Social scientists will try to measure anything, it seems, and in the most recent issue of Psychology of Women Quarterly, a professor at Arizona State has published a paper that attempts to quantify the disgust women feel with regard to body hair – their own, and that of other women. The scholar, named Breanne Fahs, conducted two experiments. In one, she measured women’s responses to the thought of body hair, asking 20 women how they felt about shaving, not shaving, and about other women’s relative hairiness.

Respondents said that shaving was a minor inconvenience and a personal choice, but that overall the idea of body hair was revolting. “I think women who don’t shave are a little gross,” said one interviewee, a 22-year-old Caucasian lesbian. “Because sometimes, like if people don’t shave their entire lives, that’s just a little to much to handle for me. I always shave. I don’t like hair. I shave everything.”

In the next experiment, Fahs gave her own students extra credit if they agreed to grow their armpit and leg hair for ten weeks and keep a journal about it. Boyfriends were furious and mothers disapproving. “I constantly thought about my gross hair,” wrote one participant. “I will never ever show anyone my pit hair,” wrote another. Participants felt or were told that they were gross, disgusting, unclean, sloppy, and “ew.” The paper, which has the alliterative title “Perilous Patches and Pitstaches: Imagined Versus Lived Experiences of Women’s Body Hair Growth,” had a feminist perspective, which was, in the author’s words, to “highlight the invisibility of omnipresent sexism directed toward those who violate practices to ‘maintain’ the female body.” The compulsion to shave, in other words, is an example of how women have internalized patriarchal ideals of femininity.

These experiments are interesting mostly in their small-bore attention to a habit so mundane and so universal that most of us don’t give it a second thought. But if one continues in this vein, and scrutinizes with the seriousness of a scientist an issue usually left to beauty editors and waxing professionals, certain provoking questions arise. In an era of widespread porn, when teenagers shrug at the sight of anuses and scrotums and wide-open vaginas, why is disgust at female armpit hair, an anatomical reality so comparatively innocent, so widespread?

What is the psychological basis of that revulsion, and the corresponding need to cleanse, depilate, purify, denude? Is hygiene really the endgame, here? For bacteria do grow in unwashed armpit hair and they can cause a stink. But the answer is, probably not. Evolutionary psychologists remind me that the emotion of disgust – which protects humans by reminding them not to get too close to things that can carry disease, like excrement or corpses — can be triggered by learned prompts as well, a primitive way of preserving cultural norms or in-group or tribal status (i.e. don’t sleep with or date this or that kind of disgusting person). 

And in the developed West, revulsion at the sight of female armpit hair has to be cultural, because the places that place the highest value on hairlessness are also those where frequent if not daily bathing is routine. Most pits in 21st century America are not carrying harmful or even particularly smelly bacteria, in other words. And all the evidence you need for that is that most men continue to leave their pits au naturel, and no one fears contagion from them.

An investigation into the nature of our collective disgust seems particularly pressing now, three months after Madonna exposed the growth under her arms in a widely retweeted selfie, as summer weekends make the unveiling of America’s armpits ubiquitous. 

So, to start, what is the evolutionary explanation for body hair, particularly in the armpits? Why do humans, who more or less discarded somewhere in the mists of evolutionary time the full suit of hair that blanketed their primate ancestors, retain hair under their arms at all? (Pubic hair has an obvious, protective function, keeping unwanted flotsam from entering sacred zones, but armpits?) And if the hair under one’s arms does have an evolutionary purpose, then what is the psychological  basis of the nearly universal assault against it? In Australia, more than 90 percent of women shave their legs and armpits. In England, 99 percent of women have depilated at some point in their lives. In the U.S., writes Michael Scott Boroughs in 2010 his dissertation for a psychology degree at the University of South Florida, depilation is “so normative that it goes almost unremarked in casual discourse or in the research or casual literature.” Among men, the practice is growing: “data collected from a sample of 118 men at a large southeastern American university resulted in an estimated prevalence of 63.6% for body depilation at one or more body sites.”

