Like many people, I ask myself continuously about some of life’s biggest mysteries. Why are we here? What is the meaning of life? Why do we have those strangely sparse, wiry little hairs growing around our genitals—hair that is singularly different from all the other hair on our bodies? Fortunately, a group of leading-edge scientists have managed to put my mind to rest on at least one of these daunting existential questions. In recent years, it seems, researchers have made some tremendous advances in the study of pubic hair.

So, let’s start with what we already know about pubic hair. It’s a signature mark of sexual maturity, sprouting up around our groins sometime in early adolescence. If it appears on a person’s body any time earlier than this in development (say, prior to the age of nine years), something is clearly the matter. There are some things that just don’t go together in this world. Pickled quail eggs and peanut butter. Bernie Madoff and sincerity. Plaid and polka dots. Babies and—pubic hair?

Precocious puberty is no laughing matter, of course, because children who begin to develop secondary sexual characteristics prior to the age of nine years may in fact have some significant underlying health problem, such as a lesion on the central nervous system that prematurely activates the hypothalamus. But for one young couple in Alabama, the term “precocious puberty” hardly does justice to what they were observing with their infant son a few years ago. Imagine changing your six-month-old’s diapers and noticing what appears to be a tuft of light-colored pubic hair on his groin. Over the next ten months, the pubic hair would become progressively darker and adult-like, which—when accompanied by an oddly large penis for a 16-month-old and, ahem, frequent erections—was finally enough to prompt this less-than-proactive pair to seek medical advice.

This was the background of the case as it was presented to a group of pediatricians at the University of South Alabama and as reported by these physicians in a 2007 issue of the journal Clinical Pediatrics . Upon examining the child, Samar Bhowmick and his colleagues noted with some astonishment that, “the pubic hair was [that of an adolescent], mostly around the base of the phallus and was dark and curled.” Further inspection revealed a healthy, bouncing baby boy—completely normal and age-appropriate in all other respects—but the laboratory results indicated an abnormally high level of testosterone . Eventually, the doctors cracked the case. It seems that the boy’s father had been spreading testosterone gel twice daily over his shoulders, back and chest, having been prescribed this treatment by his doctor to treat a low libido caused by depression. Because the little boy slept in the same bed with his parents, with his father cuddling and hugging his child just after applying the gel, the bare skin contact was causing his son to become a man much earlier than nature intended. (A follow-up visit later revealed, fortunately, that the pubic hair had all but disappeared once the cause was determined, and the doctors were hopeful that the child would have no long-term complications from the testosterone exposure.)

The peculiar case of the still-suckling pubic-haired infant is so striking, obviously, because this type of distinctive groin pelage tends to be coincident with sexual maturation, not the developmental stage in which we’re just learning how to walk. The case also highlights the oddity that is human pubic hair more generally. After all, we appear to be the only species of primate (perhaps the only species, period) that bears this type of strange hair around our genitals. Robin Weiss , a researcher from University College London’s Division of Infection and Immunology, found himself standing in the shower one day, looking down, and asking this very question:

Although naked apes [humans] have pubic hair, surely our hairy cousins don’t? How could I test my hypothesis? I knew that there was a stuffed chimpanzee in the Grant Zoological Museum at University College London and I called in on the way to my laboratory. Alas, he was a juvenile, which left the question open. A brisk walk across Regent’s Park to inspect the adult gorillas in their splendid new pavilion at London Zoo strengthened my suspicion, and this was later confirmed by a visit to the chimpanzees at Whipsnade Zoo north of London. Indeed, all the species of apes, Old World monkeys and New World monkeys seem to be less hairy in the pubic region than elsewhere; fur is present but it is short and fine.

Weiss speculates that one of the main reasons that human beings uniquely evolved a “thick bush of wiry hair” around their genital regions is its visual signaling of sexual maturation. (It also likely serves as a primitive odor trap and aids in the wafting of human pheromones.) So pubic hair acts as a sort of blinking marquee, indicating for prospective sexual partners that mating with that individual could be potentially a fruitful exercise in genetic perpetuity. Weiss believes that the advertisement of our fecundity suggests that pubic hair would have arisen only after we became “naked apes,” causing it to stand out so vividly against the backdrop of an otherwise hairless body.

Another fascinating thing about pubic hair is its unusual texture and composition compared to the rest of the hair on our bodies and heads. You can’t quite use it to floss with, but pubic hair is considerably thicker than either axillary* (underarm) hair or that on our legs, chests (for some, backs) and scalps. I’m probably not the only one who shudders to think of an alternative path of natural selection , one in which the hair on our heads evolved to be of pubic proportions—just consider what the average barber shop floor might look like at the end of the day. It’s not entirely clear why pubic hair is so distinctly thick, short and, usually, curly, but a friend of mine, the biologist Anne Clark from SUNY-Binghamton, did point out to me last week (while we were hiking on Kapiti Island in New Zealand, which made it all the more memorable) that anything else would be rather impractical. To have long, flowing, stylish locks growing down there wouldn’t be terribly convenient, especially given the logistics of sexual intercourse.

