"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.
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.
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 asubconscious 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."
If you were on the pill when you met your current partner, the situation is more complicated. Once a relationship has progressed to long-term commitment, says Herz, a woman's perception of her partner's smell is so intertwined with her emotional reaction to him that it could be difficult for her to assess his scent as if he were a stranger. "If she's in love, he could smell like a garbage can and she'd still be attracted to him."