[Eli5] Why do people get bald on the back or front of their head rather than their face, chest or genitals?
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Male pattern baldness is linked to a hormone called dihydrotestosterone (DHT), as well as genetics.
The amount of DHT in your system affects both hair growth and hair loss. DHT is partly responsible for you starting to get more body hair and a beard during puberty.
Oddly enough more DHT in your system is damaging to the hair folics on the top of your head causing them to shrink and eventually stop producing hair all together.
But at the same time DHT stimulates hair growth on your chest, back, armpits, etc.
So the same hormone that makes you bald, also causes you to get hairier as you get older.
Exactly why we have 2 different kinds of hair follicles, and why the sensitive ones are mostly on the top of your head we don’t know.
As you get old your body doesnt have the strength to push the hair out the top of your head and thats why more of it comes out of your nose and ears
I have a full beard but no hair on my head. I like my beard but would trade it in an instant with a full haired head lol
Whatever it is, know that a high testosterone level isn’t linked to male pattern baldness.
There’s some other gene at work.
However, if you are bald, then you are at a higher risk of a bunch of diseases.
Androgens, specifically DHT (dihydrotestosterone). Testosterone is converted by 5 alpha reductase to DHT. It’s not how much DHT you have that causes MPB, but your sensitivity to it, which is genetic. DHT binds to the hair follicle and shrinks it, so every time the hair falls out it produces a weaker hair until the hair can no longer break the scalp and there is no visible hair. Now for reasons not entirely known only the follicles on the top and crown of the scalp are susceptible to miniaturization; the back and sides of the scalp are not affected and neither is any hair on the body/face. That is why when people get hair transplants they transplant the hair from the back of the head and place it on the top of the call since that hair does not miniaturize it will stay there for life.
Gene expression in certain regions of the scalp makes the follicles want to kill themselves after a certain point when exposed to DHT or Testosterone (which bind to the same receptor). We have no idea why they do that.
Well, according to my Dad, men who are balding in the front are thinkers. Men who are balding in the back are lovers. If they are balding in the front and back they just think they are lovers.
So you see, when a man gets to certain age the hair on his head begins to grow inside-out, first turning his brain into Swiss cheese, then sprouting from the ears and nose.
There’s quite a few good mechanistic answers here. In terms of the evolution drivers there’s a few possibilities.
It could be this is a secondary trait that has been selected for. That is, it might be that being hairier on the chest has been selected for, and balding is just a side effect of the way that the mechanism works.
Or baldness itself could confer a fitness advantage. Given that males of other closely related primates develop obvious physical features to show they are older, you could have the same thing in humans. It might be a social signal to indicate ‘I’ve been around awhile. My genes are good and I probably have decent social standing now too.’
Or it could be genetic ‘drift’. That is mutation over time has resulted in baldness, and although there’s no advantage there’s also no disadvantage, so it ends up a part of the population for no reason except that it happened to occur.
There are probably some other possibilities too.
Edit, typos
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