But, as Weiss points out, although pubic hair had its signaling advantages, it also came with a cost. And this cost goes by the name of Pthirus pubis —more commonly known as “crabs.” The evolutionary story of crabs is remarkable, and it’s one that Weiss relays humorously in a recent issue of the Journal of Biology . If you’ve ever marveled at the similarity between human pubic hair and the coarse texture of gorilla hair (and let’s face it, who hasn’t), you’re already on the right track:

On the basis of morphology, human Pthirus pubis is closely related to the gorilla louse, Pthirus gorillae . Molecular phylogeny indicates that human pubic lice diverged from gorilla lice as recently as 3.3 million years ago whereas the chimp and human host lineage split from the gorilla lineage at least 7 million years ago. Thus, it seems clear that humans acquired pubic lice horizontally, possibly at the time of the Pthirus species’ split and probably directly from gorillas. Because they were already adapted to the coarse body hair of the gorilla, crabs would have found a suitable niche in human pubic hair.

That’s right. We got crabs from gorillas. But get your head out of the jungle gutter. Weiss speculates that our ancestors acquired these ravenous parasites not through interspecies sex, but rather as a consequence of ancient humans butchering and eating gorillas. This close contact with gorilla carcasses would have enabled the gorilla louse (Pthirus gorillae ) to jump hosts and mutate in accordance with the eventual evolution of human pubic hair—what must have seemed a cozy and familiar environment—to become the Pthirus pubis species we know and loathe today (much as bushmeat slaughter practices allowed retroviruses to invade humans from chimpanzees in modern times).

Regardless of how they came to be there, crabs have unfortunately become part and parcel of our species’ pubis. Intriguingly, however, recent innovations in our species’ cultural evolution—particularly, modern grooming habits and the aesthetic stylizing of our pubic hair regions—have begun prying loose these pesky critters’ grip on us. Some health clinics have begun noticing a significant fall in the occurrence of pubic lice, especially among patients that shave all or some of their pubic hair. (Even if only their sexual partners are shaved, the risk of acquisition in the patients themselves would still be substantially less than for those who mate with partners whose genitals are hidden in the type of thick copse that crabs delight in.) This isn’t an entirely new phenomenon. Prostitutes in medieval times would often wear “merkins” (or pubic wigs) after shaving their genitalia to help control their pubic lice.

But before you go scheduling your next Brazilian wax, consider that pubic hair does appear to offer some degree of protection against even nastier bacterial and viral infections. Although the diagnosis of pubic lice has seemingly plummeted as a direct result of human vanity in both sexes, cases of gonorrhea and Chlamydia have increased over the same period, a correlation that may not be merely coincidental. Damned if you do, damned if you don’t.

Still, in Western countries, the “hairlessness norm” is gathering a lot of steam. Several recent studies reveal just how common shaving one’s nether regions has actually become. In a 2008 issue of Sex Roles , Flinders University psychologists Marika Tiggemann and Suzanna Hodgson, for example, found that more than three quarters (76 percent) of a sample of 235 female undergraduate students from Australia reported ever having removed their pubic hair. Sixty-one percent currently did so and half of this sample said that they routinely removed all traces of their pubic hair. The current trend for men appears to be no different. In a separate study the same year, with colleagues Yolanda Martins and Linda Churchett, Tigemann reported in Body Image that of 106 gay men, 82 percent had removed their pubic hair at least once. And lest you think that this is an artifact of gay male culture , straight men weren’t far behind on this measure. Out of a sample of 228 heterosexual men, 66 percent reported doing the same.

Irrespective of sexual orientation or gender, the investigators discovered that the primary motivation for pubic hair depilation is related to concerns with one’s appearance (in contrast to health-related motivations). And for young women, at least, the removal of pubic hair is significantly correlated with having a sexual partner, something that Tiggemann and Hodgson find more than a little troubling:

The complete removal of pubic hair is also removing a key marker of adult female sexuality. The result is a prepubescent-like body that is highly sexualized. Thus it is another practice that may contribute to the increasing objectification and sexualization of young girls.

Pubic hair trends do make one wonder about unspoken human sexual proclivities. It is tempting to speculate, as my friend and fellow evolutionary psychologist Gordon Gallup reminded me recently, that those who prefer their sexual partners to be bare down there might actually be latent pedophiles. Surely there are a host of reasons why people might opt to shave their pubic hair, however, or attempt to promote pubic hair removal. For example, many individuals are put off by the idea of cunnilingus or fellatio because of those pesky pubic hairs that can lodge inadvertently in their gratifying throats. This was the theme of an episode from Larry David’s Curb Your Enthusiasm , where Larry had to embarrassingly explain this bothersome tickle to a rather serious-faced doctor.

But now this is turning into a different type of story altogether. In any event, pubic hair coiffure is not a zero-sum game. Typing “pubic hair styles” into my Google search bar (dangerously so on my work computer!) yielded 467,000 hits, every single one of which I am hesitant to click on—until I get home, that is.

*Editors' note 03/02/10: This word was originally misspelled "axiliary." Thanks to the commenters who pointed out the error.

In this column presented by Scientific American Mind magazine, research psychologist Jesse Bering of Queen's University Belfast ponders some of the more obscure aspects of everyday human behavior. Ever wonder why yawning is contagious, why we point with our index fingers instead of our thumbs or whether being breastfed as an infant influences your sexual preferences as an adult? Get a closer look at the latest data as “Bering in Mind” tackles these and other quirky questions about human nature. Sign up for the RSS feed or friend Dr. Bering on Facebook and never miss an installment again. For articles published prior to September 29, 2009, click here: older Bering in Mind columns.

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