Desires and emotions can be fleeting, mayflies in the brain. They're born and they're gone. But they can also be persistent. They can change from whims to obsessions. If an emotion or drive is going to persist and resonate, a hormone is a useful object to turn to for the task. In the brain, steroid hormones generally work together with one or more of the neuropeptides. A neuropeptide is quick and transient. A steroid hormone is resilient and insistent. They work synergistically on neural circuits that subserve motive and behavior, integrating psyche with body. Take the sensation of thirst. When your body is low in water and salt, it reacts vigorously, because we all once dwelled in the sea and our cells still must be bathed in salty water to survive. Among the responses is the activation of the adrenal glands, which secrete steroid hormones such as aldosterone. Aldosterone is a practical hormone, and it seeks to conserve the supplies that exist—for instance, by reabsorbing salt from urine or gastric juice and returning it to the fluid between cells. Aldosterone also infiltrates the brain, where it galvanizes the activity of a neuropeptide, angiotensin. The neuropeptide in turn arouses the brain's circuitry of thirst. You feel thirsty. You have an urge to drink. The sensation can usually be satisfied with ease, with a glass of water, and the adrenals and the thirst locus settle down. But if your requirements for fluid and sodium are unusually large, as they are during breastfeeding, you will be awash in aldosterone and very efficient in your use of water and salt, but you will also feel chronically parched, and you will wonder if the Nile itself is large enough to slake you, and you will love salty foods as you never did before.
An emotion is a piece of information. It is a signal of need, of a temporary lapse in homeostasis. It is the body's way of encouraging or inhibiting behaviors, which the body hopes will fulfill the need and restore balance. We don't usually think of thirst as an emotion, but that's what it is, an emotion of the body's interstitial spaces. As an emotion, thirst can be disregarded or overruled by competing demands. If you are running a race in the heat and feel thirsty, you might ignore the desire rather than stop to drink and lose precious time and weigh your belly down with fluids. Panic can bring on enormous thirst, in part because the adrenal activity that comes with fear unleashes the flow of angiotensin in the brain; but panic can also clench the throat and stomach and make the thought of drink or food repulsive. Still, thirst gives you a comparatively short leash. You can only ignore it so long—a week without water, and you will die of dehydration. The synergistic impact of neuropeptide and steroid hormone on the circuitry overseeing the behavior of fluid acquisition is therefore quite extreme. The longer you refuse to engage in the requested behavior, drinking, the more exaggerated your adrenal output becomes and the more overwhelming the desire is. At some point, as you near death, you will drink anything—poisoned water, sea water that is too salty for your body to use. Even Jesus could not conquer thirst, and died with vinegar moistening his lips.
If, however, you don't reproduce during a particular cycle, it won't kill you. Humans are long-lived creatures who operate on the implicit assumption that they will have many opportunities to breed and can afford to override the whims and impulses of Eros for months, years, decades, and, oops, a lifetime if conditions of the moment are not quite optimal. Animals in whom reproductive drives are as relentless as thirst are short-lived species who may have only one or two breeding seasons in which to leave their Mendelian badge on the world. A corollary of longevity is a rich emotional life and a complex sexuality. We mistakenly equate emotionality with the primitive and rationality with the advanced, but in fact the more intelligent the animal, the deeper its passions. The greater the intelligence, the greater the demand on the emotions, the portmanteaus of information, to expand their capacity and multiply their zippers and compartments.
We impugn emotions, but we are lucky to be so thick with them. They give us something to think about and decode. We are brilliant because of them, not in spite of them. Hormones are part of the suitcase, and they are part of the contents. They relay information about themselves, and they carry information about others. They do not make us do anything, but they may make the doing of something easier or more pleasurable when all else conspires in favor of it.
