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Studies show that laboratory mice can smell too-similar MHC in the urine of other mice and will avoid mating with those individuals. In later work conducted at the University of Bern in Switzerland, human females were asked to smell T shirts worn by anonymous males and then pick which ones appealed to them. Time and again, they chose the ones worn by men with a safely different MHC. And if the smell of MHC isn't a deal maker or breaker, the taste is. Saliva also contains the compound, a fact that Haselton believes may partly explain the custom of kissing, particularly those protracted sessions that stop short of intercourse. "Kissing," she says simply, "might be a taste test."
Precise as the MHC-detection system is, it can be confounded. One thing that throws us off the scent is the birth-control pill. Women who are on the Pill—which chemically simulates pregnancy—tend to choose wrong in the T-shirt test. When they discontinue the daily hormone dose, the protective smell mechanism kicks back in. "A colleague of mine wonders if the Pill may contribute to divorce," says Wysocki. "Women pick a husband when they're on birth control, then quit to have a baby and realize they've made a mistake."
Less surprising than the importance of the way a partner smells is the way that partner looks and sounds. Humans are suckers for an attractive face and a sexy shape. Men see ample breasts and broad hips as indicators of a woman's ability to bear and nurse children—though most don't think about such matters so lucidly. Women see a broad chest and shoulders as a sign of someone who can clobber a steady supply of meat and keep lions away from the cave. And while a hairy chest and a full beard have fallen out of favor in the waxed and buffed 21st century, they are historically—if unconsciously—seen as signs of healthy testosterone flow that gives rise to both fertility and strength.
A deep voice, also testosterone driven, can have similarly seductive power. Psychology professor David Feinberg of McMaster University in Ontario studied Tanzania's Hadza tribesmen, one of the world's last hunter-gatherer communities, and found that the richer and lower a man's voice, the more children he had. Researchers at the University of Albany recently conducted related research in which they had a sample group of 149 volunteers listen to recordings of men's and women's voices and then rate the way they sound on a scale from "very unattractive" to "very attractive." On the whole, the people whose voices scored high on attractiveness also had physical features considered sexually appealing, such as broad shoulders in men and a low waist-to-hip ratio in women. This suggests either that an alluring voice is part of a suite of sexual qualities that come bundled together or that simply knowing you look appealing encourages you to develop a voice to match. Causation and mere correlation often get muddied in studies like this, but either way, a sexy voice at least appears to sell the goods. "It might convey subtle information about body configuration and sexual behavior," says psychologist Gordon Gallup, who co-authored the study.
The internal chemical tempest that draws us together hits Category 5 when sex gets involved. If it's easy for a glance to become a kiss and a kiss to become much more, that's because your system is trip-wired to make it hard to turn back once you're aroused. That the kiss is the first snare is no accident.
Not only does kissing serve the utilitarian purpose of providing a sample of MHC, but it also magnifies the other attraction signals—if only as a result of proximity. Scent is amplified up close, as are sounds and breaths and other cues. And none of that begins to touch the tactile experience that was entirely lacking until intimate contact was made. "At the moment of a kiss, there's a rich and complicated exchange of postural, physical and chemical information," says Gallup. "There are hardwired mechanisms that process all this."
What's more, every kiss may also carry a chemical Mickey, slipped in by the male. Though testosterone is found in higher concentrations in men than in women, it is present in both genders and is critical in maintaining arousal states. Traces of testosterone make it into men's saliva, particularly among men who have high blood levels of the hormone to start with, and it's possible that a lot of kissing over a long period may be a way to pass some of that natural aphrodisiac to the woman, increasing her arousal and making her more receptive to even greater intimacy.