How does one broach such an indelicate topic as body odor except perhaps to borrow from the immortal words of the Roman playwright, Terence, who famously said that, “nothing human is alien to me.” True, Terence probably wasn’t referring to flatulence, armpit secretions, halitosis, foot odor, and the many and unspeakably loathsome scents associated with various fungal infections in the body’s hinterlands when he wrote this. Still, his axiom covers a lot of territory on the human condition, including our somewhat smelly natures. And as mature science-minded adults, perhaps we shouldn’t be so shy about our stenches, anyway, since it turns out that our social behaviors—particularly human sexual instincts—are driven by our perceptions of each other’s aromas more than we tend to realize.

Compared to the brains of other mammals, the primate olfactory cortex (the brain region associated with processing smells) has decreased in size and relative importance over the course of evolution, being outranked in functional priority by the visual system. But we do have noses for a reason. In fact, for many reasons, a sizeable proportion of which involves gathering useful information from the environment in the form of “chemosignals,” more commonly known as pheromones. As recent findings tell us, other people’s apocrine glands—that is to say, their armpits—are routinely piping out a lot of important social information. These armpit odor molecules are sucked up into our sinuses, processed by our brains, and translated into some rather interesting psychological and behavioral reactions.

Men’s body odors tend to be more pungent than women’s, and this peculiarly strong punch can communicate a lot of information about the individual’s genetic quality. Women, in turn, have an almost preternatural olfactory sense, one that appears designed for unconsciously sniffing out the mate value of prospective reproductive partners. This sex difference is probably owed to the fact that women evolved to be more discerning in their choice of sexual partners given the relative risk imbalance of casual intercourse between men and women. Historically, a man’s thirty or so seconds of physiological bliss often amounted to nine months of dangerous physiological stress on the woman’s body, not to mention a lengthy forfeiting of other reproductive opportunities. Fortunately, nature gave women a set of helpful olfactory tools to hound out whose seed was “worthy” (which means highest in adaptive value) of their uterine investment.

For example, women tend to find the smell of high-testosterone males more attractive when they’re in the most fertile phase of their menstrual cycles. Also, in controlled tests where female judges have no idea what the man looks like, women rate the body odors of attractive men as being “sexier” than those of men who aren’t as fortunate in the looks department. Think of those old Coke versus Pepsi taste test commercials and you get the idea, except here married women can’t easily alter their shopping habits to suit their preferences. (Note that I didn’t say “can’t”—just can’t easily.) So, in honor of your overworked and underappreciated nose, which has certainly experienced its share of disagreeable people in its years of service to you, what follows is just a sample of some ripe data appearing in the study of body odor perception.

To begin with, however, it’s worth pointing out there’s really no such thing as an intrinsically “bad smell.” Rather, there are only smells; and how we perceive them is largely an artifact of our particularly human evolutionary heritage. To say that rotting flesh smells disgusting is similar to saying that the sunset looks beautiful—there’s no “beautiful-ness” quality intrinsic to the sunset just as there’s no “disgusting-ness” intrinsic to rotting flesh. Rather, rotting flesh and sunsets are only perceived this way by the human mind; as “phenomenological” qualities, adjectives such as “beautiful” and “disgusting” merely describe how we subjectively experience the natural world. I can assure you that whatever particular scents you find repulsive, my dog, Gulliver, would likely perceive as irresistibly appealing. And I mean rotting flesh and just about anything else you can think of, with the exception perhaps of skunk odor and his own feces, for which I can only hope you’d share a mutual disdain.

But back to human armpits. One of the most important target chemicals believed to play a role in modulating people’s attraction toward others is called androstadienone, a compound found in axillary secretions. When women are exposed to this “chemosignal,” it activates regions of their brains associated with attention, social cognition, emotional processing and sexual behavior. The effects of androstadienone on female arousal were clearly documented in a 2008 article in the journal Hormones and Behavior. Researcher Tamsin Saxton from the University of Liverpool and her colleagues organized a series of speed-dating events in which—as typically happens with these things—women were stationed at numbered tables and male suitors moved from table to table at regular three-minute intervals. As an “ecologically valid” study, behavior between the men and women was allowed to occur naturally. That is to say, there were no experimental scripts and, indeed, although women were aware that they were participating in a study, all participants were rightly under the impression that this was an actual opportunity to meet potential partners.

