Sexual attraction is a mysterious process that we almost always try to rationalize. After all, partner choices can be the most important decisions we'll ever make and it's maddening to think that we may not be in control of our own destiny ("Oh, he's a great guy but I can't stand the way he steers his car with his index finger."). But mating decisions were being made on this planet long before abstract thought evolved. Across the animal kingdom, these decisions are predominantly mediated by the sense of smell and there is a growing body of evidence to suggest that humans are no different.
There are undoubtedly many ingredients that go together to make up what we call romantic attraction. We're visual creatures and we all want our potential mates to be as visually attractive as possible. We also have our checklist of things we like and don't like: "Wipes nose on sleeve - one demerit; petted my dog - bonus point!" It might sound a little cold-blooded, but romance is a game in which we seek to score the highest value mate. We tend to tally up value for value. As Dr. House once said; "I've seen an 8 with a 7. Maybe even a 6. But a 2?" Henry Kissinger made his "power is an aphrodisiac" remark because, let's face it, it was the only possible explanation for his romantic success.
But our position in the value matrix is not the only determinant of who we're likely to mate with. We've all encountered wonderful, attractive women who would be perfect mates for anybody but ourselves. And most of us have at some time been relegated to "just a friend" status by women who should, by any objective measure, be madly in love with us. What explains this? It's easy to write off the laws of attraction as hopelessly complex, but could it actually be something very simple, something right under our, ahem, noses?
The idea that chemicals play a key role in sexual communication is not new. The term pheromone, from the Greek pherein (to transport) and hormon (to stimulate), was coined by Adolf Butenandt in 1959 to describe a molecular attractant he discovered in the silkworm. Since then, dozens have been identified (mostly in the insect world) and many others have been postulated. In insects, the receptors for these chemicals are located throughout the body. In mammals and reptiles, pheromones are detected almost exclusively by the vomeronasal organ (also called Jacobson's organ) which is located in the nasal cavity. In humans it appears as two depressions, one on each side of the nasal septum.
It's a little known fact that most land vertebrates have two distinct and separately evolved senses of smell. One, the olfactory which we identify as smell, is primarily a means of tasting the air. This sense is mediated by neurons that line the inside of the nose. Basically, if a molecule happens to match up with a receptor molecule on the neuron, the neuron fires and sends a signal to the olfactory bulbs on the underside of the brain where the signal is interpreted. This is turn generates brain activity touching on memory, emotion and finally the language center, which directs the mouth to form around the words; "Alright, who farted?"
The auxiliary sense of smell that is picked up by the aforementioned Jacobson's organ appears to deal largely with pheromones (although cats apparently sense catnip through their Jacobson's organ). Rats and pigs deprived of Jacobson's organ can't differentiate between male and female. Female prairie voles without it fail to present themselves sexually and don't reproduce.
In humans and higher primates, the vomeronasal organ is thought to be vestigial (an evolutionary leftover). While the organ is well developed in human and primate embryos, it recedes as we grow in the womb. To date, no one has been able to identify a connection between what's left of our vomeronasal receptors and our brain. There has been much speculation as to why the vomeronasal organ became obsolete in humans, but the best explanation is that we developed other means (visual, linguistic etc.) of relaying information. Use-it-or-lose-it seems to be the evolutionary rule with neural tissue like Jacobson's organ, and in this case, it appears that we lost it.
So, if we lack the organ that in most other mammalian species is the conduit for pheromonal communication, how is it that we humans are receptive to pheromones? Well, there are three schools of thought. The first, proposed by Louis Monti-Bloch and popularized by Lyall Watson in his book Jacobson's Organ and the Remarkable sense of Smell, speculates that the human vomeronasal organ is, in fact, functional after all. Monti-Bloch claims that in clinical experiments, vomeronasal stimulation generated "gender-specific behavioral changes, modulation of autonomic nervous system function, or the release of gonadotropins from the pituitary gland." In other words, even though everybody insisted the organ was broken, they plugged it in and it worked. To date, the experiments haven't been duplicated, so the jury is still out, but Monti-Bloch's results are certainly intriguing.
The second hypothesis is that the receptors for human pheromones are actually part of the olfactory system. And there is some evidence for this. Pheromone receptors have been found in the olfactory epithelium of mice. So it's possible that humans have evolved similar receptors.
The third school of thought is that humans are not affected by pheromones and all the studies purportedly demonstrating their effects are statistically and methodologically flawed. These include the now famous menstrual synchrony experiments conducted by Martha McClintock in the early seventies that generated the first wave of interest in human pheromones. While the arguments of the pheromone naysayers are strong, the preponderance of evidence still suggests that humans do respond to unconscious chemical signals.
Humans seem to be most receptive to a class of hormones produced by the apocrine glands found in the armpit. The hormones themselves are odorless (armpits only smell because hormonal secretions are immediately set upon by hordes of bacteria) but they appear to affect us both psychologically and physiologically. In double blind "smell tests", both women and men report a strong preference for the armpit secretions of strangers over those of family members. In brain imaging experiments, exposure to estrogen based pheromones activated the hypothalamus (the area of the brain governing sexual function) in men. Exposure to the male sweat chemical produced the same effect in women.
The evidence for the existence of human pheromones is sufficiently strong that there has been something of a gold rush amongst pharmaceutical companies to develop a synthetic pheromone. Results so far have been mixed, although at least one study conducted in 2000 by researchers at San Francisco State University concluded that women who wore a perfume laced with synthetic pheromone had reported a significant increase in socio-sexual behavior, including being approached by men. Given the huge success of erectile dysfunction treatments, it's likely to be just a matter of time before the big pharma companies develop a synthetic pheromone for the general public; after all sex clearly does sell.
Will the availability of such chemical attractants fundamentally change society? Probably not. If pheromones exist, we're already pumping them out. We're breathing in clouds of other people molecules all the time. I suspect that what makes one person smell more attractive than another is highly subjective and almost entirely unconscious. We'll probably never fully understand how it all works let alone create a concoction that works like cupid's arrow.
But for those who've been rejected or relegated to be "just a friend", it might help to know that love is not all about perceived value, it's also largely - maybe mostly - about chemistry. Every person has his/her own molecular key. Sooner or later you're bound to find the lock it opens.
