Decoding Love - Andrew Trees [77]
Now, back to those pesky rats. It turns out that when a female mouse was offered two different males with which to mate, she always chose the one whose MHC genes were most different from her own. This makes sense as that choice will increase the immunity of her offspring. Her method of selection? Smelling the urine of the male rats. This put sex researchers off the scent, as it were, because humans do not typically make a habit of smelling one another’s urine. But then researchers stumbled on a very interesting discovery—humans can smell the difference among mice that differ only in their MHC without having to rely on urine at all. With that, a Swiss biologist named Claus Wedekind designed an experiment to see if he could discover a similar ability in women smelling the MHC of men (women have greater smell sensitivity than men), and thus, the great smelly T-shirt experiment was born. Although Wedekind’s findings are better known than most of the studies I discuss, they still deserve close attention because the key element driving smell preference is often not explained, and the shocking results relating to contraception are frequently left out.
More than eighty college students participated in the study. The men were given cotton T-shirts to sleep in for two consecutive nights. To guard against any wayward smells creeping into the picture, they were told not to eat spicy foods, not to smoke, and not to drink any liquor. They also had to avoid any deodorants, cologne, or perfumed soaps. And, of course, no sex. During the day, their T-shirts were kept in a sealed plastic container.
The women were also primed for the experiment. For two weeks prior to the test, they used a nasal spray to protect the mucous membranes lining their nose. Around the time they were ovulating, when their smell was enhanced, they were put to the test: a row of boxes with a hole cut in the top, each containing the T-shirt of one of the men. After inhaling deeply from each box, the women rated each man’s shirt for sexiness, pleasantness, and intensity of smell. What the researchers discovered demolished any lingering doubts about the role of smell in attraction.
Wedekind and his staff found that how a woman rated a man’s smell depended entirely upon how much of their MHC profiles overlapped. Because a person’s MHC profile is incredibly idiosyncratic, what smells good to one woman will not necessarily smell good to another, and short of genetic testing, there is no easy way to predict what a woman will find appealing. For instance, race does not exert much influence on it. All of this made the results of the study even more astounding. The more a man’s MHC profile differed, the more the woman rated his smell as pleasant and sexy (a later study found that you can be too different—if there are no MHC genes in common, women are not attracted to the man’s smell).
The evidence also suggested that this attraction was not confined to the lab. Women said that the smells they preferred reminded them of current or ex-boyfriends roughly twice as often as men with similar MHC profiles, so smell had likely played an important role in their real world mate selection as well. No matter what the MHC profile, though, strong body odor was a turn-off. Researchers theorized that a strong body odor is often an indicator of disease, so that women may have evolved an aversion to strong smells as a means of avoiding a genetically unfit mate. In fact, odor appears to be yet another marker of genetic fitness. In a different study, women judged symmetrical men to be better smelling than their nonsymmetrical counterparts.
There was one other surprising finding: women taking oral contraceptives