The catch was that prior to their interactions with the men, the researchers had randomly assigned the women to one of three experimental conditions in which they were asked to dab a saturated cotton pad in that little crevice between their nose and their lips. Without the participants knowing which condition they’d been assigned to, the pad had either just been dipped in water alone, in clove oil (one percent clove oil in propylene glycol), or in a concentration of androstadienone mixed with the clove oil solution to mask the scent. The gist of the results was that the women who’d been randomly assigned to the androstadienone condition found the men significantly more attractive than did those in the water and clove oil only conditions, who didn’t differ from each other.

One very obvious lesson to learn from this research is that although physical attractiveness can be “operationalized” as a construct, such as in evolutionary psychologists’ findings that people who have symmetrical faces and who have prototypically sized features are universally regarded as physically attractive, there are also important olfactory caveats to consider. Namely, the smells attending our perception of someone else—the “ambient odors” that are paired in our minds with that person—can modulate our feelings of attraction toward them. Once, I was introduced to a very good-looking person who, with his symmetrical features and almost computer-generated prototypical face, from several feet away admittedly made my heart patter. But as I neared him I inhaled a breath of air so foul that it could only be described as coming from the intestinal tract of a sick goat. Needless to say, in spite of his handsome appearance my sexual alchemy with this person has been rather soured as a consequence. (There is some evidence that the same hypothalamic regions activated by androstadienone in the brains of heterosexual women are also activated by these pheromones in homosexual mens’ brains). So although physical attractiveness may be reduced to very specific geo-facial algorithms reflecting genetic value, we live in an odorous world where pretty faces aren’t always paired with pretty smells, and a proper science of attraction must account for these dynamic factors.

In fact, University of Oxford psychologist M. Luisa Demattè and her colleagues have started to do precisely this. In a 2007 article published in the journal Chemical Senses, Demattè and coauthors Robert Österbauer and Charles Spence point out that, “despite the fact that virtually all humans use some sort of fragranced products on their bodies, there are surprisingly few studies that have directly investigated the question of whether the presence of an odor can cross-modally influence a person’s judgment of another person’s physical attractiveness when assessed visually.”

Thus, the investigators adopted an experimentally rigorous psychosocial paradigm in which sixteen undergraduate female participants were placed in a custom-built, computer-controlled “olfactometer” inside of which male faces appeared sequentially before them on a screen. One of five odors (two “unpleasant”: synthetic body odor, rubber; two “pleasant”: geranium, male cologne; and one “neutral”: clean air) was randomly pumped into the experimental chamber and paired with the face. There were forty faces in total, and each face was randomly presented three times so that every participant experienced it paired with at least one odor from each of the three categories (unpleasant, pleasant and neutral).

As you might expect, the young women in this study consistently rated the male faces as being significantly less attractive when presented with the unpleasant odors. In addition to these findings justifiably putting a smile on the faces of perfume company executives everywhere, there are other fascinating implications for such data, too. According to Demattè and her coauthors:

In the years to come, our findings might also be relevant in the technology sector, given recent developments in the area of multisensory applications, such as the possible use of olfactory cues in messaging applications, electronic picture storage/retrieval and enhancing the sense of presence in virtual reality.

I must say, I’m not entirely sure how that would look in practice (perhaps a spritz of sassafras from a designated pinhole in your iPhone to match your husband’s ring tone, or a scroll-down menu of scent accoutrements for profiles), but I do like the general idea.

As far as natural body odors go, scientists have found repeatedly that the less a person smells like you, the more attractive—or rather, the less repulsive—you find their armpit aromas. To evolutionary psychologist Glenn Weisfeld from Wayne State University, the reason we prefer dissimilarly scented others is obvious: since olfaction plays a key role in identifying who is genetically related to us and who isn’t (in some studies, participants with especially strong odor detection abilities can even successfully discriminate between the body odors of identical twins raised apart and those of fraternal twins raised together), being specially repelled by the malodorous fumes of our biological kin functions to promote incest avoidance.

