The Real Reason Men Grow More Facial Hair
All people grow facial hair, but while males can produce beards and mustaches, females typically cannot. Why is this? The reason has been linked to the hormone testosterone, which males produce at much higher levels than females. In males, testosterone is converted to the metabolite dihydrotestosterone (DHT), which is responsible for many of the characteristics we associate with masculinity like a deeper voice, more body hair, and yes, more facial hair (via Medical News Today).
According to a 2020 study, DHT regulates many of the cycles involved with hair growth that have a paradoxical effect: promoting beard growth in males while also causing balding in the same person, for example (via Frontiers in Pharmacology). As it stands, facial hair growth in males versus females is well understood from a biological standpoint, but scientists are still looking at the evolutionary purpose of facial hair. In other words, what benefit does growing facial hair give the male sex?
Do beards give men an evolutionary advantage?
Females typically cannot grow thick mustaches and full beards like males can and don't seem to be at a disadvantage. So is there any advantage to having the ability to grow more facial hair? A 2012 report published in Behavioral Ecology cites a 2010 study suggesting that the human beard evolved to serve a protective function, much like a lion's mane, but studies of attack patterns of male lions have rejected the hypothesis that the mane exists as a safeguard against injury.
Other hypotheses link beard growth to high levels of testosterone and a sign of social dominance, but a 2021 study published in Archives of Sexual Behavior has put this idea to rest. According to the study, beards do not reliably convey biological (testosterone levels) and psychological (self-reported dominance) characteristics. As it turns out, facial hair doesn't serve any practical function and doesn't reveal any particular personality trait or evolutionary advantage. In fact, that beard is purely ornamental (via Wired). Though "ornamental" might very well be one kind of advantage. A 2016 study published in the Journal of Evolutionary Biology reveals that heterosexual women like men with beards, and find men with facial hair more attractive than clean-shaven men.