This article was written by Sam Kean and originally appeared in the 'Future of Sex' issue of mental_floss magazine (March-April 2008).


If humans can train (or genetically engineer) themselves to harness pheromones, sexual attraction in the future may depend more on how someone smells than how someone looks—or whether they're a man or a woman.

In the late 19th century, one of Sigmund Freud's nuttier friends, Dr. Wilhelm Fliess, classified the nose as the body's most potent sex organ. According to his nasal reflux neurosis theory, "genital spots" inside the nose could excite the male libido and kick-start ovulation in women. His colleagues, however, scoffed at the idea, and Fliess died in obscurity.

But history may judge Fliess more favorably. While sex researchers have abandoned the more lurid aspects of nasal reflux theory, they agree that the nose's sensory detectors can affect—if not flatly determine—human sexuality. There's now reason to think that, as the nose goes, so goes sex.

Scent and Sexuality

sense2.jpgMillions of years ago, humans and other primates could detect lots of pheromones, and we still have vestiges of such detectors in our DNA. The airborne chemicals helped people choose mates by enabling them to "smell" the genetic material of others. But after we evolved to have color vision, we were able to pick up on emotional changes, such as blushing. Our ability to perceive attraction via pheromones died out; there was simply no point in having two ways to do the same thing.


Most colorblind land animals, however, still map the world through scent. What their noses "see" around them are pheromones, and those pheromones determine their behavior. For example, when female wild hamsters are in heat, they secrete a sticky, pheromone-infused fluid that sends randy males scurrying long distances to get to them. But in the 1980s, researchers at Florida State University decided to see what would happen if they surgically removed the pheromone-detecting organs of virgin male hamsters and released them into a cage full of willing females. Without their pheromone-sniffers, the poor guys had no idea what to do.

Then, in 2006, molecular biologists at Harvard University released one of the most mind-blowing studies ever to hit the field of animal sexuality. The scientists genetically disabled the pheromone-detecting organ in a group of female mice. For the most part, the mice behaved normally. But in the presence of "normal" female mice, the altered females stormed them like sex-starved males. Despite lacking the proper equipment, the altered females mounted the normal ones and began thrusting their hips. They even groaned like males at climax, emitting an ultrasonic squeal that, until then, had only been documented in males.

We Are What We Sniff

The Harvard experiment overthrew basic assumptions about physical attraction by showing that sexual orientation in mice is malleable. The big question, now, is how much of this pheromone stuff applies to people.

For much of the 20th century, scientists dismissed the importance of human pheromones. The thinking was that, if they had any real effect, then surely we'd be able to smell them. And humans don't just breed; we fall in love—a process much too complicated to be driven by chemicals.

Still, a few dogged scientists continued pursuing the idea, and a breakthrough came in 1971. Psychologist Martha McClintock came forward with evidence supporting an old wives' tale: Women who live together menstruate together. Suspecting that sweat pheromones were the cause, McClintock forged ahead with decades of follow-up work (some of which required collecting sweat on cotton swabs and wiping it on the upper lips of unsuspecting subjects). Eventually, she confirmed her notion that pheromones influence us biologically, even though we can't detect them.
pheromone3.jpg

Following McClintock's lead, other sex researchers explored pheromones via the decidedly unsexy territory of the armpit. They found that pheromones in sweat could make skin temperatures and hormone levels fluctuate, indicating arousal. And later, MRI studies showed that pheromone concentrations as low as one-trillionth of a gram lit up people's brains like fireworks—especially the parts responsible for sexual behavior and processing emotions.

But perhaps nothing could have prepared the scientific community for the bomb dropped in 2005. A group of doctors in Sweden published an experiment that had exposed straight women, straight men, and homosexual men to certain smells. They found no difference in the brain scans when the subjects sniffed things like lavender. But when they exposed the three groups to a pheromone in male sweat, an enormous gap emerged. The brains of straight women and gay men looked aroused, while straight men showed no reaction. Similarly, a follow-up experiment revealed that pheromones in female urine stimulated the straight men and gay women, but not the straight women.

The Swedish study proved that pheromones unconsciously influence human sexuality, and it points to a deeper role they might play in driving sexual behavior. But for humans to actually harness pheromones, we first need to become aware of them. And that would require developing the proper biological equipment—or, more accurately, relearning to use the equipment from our prehistoric days.

The Flip of a Genetic Switch

To detect pheromones, most vertebrates use the vomeronasal organ (often called the VNO, or Jacobson's organ). It's the thing that, when disabled, made female mice thrust their hips and also turned male hamsters impotent. Since the early 1990s, scientists have fiercely debated whether or not humans have functional VNOs.

We know we have something similar in our nasal cavities. It's a ridge a few millimeters long with a hollow pit on each side. You can see it with a flashlight if you peer up someone's nostrils. And although some MRI evidence has shown reactions to VNO stimulation in subjects, most scientists dismiss the VNO as a vestige, like the appendix. They argue that we already detect pheromones with our normal sense of scent, which makes VNOs unnecessary. Also, they point out that it's regularly removed during nasal reconstructive surgery without any obvious side effects.

Perhaps the most damning evidence against the human vomeronasal organ is that no one can find any nerves connecting it to the brain. It seems the human VNO is unplugged—at least in adults. Fetuses possess a functioning VNO, complete with wires to the brain, but it deteriorates around the 17th week of pregnancy. Now scientists just have to figure out how to keep that system intact. Trolling the human genome and hitting the right switches would be difficult, but geneticists can already turn genes "on" and "off" in other species. So, most scientists concede it could work.

The Sense of Sex

What would it be like to have a working VNO? Think of it as a sixth sense, or a "sex sense," to complement the traditional five. It could allow men to smell when women are ovulating, and decide whether or not it's the right time to have sex. Researchers are already working on a pheromone-based version of birth control pills. After all, the ability to "read" ovulation cycles consciously could be the most effective birth control ever devised.

Having a real pheromone sniffer could also help people judge their compatibility with potential mates. Every person has different markers on the surface of their cells called Major Histocompatibility Complex (MHC) markers that indicate immunities or susceptibilities to various diseases. Like fingerprints, your cellular protein signatures—MHC markers—are unique. To create the fittest offspring, you need to mate with someone with strengths and weaknesses very different from your own. That's why the body has evolved to advertise its MHC through sweat. Research suggests that humans mate with people with different MHCs far more often than chance would dictate. (Which also explains why you're not attracted to relatives.) An enhanced pheromone detector could make that attraction stronger and more explicit—leading to healthier children with better immune systems.

Even if humans never grow pheromone detectors, scientists have learned that human sexuality and animal sexuality aren't so far apart. Consider the mice in the Harvard experiment. Disabling pheromone detectors made otherwise normal females behave like males, and it's believed male mice have a similar neurological switch. So, what if the human brain has one, too?
Meanwhile, the Swedish experiment suggests that the brains of gay and straight people process pheromones differently. If scientists can understand the pathways involved—and they're working hard to do so—that might suggest ways to temporarily control, or even disable, them. It's a leap, but it's possible that the right drug cocktail could blur your sexuality. You'd probably still prefer redheads or button noses, because you'd be the same person, but you might find those features attractive in males one night, females the next.

Note that this wouldn't be the same as turning heterosexuality and homosexuality "on" or "off." Sexuality isn't binary. In fact, some scientists are speculating whether the most important effect of pheromones in 50 years could be to teach us that deep down, like the mice, we're all wired as bisexuals. If that's true, then a preference for men or women could turn out to be as arbitrary as preferring cedar to musk or Chanel No. 5 to Old Spice.