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Thursday, January 17, 2008Sexual chemistry?
"Sexual chemistry" is more than just a way of talking about heated attraction. Subtle chemical keys actually help determine who we fall for. But here comes news that our lifestyles may unwittingly undermine our natural sex appeal.
By: Elizabeth Svoboda
Psychologists Rachel Herz and Estelle Campenni were just getting to know each other, swapping stories about their lives over coffee, when Campenni confided something unexpected: She was living proof, she said, of love at first smell. "I knew I would marry my husband the minute I smelled him," she told Herz. "I've always been into smell, but this was different; he really smelled good to me. His scent made me feel safe and at the same time turned on—and I'm talking about his real body smell, not cologne or soap. I'd never felt like that from a man's smell before. We've been married for eight years now and have three kids, and his smell is always very sexy to me."
Everyone knows what it's like to be powerfully affected by a partner's smell—witness men who bury their noses in their wives' hair and women who can't stop sniffing their boyfriends' T-shirts. And couples have long testified to the ways scent-based chemistry affects their relationships. "One of the most common things women tell marriage counselors is, 'I can't stand his smell,'" says Herz, the author of The Scent of Desire.
Sexual attraction remains one of life's biggest mysteries. We might say we go for partners who are tall and thin, love to cook, or have a mania for exercise, but when push comes to shove, studies show, the people we actually end up with possess few of the traits we claim to want. Some researchers think scent could be the hidden cosmological constant in the sexual universe, the missing factor that explains who we end up with. It may even explain why we feel "chemistry"—or "sparks" or "electricity"—with one person and not with another.
Physical attraction itself may literally be based on smell. We discount the importance of scent-centric communication only because it operates on such a subtle level. "This is not something that jumps out at you, like smelling a good steak cooking on the grill," says Randy Thornhill, an evolutionary psychologist at the University of New Mexico. "But the scent capability is there, and it's not surprising to find smell capacity in the context of sexual behavior." As a result, we may find ourselves drawn to the counter attendant at the local drugstore, but have no idea why—or, conversely, find ourselves put off by potential dating partners even though they seem perfect on paper.
Though we may remain partially oblivious to scent signals we're sending and receiving, new research suggests that we not only come equipped to choose a romantic partner who smells good to us, but that this choice has profound biological implications. As we act out the complex rituals of courtship, many of them inscribed deep in our brain, scent-based cues help us zero in on optimal partners—the ones most likely to stay faithful to us and to create healthy children with us.
At first blush, the idea of scent-based attraction might seem hypothetical and ephemeral, but when we unknowingly interfere with the transmission of subtle olfactory messages operating below the level of conscious awareness, the results can be both concrete and devastating. When we disregard what our noses tell us, we can find ourselves mired in partnerships that breed sexual discontent, infertility, and even—in extreme cases—unhealthy offspring.
The Scent of Desire
When you're turned on by your partner's scent, taking a deep whiff of his chest or the back of her neck feels like taking a powerful drug—it's an instant flume ride to bliss, however momentary. Research has shown that we use scent-based signaling mechanisms to suss out compatibility. Claus Wedekind, a biologist at the University of Lausanne in Switzerland, created Exhibit A of this evidence by giving 44 men new T-shirts and instructing them to wear the shirts for two straight nights. To ensure that the sweat collecting on the shirts would remain "odor-neutral," he supplied the men with scent-free soap and aftershave.
After the men were allowed to change, 49 women sniffed the shirts and specified which odors they found most attractive. Far more often than chance would predict, the women preferred the smell of T-shirts worn by men who were immunologically dissimilar to them. The difference lay in the sequence of more than 100 immune system genes known as the MHC, or major histocompatibility complex. These genes code for proteins that help the immune system recognize pathogens. The smell of their favorite shirts also reminded the women of their past and current boyfriends, suggesting that MHC does indeed influence women's dating decisions in real life.
Women's preference for MHC-distinct mates makes perfect sense from a biological point of view. Ever since ancestral times, partners whose immune systems are different have produced offspring who are more disease-resistant. With more immune genes expressed, kids are buffered against a wider variety of pathogens and toxins.
But that doesn't mean women prefer men whose MHC genes are most different from theirs, as University of Chicago evolutionary biologist Martha McClintock found when she performed a T-shirt study similar to Wedekind's. Women are not attracted to the smell of men with whom they had no MHC genes in common. "This might be a case where you're protecting yourself against a mate who's too similar or too dissimilar, but there's a middle range where you're OK," McClintock says.
