The Scent of Sexiness

Scents are famously known to carry strong psychological associations—think about how a whiff of a loved one’s favored perfume or cologne can bring to mind the person who wore it, along with a cascade of emotions. Partly, this is due to the unusual design of the olfactory nerve, which extends in a network throughout the brain—unlike the nerves carrying information for the other major senses, which are less wide-ranging. This architecture helps the brain to tie memories of emotional events with olfactory information. The emotion-stirring aspect of smell is important; but smell also turns out to be surprisingly important to women when it comes to basic sexual attraction.

 

Using an instrument called the “Sensory Stimuli and Sexuality Survey,” researchers at Brown University found that women rate how someone smells as the most important of the senses in choosing a lover, edging out sight (a close second), sound, and touch. One woman in our study ranked the attractions of a sexual partner:

How a woman smells to a man, in contrast, figures less heavily in his sexual attraction. Perhaps it is because men’s sense of smell is less acute than women’s. Perhaps it is because visual cues loom so much larger in what turns men on. And it’s not just that women think smell matters in whether they are attracted to someone, it’s that women’s sexual arousal is enhanced by good body odours—and killed by bad ones.

One reason why body odours play such an important role in women’s sexual attraction has come to scientific light only recently. The first clue came from an unusual discovery: that a woman’s olfactory acuity reaches its peak around the time of her ovulation, the narrow twenty-four-hour window during the monthly menstrual cycle in which she can become pregnant. This led scientists to suspect that women’s sense of smell might play a role in reproduction. It was not until researchers began to explore the body’s defences against disease, however, that the connection was made.

The genes responsible for immune functioning—fighting off disease-causing bacteria and viruses—are located within the major histocompatibility complex, or MHC, found on chromosome 6. Different people have different versions, or alleles, of these MHC genes; in the jargon of geneticists, the MHC genes are “polymorphic.” It turns out that women can benefit in two ways from mating with men who are dissimilar to themselves in MHC genes. First, a mate with dissimilar MHC genes likely has more dissimilar genes in general, and so finding an MHC-dissimilar person attractive might help to prevent inbreeding. Reproducing with close genetic relatives can be inbreeding. Reproducing with close genetic relatives can be disastrous for the resulting children, leading to birth defects, lower intelligence, and other problems. But a second benefit of mating with someone with complementary MHC genes is that any resulting children will have better immune functioning, making them better able to fight off many of the parasites that cause disease.

The puzzle is how women could possibly be able to choose mates who have complementary MHC genes in order to give these benefits to their offspring. In a revealing study, Brazilian researchers had twenty-nine men wear patches of cotton on their skin for five days to absorb their sweat—and thus their body odours. A sample of twenty-nine women then smelled each cotton patch and evaluated the odour on a dimension from attractive to unattractive. Scientists identified the specific MHC complex of each man and woman through blood assays. Women found the aromas of men who had an MHC complex complementary to their own smelled the most desirable. The odours of men who had an MHC complex similar to their own made them recoil in disgust. Amazing as it may seem, women can literally smell the scent of a gene complex known to play a key role in immune functioning.

This highly developed sense of smell can have a profound effect on women’s sexuality. University of New Mexico evolutionary psychologist Christine Garver-Apgar and her colleagues studied MHC similarity in forty-eight romantically involved couples. They found that as the degree of MHC similarity between each woman and man increased, the woman’s similarity between each woman and man increased, the woman’s sexual responsiveness to her partner decreased. Women whose partners had similar MHC genes reported wanting to have sex less often with them. They reported less motivation to please their partner sexually compared to the women romantically involved with men with complementary MHC genes. Perhaps even more disturbing to their mates (if they knew), women with MHC-similar partners reported more frequent sexual fantasies about other men, particularly at the most fertile phase of their ovulation cycle. And their sexual fantasies about other men did not just remain in their heads. They found themselves in the arms of other men more often, reporting higher rates of actual sexual infidelity—a 50 percent rate of infidelity among couples who had 50 percent of their MHC alleles in common.

So when a woman says that she had sex with a man because he smelled nice, her sexual motivation has hidden roots in an evolutionary adaptation. At an unconscious level, women are drawn to men with whom they are genetically compatible.

Another reason why a man’s scent is so important comes from the unusual discovery that body symmetry has sexual allure.

Most human bodies are bilaterally symmetrical: The left wrist generally has the same circumference as the right wrist; the left ear is generally as long as the right ear; from the eyes to the toes, the left and right halves of people’s bodies roughly mirror each other. Each individual, however, carries small deviations from perfect symmetry. Two forces can cause faces and bodies to become more asymmetrical. One is genetic—the number of mutations an individual has, which geneticists call mutation load. Mutations an individual has, which geneticists call mutation load.

Although everyone carries some genetic mutations (estimates are that the average person has a few hundred), some people have a higher mutation load than others, and those with more mutations tend to be more asymmetrical. The second force is environmental. During development, some individuals sustain more illnesses, diseases, parasites, and bodily injuries than others, and these environmental insults create asymmetries in the body and face. Symmetry, in short, is a sign of good health—an indication that a person carries a low mutation load and has experienced few environmental injuries, or at least possesses the capacity to sustain environmental injuries without their leaving much of a mark.

If body symmetry is attractive because of how we evolved, so is the fact that women are able to detect the scent signature for symmetry, a useful skill when you consider that some asymmetries may not be immediately visible.

But could a woman possibly smell body symmetry? In one study, men wore white cotton T-shirts for two nights. The T-shirts were then sealed in plastic bags. In the laboratory, scientists used callipers to measure the various physical components of the men’s bodies, including their wrists, ankles, and earlobes, in order to evaluate their degree of symmetry.

Then women smelled each T-shirt and provided a rating of how pleasant or unpleasant it smelled. Women judged the T-shirt odours of symmetrical men to be the most attractive and deemed the odours of asymmetrical men to be repulsive. Four independent the odours of asymmetrical men to be repulsive. Four independent studies have replicated the finding.

Women find the scent of symmetry particularly attractive when they are in the fertile phase of their ovulation cycle—precisely the time in which they are most likely to conceive. This apparently reflects an evolutionary adaptation in women to reproduce with men possessing honest signals of good health, including high-quality genes. When women have extramarital affairs, they tend to choose symmetrical men as partners—yet another indication of the importance of symmetry in sexual attraction.