Sex Sleep Eat Drink Dream (21 page)

Why would the underarm, of all places, serve as such an excellent source of sexually attractive scents? Possibly because of our erect stance: In daily life, scents originating from the sexual organs are not usually perceptible. Because humans walk upright, the underarm is the ideal odor-generating spot—"an area that often contains hair that can greatly increase the surface area for dispersal," says Wysocki, "warmed to aid in dispersion, and positioned nearly at the level ot the nose of the recipient."

But there's a puzzle here. "If armpit odor is a turn-on, then why are we reviled by it?" Rosenberg asks. The answer, he suspects, may lie in the habits of modern civilization, which put us nose-to-pit with complete strangers in buses, elevators, waiting rooms, forcing on us their intimate odors.

Here's a new view of that party crowd. Issuing from the armpits of friends, colleagues, distant acquaintances, are waves of airborne chemicals that may be capable of affecting your perception, behavior, mood, even your libido and your choice of a mate. The word "pheromone" (from the Greek words
pherin,
to transfer, and
hormon,
to excite) was coined a half century ago to describe powerful chemical signals released and received by individuals of the same species. Mice, for instance, send vivid signals in body fluids such as urine, and even, according to one new report, in sex hormones secreted from the eyes. These invisible messengers may excite mating, block pregnancy, and accelerate puberty.

The idea that humans might participate in such invisible forms of communication has been greeted with a great deal of skepticism. But evidence is mounting to suggest that we almost certainly do. Among the first clues to the existence of human pheromones came in 1971, when Martha McClintock, now at the University of Chicago, published a paper showing that the menstrual cycles of female roommates in a Wellesley College dormitory tended to synchronize over time. Later it was found that the same effect could be achieved by merely depositing a little underarm sweat from donor women on the upper lips of recipient women.

Recently McClintock's team discovered that odors from breast-feeding women affect women who aren't lactating—influencing not only the length of their menstrual cycles but their libido. When exposed to the breastfeeding compounds, nonlactating women reported a 17 to 24 percent boost in sexual desire. The researchers suggest that pheromones of this sort may have evolved as a way of regulating fertility within groups of women—for example, by signaling one another that the environment was a good one for raising young.

As for the potency of male odors: Dab a little sweat from underarm pads worn by men under the nose of women volunteers, and perception, mood, and menstrual cycle may all fall under its aromatic influence. George Preti and his colleagues at the Monell Center exposed women to male underarm odors and then monitored both their mood and their blood levels of luteinizing hormone, which affects the length of menstrual cycles and the timing of ovulation. Normally the pituitary gland releases this hormone in pulses that increase in size and frequency at the approach of ovulation. The women subjected to male underarm secretions experienced acceleration in the onset of their next hormonal peak. They also reported feeling less tense and more relaxed when they had that sweat from males present on their upper lip.

What possible evolutionary reason could there be for this? Preti and his team speculate that early humans may have had relatively little time to spend in the company of their mates; the female reproductive system evolved so that it revved up the approach of ovulation when a woman caught a whiff of her man.

And here's news to lift the nostrils: A woman may reveal when she is ovulating not just by her facial features but by her smell. Scientists asked women to wear a T-shirt for three consecutive nights during ovulation and another T-shirt for three nights during the luteal phase of her cycle. The team found that men judged the odor of a woman's shirt worn during the fertile phase as more pleasant and sexy than the odor of a T-shirt worn during the luteal phase, even after the T-shirt had been kept at room temperature for a week.

Until lately, how we might sense such subtle pheromonal signals was a black box. Science had believed that mammals detected pheromones only with the help of a vomeronasal organ—a specialized olfactory system that does not function effectively in humans. But in 2003 the late Lawrence Katz, a neuroscientist at Duke University, overturned this view by reporting that neurons in the body's main olfactory system can detect pheromones. Since then, several studies have confirmed that we don't need a special organ at all to sense pheromonal vibes; our normal smell machinery may serve nicely to sniff out the volatile chemicals.

