Authors: Andrew Trees
Decoding Love (23 page)
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 had preferences that were the opposite of women not on the pill. They chose men who had similar MHC profiles. To give you some idea of how potentially disturbing this reversal is, women generally described the smell from men with similar MHC profiles as reminding them of their father’s or brother’s odor. Traditionally, it may have made sense for a woman to stay close to her family during her pregnancy to ensure her protection and care. But that evolutionary development is clearly being blindsided by scientifically engineered contraceptives, which fool a woman’s body into thinking that it is pregnant. As if that wasn’t bad enough, another study has shown that women on the pill see the world in a more platonic light, which is clearly counterproductive for single women looking to find a partner because it might blind them to the romantic possibilities around them.
Similar MHC genes have also been shown to cause a host of problems for couples. According to one study, partners who have more difficulty conceiving a child share significantly more of their MHC genes than couples who have dissimilar MHCs. Doctors have also found that couples with more similar MHC profiles suffer from higher rates of miscarriage. It even has an effect on sexual fidelity. As MHC profiles become more similar, researchers have found that a woman’s sexual responsiveness to her partner diminishes. She is more attracted to other men, and she is, on average, unfaithful with a greater number of men. With 50 percent of the profile in common, a woman has a 50 percent chance of sleeping with another man. These studies clearly suggest that any woman who is serious about finding a long-term partner should avoid oral contraceptives. By outwitting nature, we may have actually outwitted ourselves and undermined an important element of mate selection—smell.
If a woman does decide to go off the pill, I do have to warn that there will be one unexpected downside if she chooses to use condoms instead—she will no longer have the medicinal benefits of semen. That may sound even more preposterous than the T-shirt study, but it’s the truth. Unlikely as it sounds, semen contains powerful, mood-enhancing chemicals that are absorbed through the vaginal wall (digestion is a different matter, so this doesn’t apply to fellatio). According to a recent study, women who don’t use condoms and have regular sex are less depressed than women who use condoms or women who aren’t having sex.
Of course, women have probably intuitively understood the power of smell all along. A surprising number of women I interviewed said that smell played a crucial role in their dating, and some even claimed that they could immediately “smell” if a man was right for them. One woman even said that she liked to smell her boyfriend’s armpit—and not when he was fresh out of the shower. Not since Walt Whitman have I seen such a ringing endorsement of the sweet smell of armpits. Our hygiene-obsessed society makes it more difficult for women to smell whether a protective partner has the right stuff or not, which might help explain why kissing plays such a large role in most relationships. Our saliva also contains traces of our MHC, so kissing can reveal what our daily showering hides. You do have to exercise some caution when relying on your nose, though. Fragrances can fool even the most astute sniffer, so you need to make sure that you are smelling the unadulterated scent.
It is possible that the chemistry of the body, particularly hormones, also offers an explanation for the fickleness attributed to women. For example, a woman’s ovulation cycle can have a dramatic effect on what sort of man she finds attractive. Studies have shown that during ovulation women find masculine faces more attractive but find feminine faces more attractive the rest of the time (other traits such as voice follow a similar pattern). Women during ovulation also show a greater preference for the smell of symmetrical men. Masculine faces and symmetricality are signs of health, so there are very good evolutionary reasons why women would have developed an unconscious preference for these markers of genetic superiority. Intriguingly, women do not show a preference for the smell of symmetrical men when they are not ovulating. This may be due to the unconscious realization that men with the best genes often don’t make the most faithful partners, which suggests just how subtle our evolutionary tendencies can be. Even the smell of other women can influence female behavior. In one study, women exposed to the smell of other women who were breast-feeding found that their sexual desire spiked 17-24 percent. Truly bold women can try to pull off a daring stunt with their copulins, which are fatty acids found in vaginal secretions. In one study, while men did not rate the smell of copulins as pleasant, they had a dramatic effect on how the men viewed women. After being exposed to the smell, the men gave higher ratings for attractiveness to photos of women, and their testosterone levels spiked. The less attractive the woman, the more her rating went up. If men want to pull off a similar feat, they can try to plant some sweat under a woman’s nose. In one study, women rated men’s faces and then were exposed to a small amount of underarm sweat and asked to rate the faces again. After smelling the sweat, the women raised their ratings substantially.