There are two main theories about why humans ultimately became hairless. First, because the hair was making us too hot when we foraged for food on the African plains. Second, because ape-like body hair provided a happy haven for vermin, and the bugs were making people sick. Ergo: No hair, no bugs; no bugs, a healthier population. But the hair under our arms, on our legs, and around our genitals persisted, stubbornly announcing its presence around puberty. And something about that hair has disgusted humans for centuries, to the point where religions explicitly command its removal. According to Islamic tradition, the plucking of armpit hair and the shaving of pubic hair is required of an observant Muslim. In Leviticus, a complete body shave is mandated in the treatment of leprosy.

So, humans have a long history of being disgusted at armpit hair even though, in the modern era, that disgust is based more in culture than in any real risk to health. Then what might be its evolutionary purpose? Why didn’t we long ago discard these tufts, along with our tails and the fluff on our shoulders? One theory is that it’s protective against chafing, not unlike a sock or a pair of underwear: “It is probably there by ‘design’ in that it reduces friction between the arm and the torso,” Johan Lundström, a neuroscientist at the Monell Chemical Senses Center in Philadelphia, writes in an email. So that’s useful, though probably not critical in the age of spandex and wicking fabrics.

Here’s the best guess. Armpit hair works as a conveyor of pheromones, those scented molecules that, crucially, abet human mating, by giving each person his or her particular, alluring scent. Armpit hair (and hair around the nipples, and some groin hair) grows atop the sweat glands that produce a body’s natural smells, and it works as a kind of human fragrance diffuser, allowing one’s personal scent to waft into the atmosphere and announce one’s presence (and one’s fertility) to potential mates. “The specific development of hair in these regions (in our otherwise largely naked bodies) is thought to aid the dispersal of odorants in sexually mature humans,” writes Mahmood Bhutta in his 2007 article “Sex and the Nose.” Humans recognize each other by their scent. They can tell, on the basis of scent, whether a sweaty T-shirt belongs to a man or a woman. Women say they feel more relaxed in the presence of male smells. In men, the smell of estrogen lights up an area of the brain that controls erection.

People say body odor matters more than almost anything else when they’re picking sex partners. In a 2001 study published in Evolution and Human Behavior, psychologists found that men and women ranked an attractive body odor at or near the top of the list of other physical traits when selecting a mate. Women put “good smell” above everything else: looks, the sound of their mate’s voice, and the feeling of their mate’s skin. Men put “good smell” second only to looks. Both men and women said they preferred a good natural smell to perfumes or other fragrances.

So if hair conveys smell, and smell is so crucial to mating, then why this insistence on shaving?

The answer, it seems, is embedded deep in the evolutionary nature of disgust. And in the central role sex plays in the evolutionary story. Disgust is a protective reflex, writes Daniel Kelly, author of Yuck! The Nature and Moral Significance of Disgust, in an email. “The heart of this particular emotion’s primary job or core mission is to protect us from infectious diseases, and keep us away from the types of critters – microbes and parasites – that transmit them.” Its opposite is pleasure, or attraction. And no human experience evokes more intense pleasure, or disgust, than sex. People are drawn to and repelled by sex and sex acts in equal measure. “It’s always a delicate dance with sex and disgust. Maybe if the underarm hair as mate selection relevant scent carrier idea is right, we’re just seeing another facet of the ongoing back and forth,” Kelly writes.

Evolutionarily speaking, sex is the whole game. Sex with the wrong person can kill you and your genetic line – through disease, infertility, misfortune. With the right person, it can assure that your genes are transmitted to the next generation. Armpit hair signals sex because it grows during puberty and is one of the first signs of maturity (and fertility). And it signals sex because it transmits the scents that lead to mating. It triggers disgust because it reminds humans how dangerous sex can be. And that’s why we shave it off. Because armpit hair betrays the western fantasy about sex, which is that sex is fun, pleasurable, innocent, and inconsequential, a fantasy that elides the evolutionary truth. The revulsion at armpit hair might be evolution’s way of saying "proceed with caution," and its removal one less barrier to cross.