Just what other signals are you broadcasting as you buzz through the beehive of a social gathering? Nothing less than your deepest personal identity—and, possibly, your status as an acceptable genetic mate. We've known for decades that mice possess individual odor "signatures," which can be read in great detail by other mice and used to pick their partners. Now it seems the same is true for humans.

Each of us bears a chemical calling card that confers on us our own unique odor and reflects our subtle genetic differences. Moreover, this "odor print" signature can be detected by others. Women are especially adept at identifying the odor of relatives, their children, and their mates, says Mel Rosenberg; males to a lesser extent. The source of our singular odor is a key set of genes known as the major histocompatibility complex (MHC), which plays a big role in our ability to fight disease. These are the most diverse of all the body's genes, the better to deal with the multiplicity of bacteria, viruses, and other potentially harmful germs. Women tend to prefer the odor of men whose MHC genes differ from their own. One study found that women rated these odors as "pleasant" and the odors of men with genes similar to theirs as "less pleasant." By selecting partners with genes different from our own, say the researchers, we can avoid inbreeding or enhance our children's ability to fight disease.

To confound this tale, however, is a surprising new addendum: Women look for a fleck of their own father in their partner's MHC genes. Martha McClintock and colleagues found that in populations with plenty of genetic diversity, a woman prefers odors from males carrying MHC genes that match some of those she herself inherited from her father. Why? Perhaps women prefer a partner who shares some of their own vigorous immune genes rather than someone with completely unknown immune genes. Or they may be avoiding too much of a good thing. While diversity in these genes is generally considered beneficial, too much variation may make the immune system trigger-happy, increasing the risk of autoimmune disorders—the body turning against itself. In any case, the best choice seems to be a small number of matches. What's remarkable, as the scientists point out, is that women possess an olfactory system so exquisitely sensitive that it allows them to perceive these tiny genetic differences.

Such scientific revelations suggesting the possibility that attraction is influenced by a well-timed shot of pheromones or the keen detection of MHC genes may seem ultimate examples of the power of science to "clip an angel's wings, conquer all mysteries by rule and line," as Keats wrote. But I don't find them so. For me, these revelations have a way of enhancing the mystery. We think we make our choices willfully, consciously, through careful contemplation of the options; we think we know all that sways us. But in truth, part of what makes your marrow beat and your blood leap in wordless song may be a deep-down, chemical intuition aimed at protecting your unborn children.

Night

At night, every cat is a leopard.

ITALIAN PROVERB

10. BEWITCHED

P
ERHAPS YOU'RE HOME NOW
, settled into your nest with partner or mate. Darkness has fallen, the hours of intimacy when smell, hearing, and touch rule. Night has always yielded pleasures denied by daylight, offering privacy and refuge. As Shakespeare wrote, "Light and lust are deadly enemies."

The hour or so after 11
P.M
. is the most popular time for sex, but not because of any intrinsic natural rhythm. When scientists studied the circadian distribution of human sexual behavior, they found that the majority of sexual encounters took place at bedtime solely because of the rigidity of work schedules and family obligations. (Just what qualifies as bedtime may be an issue for couples. Not surprisingly, research suggests that couples with incompatible chronotypes—larks paired with owls—rate their marriages as less satisfactory than well-matched couples, with more arguing, less time spent on shared activities, and less frequent sex.)

Unlike our mammalian relatives, who generally time their sexual acts to maximize reproductive success, our cultural clocks and habits have corralled sexual behavior so that our preferred time for sex is rooted in expediency rather than drive; it doesn't mesh particularly well with our natural hormonal rhythms or fertility cycles. Levels of testosterone, for instance, are significantly lower in the late evening and higher in the morning, cresting at about 8
A.M
. Semen quality, on the other hand, peaks in the afternoon (with 35 x 10
⁶
more spermatozoa per ejaculation than in the morning). This higher sperm concentration in seminal fluid probably derives not from circadian variations in sperm production and maturation, say researchers, but from variations in the nerve-muscle mechanisms that control ejaculation. Whatever the reasons, some experts advise that couples attempting to conceive have sex not at night but in the afternoon.

