This Is Your Brain on Sex (16 page)

In the fifty-odd years since Karlson and Lüscher first came up with the term, there has been some
debate about whether there is a human pheromone. There are many reasons for this, and most are beyond the scope of this book. Some say pheromones work through the vomeronasal organ, a specialized olfactory organ that interfaces with the brain in lower mammals. But humans don’t have this organ; ergo, some say, we don’t process pheromones. Some say human olfactory processing has evolved beyond the simple molecule. Like our big frontal lobes, human chemosensory signals are more complex than simple pheromone molecules. But perhaps the most important criticism of human pheromones is that, as of yet, a unique human pheromone has not been identified. The only candidate, discovered by Martha McClintock at the University of Chicago, is a chemical that helps to sync the menstrual cycles of cohabitating females. Though studies like Zhou and Chen’s have demonstrated that humans are susceptible to chemical messages from smells, it’s unclear if those chemicals are pheromones in the strict sense of the word.
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One chemosensory compound that has been the subject of intense study is the major histocompatibility (MHC) complex. As the name says, it’s a complex: a mixture of hundreds of different compounds, perhaps more. Although the MHC complex is often talked about as if it were a pheromone, it doesn’t fully meet the criteria. It is a group of genes that leads to a specific odor-print. As it turns out, every human has a signature odor—like a fingerprint. In humans the MHC complex is based on the human leukocyte antigen (HLA) system. And this group of genes that regulate the human immune system are also responsible for our unique odor-prints. The variability in the HLA cluster of genes results in one’s own personal and particular bouquet.

The MHC complex aids animals in identifying their own offspring and family members. It also helps them assess a potential mate. The variability seen in the MHC complex is not just responsible for that odor-print; it is also linked to a healthy immune system. It’s no different in humans: the more variability in your HLA, the better your immune system. Because of this, it was long hypothesized that folks were most attracted to those who had an MHC complex as different from their own as possible—a little sniffable yet unconscious note to lock on to the mate that will help us produce the strongest, healthiest offspring.

Over ten years ago Swiss researchers at the University
of Bern undertook what I have now come to think of as the “stinky tee” experiment.
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The group genotyped forty-nine female students and forty-four male students, looking specifically at their HLA gene sequences. The men were then asked to live “odor-neutral” for a few days, avoiding sexual activity, odor-producing foods, and cigarettes; on two consecutive nights they slept in a T-shirt provided by the researchers. When the T-shirts were returned to the experimenters, the women were asked to sniff six different shirts and rate them for intensity, pleasantness, and sexiness.

The researchers found that HLA mattered: women rated the odor of the men whose HLA systems were more dissimilar from theirs as more pleasant and sexy, compared to the men whose HLA systems were similar to theirs. This trend was reversed if the woman was taking oral contraceptives. The findings led the researchers to conclude that HLA-linked genes do influence female mate choices, supporting the hypothesis that divergent MHC complexes will result in attraction.

A more recent study, led by McClintock, challenged the notion that the more different, the more attractive. The group did their own “smelly tee” test with forty-nine women. In this case the researchers selected a diverse group of male sweaty T-shirt donors, but made sure there was some overlap with common HLA alleles found in the female smellers’ families. McClintock and her colleagues discovered that women were able to discriminate the differences in HLA genotypes. However, they did so based on the HLA alleles they inherited from their father. What does this mean, exactly? These women did not prefer odors that were completely different from their own HLA system–influenced odor-print. Rather, they preferred odors that had a couple of alleles in common with dear old Dad. It would seem that women are programmed to prefer a man who retains some genetic similarity to their own paternal ancestry.

“Women need to optimize a choice of mate in hopes of providing the best possible immune system for offspring,” said Charles Wysocki, a researcher at the Monell Chemical Senses Center in Philadelphia. “She can do this by choosing a male whose immune system genes are different from hers. But not maximally different, optimally different.”

So if at last year’s family reunion you found your fourth cousin (twice removed) on your dad’s side more attractive than you thought
you should, you are not quite as weird as you thought. Your olfactory system was just on the lookout for optimal genetic diversity.
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Several online dating companies now offer HLA matching services as part of their programs. For a cool grand, you can have potential matches genotyped to see how closely related your HLA systems are. While doing so probably won’t hurt your dating scorecard, there’s no guarantee that similar HLA genes make for a happy relationship. And the HLA complex is not the only chemical being released by the body. Wysocki argues that there may be more olfactory cues that can help women suss out optimal partners. They simply haven’t been characterized yet.