To explore his basic evolutionary hypothesis, Weisfeld and his colleagues asked family members to smell each others’ body odors—not the most pleasant scenario, you’ll probably agree (for me an outright nightmare), but a necessary one for scientific purposes. A total of twenty-one families from the Ontario area participated in this study published in a 2003 issue of the Journal of Experimental Child Psychology. Each person in the family was given a new, all-white, identical t-shirt and was instructed to wear this shirt to bed for three consecutive nights. No scented soaps or perfumes were used during this odor absorption period, and shirts were kept in their own sealed plastic envelopes during the day.

After three nights, each family member was tested in isolation and asked to compare the shirt worn by another family member with that of an unrelated control subject (matched by gender and approximate age). The participants were asked first which odor they preferred, and then which shirt was that of their mother, father, child or sibling. A few interesting findings emerged from this study, some of which lends support to Weisfeld’s incest aversion hypothesis. First, both mothers and fathers could distinguish the smell of their own child but, somewhat curiously, mothers “strongly preferred” the odor of the control child. Fathers demonstrated a similar trend in odor preference, but for them the effect failed to reach statistical significance. Older children in the study (those aged 9-15 years) likewise preferred the body odor of the control adult man and woman to that of their parents. Furthermore, these older children strongly preferred the scent of control children in opposite-sex trials, but showed no preference for the body odors of their same-sex siblings versus controls. That is to say, brothers found their brothers’ body odors as mutually offensive as another boy’s body odor, and the sister pairs showed the same pattern of indifference response.

A final recent study worth mentioning was one conducted by Charles University of Prague anthropologists Jan Havlicek and Pavlina Lenochova, who actually answered the age-old question of whether carnivorous people smell worse to us than vegetarians. In other words, “the aim of this study was to test the effect of red meat consumption on axillary odor hedonicity.” In a 2006 article published in the journal Chemical Senses, Havlicek and Lenochova convinced seventeen disease-free, nonsmoking male students ranging in age from nineteen to thirty-one years to eat strictly defined diets. (They were paid about $35 for their participation in the study, a real bargain.) Half of the participants were assigned to the “meat” condition for two weeks, and the remaining half to the “non-meat” condition for this period.

During the first ten days, these males could choose from a list of prepared meals and had to select at least one main dish out of the list every day. “The individual dishes were elaborated,” write the authors, “to differ in meat content only (e.g., vegetable risotto/pork risotto).” The final three days of the special diet were even stricter; during this time, the participants were hand-served lunch and dinner to precisely control for their dietary intake—the meat group, for example, was observed wolfing down a 100-gram red meat dish for each main course. To rule out other possible factors, Havlicek and Lenochova forbade the participants from sleeping in the same bed as their partner, engaging in rigorous exercise, using perfume, deodorants, antiperspirants, aftershave and shower gels, eating garlic, onions, chili, pepper, vinegar, blue cheese, cabbage, radish, fermented milk products and marinated fish, consuming alcohol or other drugs—and even from having sex (those who confessed to such venal sins were excluded from the final data analyses). After two weeks on either the “meat” or “non-meat” diet, participants then switched to the other diet for the following two weeks, so that each man’s body odor was assessed on both diets.

At each two-week interval, thirty female students from the same university were asked to inhale the scent of cotton pads that had been affixed to the men’s underarms the entire day before. I don’t know whether it would have been worse being one of the male “odor donors” in this study and having to change my diet so drastically for such an extended length of time, or being a female odor judge and having my delicate olfactory sense pierced by even a moment of such miasmic horrors. (By the way, the women received a perfume tester and a chocolate bar for their help.) The ends justified the means, though, at least for the sake of vegetarian males in Prague. We now know that, even independent of where women are on their menstrual cycles, the “odor signatures” of men on a non-meat diet are perceived by fertile women to be less intense, more pleasant and more attractive than their meat-eating peers.

Now, if you’ll excuse me, I have the sudden urge to take a shower.

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.