Swapping spit: The term takes on a more refined meaning at the new dating site ScientificMatch.com. A prerequisite for signing up -- in addition to having a bit of cash to spare -- involves swishing a cotton swab inside your cheek and mailing a juicy sample of skin cells and saliva.
What do you get in return for your DNA-laden drool? A chance at genetic and olfactory harmony. ScientificMatch.com -- perhaps the first company to combine the commercial potential of genetic testing, dating and the Internet in one package -- offers to find you a lover who smells good.
But not simply a bathed-and-used-deodorant kind of good smell. If all goes well, you'll get a lusty good smell, the kind that makes you bury your face in your mate's pillow the next morning to catch the lingering scent. The kind that, after a week of backcountry camping, actually increases your partner's sexiness quotient.
Of course, it takes more than an alluring eau de mate to build a good relationship. So the company, launched in December and now offering a discounted lifetime membership fee of $995, boasts a panoply of "member benefits" for its scientifically matched opposite-sex couples: better sex, increased fertility, healthier kids, less cheating and more orgasms. (Actually, only women are promised more orgasms. Men are, however, encouraged to imagine how this could be a perk for themselves as well.)
"All of the benefits that I've listed are supported by, and derived from, peer-reviewed scientific research," says Eric Holzle, the founder and president of ScientificMatch.com and a mechanical engineer by training. Though he won't reveal membership numbers, later this year he plans to expand his service -- now available only in the Boston-Providence area -- to other cities, possibly including Los Angeles.
Holzle's matchmaking efforts rely on published but still-preliminary results from a growing field of science: the genetics of mate attraction. (People looking for same-sex partners are welcome on the site, although studies have yet to look specifically at genetic attraction in gay couples.)
Researchers have long studied how certain traits -- square jaws in men, narrow waists in women, facial symmetry in both genders, for example -- seem to signal good genetic fitness to potential mates. But recently scientists have zeroed in on specific genes that might play a surprising role in how we choose hookups -- and possibly settle-downs.
Known as MHC (for major histocompatibility complex), these genes control how the immune system recognizes and fights off microscopic foreign invaders such as viruses, fungi and bacteria. Doctors also look at this portion of the genome to match up compatible organ donors and recipients.
Apparently the nose uses these genes too -- albeit for a different type of compatibility. Imagine you were to work the crowd at your singles bar by sniffing potential dates' sweaty underarms. (Urine aromas would work, too, but let's stick with armpits for now.)
Studies suggest that owners of the underarms you found to be most tolerable -- primally sexy, even -- are likely to have different histocompatibility genes than you. And those who have similar immune system types probably smell more like gym socks to you.
It's plausible that natural selection rigged the mating game in its favor, explains Randy Thornhill, a biology professor at the University of New Mexico and an immune-system-genetics researcher. If men and women with complementary immune systems are inexplicably drawn to each other, their kids will have an advantage in fighting off pathogenic nasties.
A good armpit musk, then, might be the spicy perfume of genetic diversity, an evolutionary come-hither call.
Gag-inducing body odors, on the other hand, might serve as "warning: do not copulate" signs, steering you away from mating with people who share too many genes with you. (Your kin, for example.)
Not that when you nuzzle your date's earlobe you think about the immune-system information floating in the air. (Literally in the air, probably via evaporated sweat and urine.) "All of this is done outside the realm of consciousness," Thornhill says.
Studies in mice first showed a link between chemical attraction and these genes in 1976, with results in birds and fish soon following. Two decades later, Swiss researchers found it in humans, too, as college students preferred the odor of sweaty T-shirts worn by other students with dissimilar immune-system genotypes.
That was in the aseptic environs of the lab. In 2006, researchers at the University of New Mexico (including Thornhill) looked at 48 couples where it counts: in the bedroom. Sure enough, women with genetically dissimilar partners tended to be more satisfied, responsive and adventurous in the sexual arena. And, yes, they had more orgasms -- during the most fertile days of their cycle, at least.
Women who had immune-system-similar mates, on the other hand, fantasized about other men more frequently -- again, especially during their ripe-for-pregnancy days. They also tended to cheat with other men more often.
Yet the frequency of men's orgasms, fantasies and affairs did not seem to be connected to their partner's immune system. Nor did a couple's genetic compatibility significantly correspond to their overall relationship satisfaction.
Indeed, the accumulated evidence for these genes' role in whom we marry, or with whom we dally, is far from clear-cut. Several studies have indeed found histocompatibility-odor preferences in men and women; others have found either a more complicated relationship or no link at all.
What's more, these preferences appear to actually reverse for some women on birth control pills or other hormonal contraceptives (That's why ScientificMatch.com accepts only women not using hormonal birth control.) And smell and vision may not line up, as evidence suggests that we're visually attracted to people who have similar facial features -- and similar immune-system types -- as we have.
Though the scientific picture is complicated, the ScientificMatch.com algorithm is not. Members are first genetically matched on the basis of their two copies (one inherited from Mom, one from Dad) of three major genes (sometimes known as HLA, for human leukocyte antigen). Members can choose to use results from a personal-values survey to refine the pool even further.
"We still don't understand how MHC genes relate to actual mate preferences," says Craig Roberts, a biological sciences professor at the University of Liverpool and mate-choice expert. "This is not to say I don't like the idea. . . . But I think we are a long way from really understanding what's going on."
Meanwhile, the purists among us can savor the old-fashioned method of genetic matchmaking: inhaling deeply and surreptitiously sniffing up the hotties we meet in person.
The Mating Game, a new column appearing monthly in the Health section, will explore the science of mating, dating and sex.
Health and Science Newsletter
The latest health and science updates, breakthroughs, research, and the best in investigative and informative journalism.