So much for the cold timing of making love. More than 1,500 years ago, Sappho described the physical symptoms of love itself: the blind eyes, "the elusive little flames that play over the skin and smolder under," the faintness and stupor. Since Sappho, we have not learned a great deal about the anatomy and physiology of love. Our understanding of such positive states as pleasure, happiness, and sexual arousal has not advanced nearly as spectacularly as our grasp of stress, anger, and fear. Perhaps such knowledge of love just isn't possible. "How on earth are you ever going to explain in terms of chemistry and physics so important a biological phenomenon as first love?" Albert Einstein wondered. But even the more straightforward aspects of sex are still cloaked in mystery—the brain mechanisms that control arousal, say, or the hows and whys of orgasm, which are awkward to pin down in a laboratory situation.

Of late, however, science has made valiant attempts to put under the microscope some of the more elusive aspects of love and sex, and in so doing, found glimmerings of their workings. Take the biology of a caress. Neuroscientists have lately stumbled across clues to the nature of our responses to this soft, stroking variety of contact in one who seemed to have lost touch.

Among the great physical pleasures of coupled life is the swapping of slow back rubs between partners, the methodical moving of hands along the swale of spine and up to the tight muscles of neck and shoulders; then "spoonerizing," as it's called in my household, curling into each other, comforted by the contact.

Unlike our other senses, touch is ubiquitous in the body, with receptors nearly everywhere—inside and out—that record sensations of pressure, pain, heat and cold, movement, and the awareness of where we are in space. Touch is the sense least easily fooled throughout life, the first one awakened in the developing fetus, the last to leave us at the end, and perhaps the most essential to our well-being.

Human infants deprived of touch fail to thrive. When scientists visited the overcrowded, squalid orphanages of Romania after the overthrow of the dictatorial Ceauşescu regime, they found that the hundreds of babies who were rarely or never touched were developmentally disabled and had high levels of Cortisol. While there were many causes of their trauma, deficiency of touch seemed to play a key role in exacerbating their stress.

By contrast, ample touching, especially massage, has been shown to reduce levels of stress hormones and boost oxytocin, the hormone of pair-bonding and maternal love, which has a calming effect, lowering heart rate and blood pressure. The purported positive effects of massaging touch are legion, among them diminished pain, better lung function in asthma patients, even improved alertness and performance in children with attention disorders.

Touch is as old as life itself, going back to those early single-celled creatures that gained sensitivity to dimpling or pressure on their protective outer layer. In humans it arises from nerve endings beneath the surface of the skin that sense physical strain or pressure and convert this mechanical energy into electrical signals that travel to the brain. These nerve endings are distributed all over the body but cluster most densely in the lips, tongue, fingertips, nipples, penis, and clitoris. Among them, it appears, are some specialized to detect a caress.

Not long ago, the neurophysiologist Håkan Olausson and his colleagues in Sweden studied a fifty-four-year-old female patient who had lost sensation from her touch receptors. The patient could not detect pressure or tickling and denied having any touch sensibility on her body below the nose. Yet she could detect the faint sensation of light skin-to-skin touch and reported it as distinctly pleasant. The case suggests that our bodies possess a system of touch receptors separate from the nerves that detect pressure and vibration. These "slow-conducting" nerve endings lie beneath hairy skin and are specifically tuned to soft touch; when stimulated, they activate areas in the brain involved in sexual arousal and the processing of emotions. "This kind of receptor is abundant in animals but was long thought to have disappeared during the evolution of humans," says Olausson. "The finding that we still have a special touch system dedicated to processing emotional or social aspects of skin stimulation suggests the paramount importance of tactile stimulation for human well-being."