Sexy as a Boar’s Saliva

Though scientists have not officially designated any one chemical as a “pheromone” in humans, they have identified a few in other mammals. The most well-known of these is a compound called androstenone. When female pigs in heat get a whiff of this stuff in a boar’s saliva, they immediately assume lordosis, or the arched back ready-to-get-busy position. You can even buy it in a convenient spray can, a product called Boarmate, for all your swine husbandry needs. It is used in pig farms across the world to assist in artificial insemination.

As it so happens, androstenone and a related chemical called androstadienone also reside in human male axillary compounds—that’s fancy talk for pit sweat—as well as in their saliva and urine. This inclusion has led some to suggest that these two compounds also work as pheromones in humans. Others, however, contend its girl-getting properties are unique to pigs.

This debate hasn’t stopped several companies, most Internet-based, from marketing and selling androstenone as a “pheromone” fragrance. Men can buy their own androstenone, which, according to the sellers, will assist in picking up the ladies. But Wysocki cautioned that we shouldn’t believe all the hype. “Pheromones aren’t what you read about on some of those Internet sites,” he told me. “Human evolution has stepped away from the
reflexive response to pheromones we see in moths, rats, or mice. We have a lot of cognitive input in how we respond to situations—there’s a lot more going on than pure reflex.” He paused for a moment. “We know there is some unconscious processing of human body odor. And there is some evidence to suggest body odor can help us identify individuals we know or perhaps attract us to others. But there is simply no good, reliable experimental evidence to support the claim that some pheromone spray you buy on the Internet is going to help make you more attractive to others.”

More than a few who have balked at the price tag for an androstenone cologne made for humans have instead opted to try Boarmate as a personal fragrance. Some have even written about their experiences on the Internet. These tales all seemed to be very positive. Yet I found myself wondering, if these folks were so busy getting busy thanks to this pig pheromone, how they have the time to write so eloquently (and frequently) about their many sexual exploits.

One of the most interesting things about androstenone is that not every human can smell it—and that those who can think it smells either quite pleasant, like vanilla, or absolutely disgusting. You would think that if it were a human pheromone guaranteed to attract the ladies, it would smell yummy to every woman. I was certainly curious about what it might smell like to me. I decided to find out. Though Wysocki told me I could pick up a can of Boarmate at any agricultural supply shop, it was harder to track down than I anticipated. Eventually, after a lot of searching, I found an online store in the United Kingdom willing to ship me a can.

When the package arrived in the mail, I quickly pulled it open to find a small yellow aerosol can emblazoned with a cartoonish red pig. Hardly sexy. Of course it’s meant for use with pigs, not guys wanting to pick up hot chicks at the local watering hole. Curled up on my bed, my female cat, Boo Boo, at my feet, I read the application instructions: “Spray BOARMATE™ at the gilt or sow’s snout for two seconds from a distance of approximately 60 centimeters, then apply pressure to the back of the sow.” If, after you press down on the pig’s back, she assumes the lordotic position, she’s ready to go. It’s time to inseminate. Simple enough.

I suppose I could have just sprayed
Boarmate in the air and taken a whiff to satisfy my curiosity. But Boo Boo, blissfully snoozing at the foot of the bed, gave me another idea. Yes, it was a mean idea. A despicable idea, really. But I could hardly push on my own back after application to see if I would go into lordosis. Humans don’t do lordosis. But cats do. So I sprayed near poor Boo Boo’s nose for two seconds and pushed lightly on her back.