It’s not simply how we judge looks that can be affected by things like smell and ovulation—these things can also alter what sort of personality a woman finds attractive. In a study run by Geoffrey Miller and Martie Haselton, women read stories that described either a creative, poor man or an uncreative, rich man. When women were near their peak fertility, they preferred creativity to wealth for a short-term relationship, offering evidence that ovulation can dramatically affect a woman’s mate choice by pushing her to choose good genes over a good dad, particularly when she is not making a long-term decision. Marriage itself influences a woman’s preferences. In another examination of the influence of smell on female choice, a woman with a partner preferred the smell of more dominant men, while single women showed no preference. Researchers have suggested that single women were interested in securing a partner, but once that was taken care of, women turned to the pursuit of the best genes. Ovulation may even provide an explanation for the female complaint that all the good men are taken. According to a 2008 study, women with partners found men in a relationship more attractive during periods of low fertility, but they preferred single men (if they displayed masculine traits) during periods of high fertility. Again, this shift in preference seems to correspond to a good genes/good dad split, with ovulation increasing a woman’s attraction to single, masculine men for their genes but with low fertility shifting their attraction to men who show an ability to commit to a long-term relationship (i.e., a man already in a relationship). So, the next time a woman finds herself lamenting the lack of available men, she can perhaps blame it on her fertility cycle.
Don’t worry, ladies. A man is ruled just as much by his own body chemistry, although it does not function in the same way that a woman’s does. Let’s return to that odiferous object of scientific inquiry, the sweaty T-shirt. Researchers decided to run a similar smell test on men. They gave women cotton T-shirts to wear during different phases of their menstrual cycle and then had men smell the results. What men honed in on was not a woman’s MHC profile but her ovulation cycle. Rating T-shirts for pleasantness and sexiness, men gave women who were ovulating or about to ovulate a much higher rating than women who were approaching menstruation. According to another study, women reported that their partners became more loving and attentive during their ovulations as well as more jealous of other men. In other words, men aren’t particularly focused on finding a woman with a different immune profile, but they have an uncanny ability to seek out women who are at their peak fertility. This seems to hold true in the real world as well as the experimental one. In one barroom study, men initiated touching with ovulating women more than nonovulating women. This all makes evolutionary sense. In general, men want to have sex with as many fertile women as possible, regardless of whether or not the woman is a good genetic fit. For women, though, each pregnancy represents a huge investment, so it makes sense that they would devote a lot more (unwitting) attention to ensure that the baby has the best genetic chance of success.
Men also have their own hormonal fluctuations to worry about. Their testosterone goes through a series of peaks and valleys throughout the day, but it can still serve as an indicator for more significant traits, such as mood and aggression. Some men are naturally low in testosterone, which is good for society because if all men were alpha males, the fighting would never stop. We would constantly find ourselves locked in Travis Bickle-like arguments. “Are you looking at me?” “No, but are
you
looking at
me
?” Important life changes can also influence a man’s testosterone. When men are in a stable relationship, their baseline level of testosterone drops. For example, married men have lower testosterone on average than single men, and married men with children have less testosterone on average than married men without children. But their testosterone will ratchet back up if the relationship becomes unstable. A high-testosterone man will also behave more aggressively when it comes to dating. A study was done in which different men competed for the attention of a woman. Men with high testosterone would make fun of the other man, criticize him, and refuse to laugh at his jokes. Women interested in a long-term relationship should try to avoid men who display the traits of someone with high testosterone—they tend to marry less frequently, have more affairs, abuse their wives more, and divorce more often.
you
looking at
me
?” Important life changes can also influence a man’s testosterone. When men are in a stable relationship, their baseline level of testosterone drops. For example, married men have lower testosterone on average than single men, and married men with children have less testosterone on average than married men without children. But their testosterone will ratchet back up if the relationship becomes unstable. A high-testosterone man will also behave more aggressively when it comes to dating. A study was done in which different men competed for the attention of a woman. Men with high testosterone would make fun of the other man, criticize him, and refuse to laugh at his jokes. Women interested in a long-term relationship should try to avoid men who display the traits of someone with high testosterone—they tend to marry less frequently, have more affairs, abuse their wives more, and divorce more often.
In a result that will delight Freudians, it appears that although men may not want to sleep with their mothers, they do prefer women who remind them of their mothers—at least that’s what the evidence from the animal kingdom seems to suggest. In one study, the mothers of infant male rats were doused with lemon scent. Later, the males were paired with some female rats with lemon scent and some without it. It was no contest. The males mounted the lemon-scented rats sooner and ejaculated more quickly. One man I interviewed admitted that he had a serious relationship with a woman and found himself attracted to her perfume, only to realize later the rather distressing news that the perfume was the same one his mother used.
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