Chemical Reaction

Love in the air—and the sweat glands?
Why the sense of smell is a matter of the heart
Long before the days of Internet dating, classified personal ads and matchmaking yentas, humans found love using the simplest of means. They used their eyes to look for healthy, virile types, and their ears to listen for pleasing voices. But perhaps most importantly, they used their noses to take a good strong sniff.What, exactly, are we inhaling? And what does this tell us about a potential partner? Believe it or not, we can find a few clues in funky-smelling T-shirts.In a famous experiment conducted in the late 1990s, University of New Mexico researchers found that women were attracted to specific odors in sweat. They consistently preferred the sweaty T-shirts worn by men whose so-called MHC genes were different from theirs. Having a variation in these genes, scientists say, enables offspring to fight infections more easily, and thus provides better odds for survival. Somehow, MHC genes influence the smell of sweat.But men, in a companion experiment, didn’t respond to female sweat—or, beg your pardon, perspiration—the same way. Instead of registering MHC genes, men used scent cues to detect the stage of a woman’s menstrual cycle. Most attractive were scents from women who were in mid-cycle, when they were most likely to conceive. (Men also have higher levels of testosterone after inhaling the scent women give off during ovulation.)

Here’s a whole new type of chemistry lesson: A woman with MHC genes too similar to those of her partner might be inclined look elsewhere for love. Researchers found that in 48 heterosexual couples together at least two years, those with similar MHC genes were less sexually responsive to their partners and more likely to have affairs. Male sexual behavior seemed to be unaffected by the variation or similarity in MHC genes.

Love Without the Odor

Snakes do it. Mice, moths and some monkeys do it. No, they don’t fall in love. But they do use pheromones—odorless chemicals given off and sensed by animals of the same species—to communicate with each other. The first observation of pheromones at work occurred in the 19th century, when a Frenchman noticed that male moths were attracted to a captured female living in his laboratory.

These chemicals can speak volumes to others in the group about things like food location, sexual receptivity and territory boundaries. But do humans use them, too?

Up until about 25 years ago, the answer was a definite no. Unlike most other mammals, reptiles and social insects, humans did not appear to have a special sense organ for pheromones, called a vomeronasal organ. But then someone found something that looked like one: two small slits inside the nose. The question remained, was it wired to the brain and functional, or merely a vestige from a previous incarnation, like the tail that appears and then vanishes in human fetuses?

The answer remains controversial. A major clue the vomeronasal organ doesn’t work is that no Old World primates (chimps and gorillas), our closest relatives, appears to have one. In 2003, Jianzhi George Zhang, an evolutionary geneticist at the University of Michigan in Ann Arbor, using a computer model, traced the dormant gene unique to the pheromone pathway, called TRP2. He found the gene was last active 23 million years ago, almost precisely the time when Old World primates developed full-color vision. It is thought vision might have become a more useful tool for discriminating between possible mates than pheromones, mostly because you can see from a distance rather than be required for a close-up encounter to engage your sense of smell.

But then how do you explain the research findings that women who live together synchronize the timing of their menstrual cycles? Are the cues from pheromones or odors? Perhaps it doesn’t matter. In 2003, Duke University neuroscientist Larry Katz and his team discovered that mice can sense pheromones in their primary olfactory system. So maybe humans can, too.

In any case, some scientists are positive they’ve been able to manufacture synthetic pheromones touted to do everything from enhance your sexual attractiveness to giving you more self-confidence. Other experts remain skeptical about how or even if these chemicals produce their alleged effects.

Why Love Feels So Good

Regardless of whether humans use scented or unscented chemicals to communicate sexual signals, every one of us relies on chemicals produced in the brain to fall—and hopefully stay—in love.

One of the most intriguing is the hormone oxytocin. If it could be synthesized and bottled, oxytocin would have to be made illegal. Among several other duties, it regulates the feeling of trust between people, perhaps by calming the amygdala, the section of the brain that governs fear. Its levels are known to rise during sexual arousal and even by looking at a picture of a long-term partner. It’s a bonding, feel-good chemical.

To study its effects, scientists asked participants in a National Institute of Mental Health experiment to hand over “money” to an “investor,” with no assurance that it would be returned. If they inhaled oxytocin first, the subjects were much more likely to invest.

But if some chemicals make us feel wonderful about being in love, others seem to make us feel crazy. Donatella Marazziti of the University of Pisa in Italy won a tongue-in-cheek “Ig Nobel” prize in chemistry for her 1999 discovery that couples in love have the same low levels of serotonin, the anxiety-reducing neurotransmitter, as people with obsessive-compulsive disorder.

Marazziti later went on to study testosterone levels in couples newly in love. Surprisingly, she found that the hormone, associated with puberty, sexual drive and aggression, tended to decrease in men and increase in women (men and women both produce it naturally, though men create 20 times more).

What could be the reason? Marazziti speculated to New Scientist that “men, in some way, had become more like women, and women had become like men. It’s as if nature wants to eliminate what can be different in men and women, because it’s more important to survive (and mate) at this stage.”

The science of human love and attraction, though in its infancy, is producing a great deal of fascinating data. One result: Free will doesn’t seem to play nearly as large a role in mate selection and retention as our instincts do.

Fact is, our genes seem to be masters of employing and deploying all sorts of chemicals—odors, hormones and perhaps even pheromones—in a never-ending, all-consuming quest to help us find love—and survive

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