Women consistently outperform men in smell sensitivity tests, and they also make greater time and energy sacrifices on their children's behalf than men do—in addition to bearing offspring, they look after them most of the time. These factors may explain why women are more discriminating in sniffing out MHC compatibility.
Men are sensitive to smell as well, but because women shoulder a greater reproductive burden, and are therefore choosier about potential mates, researchers are not surprised to find that women are also more discriminating in sniffing out MHC compatibility.
Unlike, say, blood types, MHC gene complements differ so much from one person to the next that there's no obvious way to reliably predict who's MHC-compatible with whom. Skin color, for instance, isn't much help, since groups of people living in different areas of the world might happen to evolve genetic resistance to some of the same germs. "People of different ethnicities can have similar profiles, so race is not a good predictor of MHC dissimilarity," Thornhill says.
And because people's MHC profiles are as distinct as fingerprints—there are thousands of possible gene combinations—a potential sex partner who smells good to one woman may completely repel another. "There's no Brad Pitt of smell," Herz says. "Body odor is an external manifestation of the immune system, and the smells we think are attractive come from the people who are most genetically compatible with us." Much of what we vaguely call "sexual chemistry," she adds, is likely a direct result of this scent-based compatibility.
Typically, our noses steer us in the right direction when it comes to picking a reproductively compatible partner. But what if they fail us and we wind up with a mate whose MHC profile is too similar to our own? Carol Ober, a geneticist at the University of Chicago, explored this question in her studies of members of the Hutterite religious clan, an Amish-like closed society that consists of some 40,000 members and extends through the rural Midwest. Hutterites marry only other members of their clan, so the variety in their gene pool is relatively low. Within these imposed limits, Hutterite women nevertheless manage to find partners who are MHC-distinct from them most of the time.
The few couples with a high degree of MHC similarity, however, suffered higher rates of miscarriage and experienced longer intervals between pregnancies, indicating more difficulty conceiving. Some scientists speculate that miscarriages may be the body's way of curtailing investment in a child who isn't likely to have a strong immune system anyway.
What's more, among heterosexual couples, similar MHC profiles spell relational difficulty, Christine Garver-Apgar, a psychologist at the University of New Mexico, has found. "As the proportion of MHC alleles increased, women's sexual responsiveness to their partners decreased, and their number of sex partners outside the relationship increased," Garver-Apgar reports. The number of MHC genes couples shared corresponded directly with the likelihood that they would cheat on one another; if a man and woman had 50 percent of their MHC alleles in common, the woman had a 50 percent chance of sleeping with another man behind her partner's back.
The Divorce Pill?
Women generally prefer the smell of men whose MHC gene complements are different from theirs, setting the stage for the best biological match. But Wedekind's T-shirt study revealed one notable exception to this rule: women on the birth-control pill. When the pill users among his subjects sniffed the array of pre-worn T-shirts, they preferred the scent of men whose MHC profiles were similar to theirs—the opposite of their pill-free counterparts.
This dramatic reversal of smell preferences may reflect the pill's mechanism of action: It prevents the ovaries from releasing an egg, fooling the body into thinking it's pregnant. And since pregnancy is such a vulnerable state, it seems to activate a preference for kin, who are genetically similar to us and likely to serve as protectors. "When pregnant rodent females are exposed to strange males, they can spontaneously abort," Herz says. "The same may be true for human females." What's more, some women report a deficit in sex drive when they take the pill, a possible consequence of its pregnancy-mimicking function.
The tendency to favor mates with similar MHC genes could potentially hamper the durability of pill users' relationships in the long term. While Herz shies away from dubbing hormonal birth control "the divorce pill," as a few media outlets have done in response to her theories, she does think the pill jumbles women's smell preferences. "It's like picking your cousins as marriage partners," Herz says. "It constitutes a biological error." As a result, explains Charles Wysocki, a psychobiologist at Florida State University, when such a couple decides to have children and the woman stops taking birth control, she may find herself less attracted to her mate for reasons she doesn't quite understand. "On a subconscious level, her brain is realizing a mistake was made—she married the wrong guy," he says.
"Some couples' fertility problems may be related to the pill-induced flip-flop in MHC preferences," Garver-Apgar adds. No one has yet collected data to indicate whether the pill has created a large-scale problem in compatibility. Still, Herz recommends that women seeking a long-term partner consider alternative birth control methods, at least until they get to know their potential significant other well and are sure they like the way he smells. "If you're looking for a man to be the father of your child," she says, "go off the pill before you start your search."