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."
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."