Being a kind and placid animal, the best of pets, she did not rear up and scratch out my eyes for experimenting on her in such a manner. Frankly, I wouldn’t have blamed her if she had. She simply sniffed at the spray with wide-open eyes, shook her head with menace, and ran out of the room as fast as her little kitty legs could carry her. But not before I could give a quick push down on her back. It probably doesn’t come as a surprise that there wasn’t any lordosis. She fought my push every step of the way, mainly because she was trying so desperately to make a break for it. Of course, it might also have had something to do with the fact that she wasn’t in heat; the Boarmate instructions said I should wait until the sow was in estrus before spraying. But as she was a cat, not a sow, I hardly felt the need to follow the instructions to the letter. Neither do most humans who use the stuff themselves: they don’t spray an unsuspecting girl at a club with androstenone, an act that might get them slapped or arrested; they spritz themselves. And I doubt they bother to wait for estrus either. Still, as I said, it was a mean idea. My quick spray did not result in an animal ready for some hot reproductive action. Instead it just pissed off a normally loving feline who refused to come near me for the rest of the day.

The experiment was a success in one respect, however: it allowed me to smell androstenone for myself. I fall into the category of humans who can smell it. Inhaling deeply, I did not think it smelled like vanilla in the slightest, nor did it smell like garbage. For me, Boarmate was reminiscent of the precursor of deodorant failure. You know how sometimes you get a whiff of what might be your own body stink, so you try to inconspicuously sniff under your underarm to check if you need to wash up? Upon careful reflection, I concluded androstenone smells most like that sweaty antecedent—the almost pit stink—to me. If I were to smell it on a guy who was trying to chat me up,
I’m fairly certain I’d try to keep some distance, or maybe walk to the ladies’ room to make sure the smell wasn’t coming from me.

Why might there be such differences in how people perceive androstenone? It all comes down to a type of olfactory receptor. “Androstenone has been known to have a very different olfactory percept for different people,” said Hiroaki Matsunami, a molecular geneticist at Duke University. “We were interested in finding out the genetic basis of that.”

Matsunami and his colleagues collected blood from nearly four hundred individuals for genotyping. The group was particularly interested in olfactory receptor genes—that is, receptors in the epithelium of the nose that perceive smells. The study participants were then asked to rate the intensity and valence of more than sixty different smells, including androstenone and its close cousin, androstadienone. The group discovered that one particular receptor gene, called
OR7D4,
was linked to whether individuals could smell these pheromones. And a particular variation, or polymorphism, of the gene in some individuals was correlated to whether they found the smell pleasant or grody.
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So there appears to be a human olfactory receptor that is specific to this chemical, a known pig pheromone. When I asked Matsunami if this work provided evidence for or against human pheromone signaling, he offered caution. “The idea is still very controversial, and there is evidence on both sides,” he said. “If my contribution about the variation in these receptors leads to understanding of how these chemicals can have a pheromonal effect in humans, that would be nice to see. But I haven’t drawn any conclusions yet.”

Matsunami was quick to point out that his study involved one receptor out of hundreds. Not to mention that the chemicals androstenone and androstadienone are only two molecules out of potentially millions. The task ahead—to try to understand how the brain makes sense of all these different chemicals and what effects they may have on behavior—is a daunting one at best.

Pheromones and the Brain

Despite the controversy over whether
pheromones can really affect humans, a group of researchers at Sweden’s Karolinska Institute, including Ivanka Savic, have used positron emission tomography (PET), a neuroimaging technique, to see how pheromones like androstadienone (AND) and an estrogen-like steroid (EST), a compound found in female urine and more common odors like lavender and cedar, affect cerebral blood flow.
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Previous studies have demonstrated that exposure to AND and EST can alter mood, heart and respiratory rate, and skin conductance in humans. They’ve been shown to do so in a gender-specific manner too. AND does this in females, while EST does so in males. Savic and her colleagues wondered if that effect extended to brain activation. To that end, the researchers scanned the brains of twelve healthy men and twelve healthy women as they smelled AND, EST, and odorless air. The group found that AND and EST activated the anterior hypothalamus in the same gender-specific manner seen in other studies. That is, AND activated the sex and reproduction region of the brain in the women, while the EST lit it up in the men. This kind of sexually dimorphic effect is not seen in common odors. Thus, Savic argues, this is evidence that humans are influenced by pheromone-like signals. “Some argue we cannot perceive pheromones because we do not have a [vomeronasal organ]. But it seems that we can read signals from pheromone-like compounds through the olfactory system,” Savic said. “It takes another path, a fairly direct path, from the olfactory mucosa up to the brain.”