Why do men have facial hair but women don't?
It's complicated.
Do you sport a Fu Manchu? Muttonchops? A soul patch? If so, you can chalk your ability to grow facial hair up to your hormones.
Sex hormones called androgens that encourage the development of male traits are the main reason why men generally grow facial hair and women generally don't. We'll get into why in a moment, but first, a warning: Hair follicles are not as simple as they seem. In some, androgens spur hair growth. In others, they reduce it (hello, male-pattern baldness!). And in others — say, in the ear canal — androgens make hair grow, but on a delayed schedule, decades after the same sex hormones trigger the sprouting of a beard.
Why this follicular diversity? The answer is complicated.
"The reality is that at the moment it's still quite challenging for researchers to conclusively answer this," said Ben Miranda, a consultant plastic and hand surgeon at St. Andrew's Centre for Plastic Surgery & Burns and a visiting professor at Anglia Ruskin University in the U.K., "but there are differences within the hair follicles themselves that are retained depending on what part of the body they come from."
This is your face on androgens
Before puberty, the hair follicles on the body produce vellus hair, the light, fine short hair you might see on the back of a woman's hand. At puberty, both males and females produce more androgens, namely testosterone and dihydrotestosterone. The male body produces far more androgens, however. These androgens stimulate the hair follicles to produce darker, thicker hair known as intermediate hair, the "peach fuzz" usually first seen on the upper lip. Over time, the androgen simulation encourages the production of even darker, thicker "terminal hair," of the same sort seen on the scalp.
Related: Why don't people have tails?
Peach fuzz on this teen's upper lip is the result of androgens.
The androgens do this by increasing the amount of time a given hair follicle spends in its growth phase in relation to its shedding or resting phases, Miranda, who also leads the St. Andrew's Anglia Ruskin Research Group, a joint effort between St. Andrew's and Anglia Ruskin, told Live Science.
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Hair on the female body changes during puberty, too, but not as dramatically as in men. The hair follicles on a woman's face near the ear transition from vellus to intermediate strands, and armpit and pubic hair transition to terminal hair.
Masculinizing hormone therapy, sometimes used by transgender individuals, has a similar effect on body hair. Within a few years of starting testosterone therapy, body and facial hair darken and thicken, according to the University of California, San Francisco.
Follicles gone wild
Nevertheless, the story isn't as straightforward as, "just add androgens." In some scalp follicles, androgens actually encourage the exact opposite pattern as in body hair. Instead of triggering a transition from vellus hair to intermediate hair to terminal hair, androgens trigger terminal hair to become intermediate hair, and then cause a transition from intermediate to vellus hair. It's called androgenic alopecia, better known as male-pattern balding. Not all men are genetically susceptible, but in those who are, the result is thinning at the front of the scalp that gradually creeps backward with age.
"The real weird thing about it is, why is it that one hair follicle on the scalp that is in one area is androgen-sensitive, and the one next door is not?" Miranda said. "It just doesn't make sense."
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The differences are specific to the site on the body where the hair follicle develops; if you transplant a non-androgen-sensitive follicle to a balding spot on the scalp, the hair will merrily grow. (This is why hair-transplant surgery works.) But the reasons behind these differences are hard to understand, Miranda said. The genes in each follicle are the same, but the control of the gene activity — a domain called epigenetics — is clearly different. There are complex differences in cell signaling, or the cascade of molecular instructions that cause a follicle to grow, rest or shed, Miranda said. Some genes become more or less active during balding, but research hasn't always found consistency in those genetic patterns.
"It's epigenetic differences, upregulation and downregulation of signaling pathways, exogenous exposure to different environments, circulating hormones within the body during those times," Miranda said.
Miranda and his team are working on developing hormone-sensitive intermediate hair follicles that can be kept alive in the lab in order to probe these factors. This research could be used to help prevent not only the common pattern of male balding, but also to treat some forms of alopecia, or the loss of both scalp and body hair, and hirsutism, a condition in which women grow unusually thick facial and body hair. Hair follicles also have their androgen sensitivity in common with the cells of the prostate, Miranda noted. Under the influence of androgens, the prostate can enlarge or become cancerous, an extremely common condition for older men. Studying how androgens affect the cellular processes in hair follicles could help reveal how the same hormones cause prostate problems in old age.
Originally published on Live Science.
Stephanie Pappas is a contributing writer for Live Science, covering topics ranging from geoscience to archaeology to the human brain and behavior. She was previously a senior writer for Live Science but is now a freelancer based in Denver, Colorado, and regularly contributes to Scientific American and The Monitor, the monthly magazine of the American Psychological Association. Stephanie received a bachelor's degree in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz.