in the length of the growth cycle ("anagen cycle"). The picture they painted was of most hairs achieving a length significantly shorter than the longest hairs. And the anagen cycle length had an average of only 1.4 years. So I'm not sure how representative these data are to the general population. Surely it does not describe what one can easily observe of many individuals.

I found one mathematical model of hair growth that started off with the premise that individual hairs each grow randomly with a 50% chance of falling out each day. The author didn't take the time to justify this starting point and offered very little in terms of evidence that it matches physical reality.

At this point my meager searching skills seem to be tapped out. Can anyone point me to more/better information?

 

[Tom.G]

Can anyone point me to more/better information?

Sure.

1) Note the date.
2) Document your hair length in a particular area.
3) Get a hair cut.
4) Document your hair length in the same area.
5) Wait until you 'need' another hair cut.
6) repeat 1) thru 4)

Process the data above to answer your question.

It is called an "Experiment."

Cheers,
Tom

 

[JT Smith]

I'm not sure if your slightly condescending post is merely an attempt at levity or is due to misunderstanding my question. I suppose it could be both. Your suggested experiment would, at best, give a rough measure of the bulk hair growth rate of one individual. But growth rate is given attention in the Wikipedia article I linked above. If that information is all one had it's hard to imagine coming up with anything more than a very simple, unrealistic model.

It's clear that hair has a growth cycle. It's the dynamics of that cycle that I wish to understand better. According to Wikipedia the length of the growth phase varies from person to person but whether it also varies for individual hairs on a given person is not addressed. The other paper I linked show data that the latter is the case but the single non-balding subject detailed seems like an outlier in terms of hair growth. So I'd like to see more.

I jokingly asked my wife if she'd mind if I closely trimmed a 1cm square on her head every month for the next several years and she said: "No."

Frankly, my expectations from PF are not particularly high. But I thought I'd give it a shot.

 

[berkeman]

I jokingly asked my wife if she'd mind if I closely trimmed a 1cm square on her head every month for the next several years and she said: "No."

No she doesn't mind, or No don't do it? Did she ask which square cm you had in mind?

 

[JT Smith]

Yeah, you're right. The way I worded it above was ambiguous. But her answer was very clear. We never got around to selecting a site.

I've thought about just trying to measure the lengths of a bunch of hairs emerging from a small area. But separating out and measuring dozens of hairs, some of which could be quite short, would be difficult. I doubt my wife would sit still for that. And doing it to myself would be even harder. Plus it would only be a single snapshot. More than one model might fit those data.

 

[FactChecker]

found one mathematical model of hair growth that started off with the premise that individual hairs each grow randomly with a 50% chance of falling out each day. The author didn't take the time to justify this starting point and offered very little in terms of evidence that it matches physical reality.

I had real trouble understanding the model that you linked to. And I am not sure that your statement that each hair has a 50% of falling out each day makes sense. It seems like practically all the hair would fall out in a week or two.

 

[JT Smith]

Yes, you're correct, it isn't each day. I should have said 50% probability p after each time interval t, where p, t can be arbitrarily chosen to (hopefully) fit reality. I was thinking of modifying my post but didn't expect anyone to actually look that closely.

 

[berkeman]

I was thinking of modifying my post but didn't expect anyone to actually look that closely.

I also had the same question, so thanks for addressing it. Yeah, it's better not to modify your OP, since that will be confusing when other folks later read the thread. It's best to just post a follow-up reply like you did with clarification. One thing that you can do is go to the OP you want to correct, and use strikethrough to deprecate the part you want to fix, and add a note pointing to your later reply that has an update in it. That way everybody stays in sync.

 

[pinball1970]

Yes, you're correct, it isn't each day. I should have said 50% probability p after each time interval t, where p, t can be arbitrarily chosen to (hopefully) fit reality. I was thinking of modifying my post but didn't expect anyone to actually look that closely.

Another idea. Dye your wife's hair and measure the new hair as it grows out.
Obviously there would have to be a contrast to the real colour otherwise it will be difficult to see in order to measure.
Women dye their hair so you should be ok, letting the roots grow out exposing the contrast may be tricky to convince her but at least no hair is cut.

 

[JT Smith]

Another idea. Dye your wife's hair and measure the new hair as it grows out.

I'm not certain that would work but it's irrelevant since my wife isn't going to let me use her hair for an experiment.

Anyway, I don't think it makes sense for me to attempt what would likely be a years-long experiment in order to answer a question about which I am merely curious. I'm just asking if anyone knows where I can find more information on the subject, assuming it exists. Maybe it doesn't?

 

[symbolipoint]

I'm not certain that would work but it's irrelevant since my wife isn't going to let me use her hair for an experiment.

Anyway, I don't think it makes sense for me to attempt what would likely be a years-long experiment in order to answer a question about which I am merely curious. I'm just asking if anyone knows where I can find more information on the subject, assuming it exists. Maybe it doesn't?

Physics Forums might have insufficient members with the kind of focus of which you ask. Inquire about your topic with cosmetologists, barbers, maybe dermatologists.

 

[JT Smith]

Barbers?