Estrogen, puckish estrogen, works through many intermediaries in the brain, many neuropeptides and neurotransmitters. It works through nerve growth factor, and it works through serotonin, a neuropeptide best known for its role in depression. It works through natural opiates and it works through oxytocin. It may be thought of as a conjoiner or a facilitator, or as leavening, like yeast or baking soda. Estrogen has no particular emotion in mind, yet it permits emoting. For years researchers have sought to link estrogen levels to women's sexual behavior. The assumption is logical. Estrogen concentrations rise steadily as the egg follicle grows each month, peaking with the moment of ovulation, when the egg is released into the fallopian tube. If the egg has a need, a desire to be fertilized, in theory it could make the need known to the brain through estrogen, and estrogen would then stimulate a neuropeptide to encourage a particular behavior—to wit, seeking a sexual partner like a thirsty pedestrian seeks a water fountain.
The difficulties of correlating estrogen to human sexual behavior are considerable. What sort of behavior are you looking at? What are the relevant data points? Frequency of intercourse? Frequency of orgasm? Frequency of masturbation or sexual fantasy? The sudden urge to buy Cosmopolitan? Here is what we know. There is no association between rate of intercourse and where a woman is in her ovulatory cycle. Women do not have sex more often during ovulation than they do at any other time of the month, unless they're consciously on the fertility quest. But the completion of a behavior tells you little about the subliminal provocations of that behavior. If you plot the incidence of intercourse among couples, you'll see an amazing statistical high point, and it's called the weekend—not because people necessarily feel sexy each Sunday, but because people have sex when it's convenient, when they're not exhausted by work, and when they have the whole day to toy with. A hormone may lead you to water, but it can't make you drink.
There is also no correlation between estrogen levels and physical arousability—the tendency of the genitals to swell and lubricate in response to an overt sexual stimulus, such as a lovemaking scene in a movie. Women have been shown to be fairly invariate in their display of physiological arousal, regardless of their cycle. But physiological arousal says little about meaningful sexual motivation or hunger, for some women will lubricate during rape, and Ellen Laan, of the University of Amsterdam, has shown that women's genitals congest robustly when they watch pornography that the women later describe as stupid, trite, and distinctly un-erotic.
We get a somewhat better kinship between hormones and sexuality when we look at desire rather than at genital performance. Some studies have taken female initiation of sex as the marker of desire. The results have varied considerably, depending on the type of birth control used, but they list in the predicted direction. Women on oral contraceptives, which interfere with normal hormonal oscillations, are no more likely to come on to their partners at the middle of the cycle than they are at other times. When the birth control method is reliable but nonhor-monal—a vasectomized husband, for example—women show a tendency to be the initiators of sex at the peak of ovulation more than they are during other times of the month, suggesting that the estrogen high is beckoning to them. Add in the complicating factor of a less trustworthy barrier, such as a diaphragm or condom, and the likelihood of midpeak propositioning subsides. No great enigma there: if you don't want to get pregnant, you might not be eager to fool around when you think you're at your most fertile. In a study of lesbian couples, who have no fear of pregnancy, don't use birth control, and are free of supposedly confounding factors of male expectations and manipulations, psychologists found that women were about 25 percent more likely to initiate sex and had twice as many orgasms during the midpoint of their cycle than at other times of the month.
The strong
est correlations between hormones and sexuality are seen when pure, disembodied desire is the object of scrutiny. In one large study, five hundred women were asked to take their basal temperatures every day for several months and to mark down the day of the month when they first noticed the stirrings of sexual desire. The pooled results show an extraordinary concordance between the onset of sexual hunger and the time that basal temperature readings suggest the women were at or nearing ovulation. Women may even express desire through unconscious body language. In a study of young women who spent a lot of time dancing in nightclubs, the scientists found that as the women approached the day of ovulation, their outfits became progressively skimpier, more flaunting of flesh: the hemlines rose with estrogen levels as if with a bull market. (Of course, it doesn't hurt that midcycle is also the best time to wear your tightest and most revealing clothing, as that is when you are free of premenstrual water retention and blemishes and any fear of leaking menstrual blood.)
A number of researchers lately have suggested that it is testosterone, not estrogen, that is the "true" hormone of libido, in men and women alike. They point out that the ovaries generate testosterone as well as estrogen and that androgen levels spike at midcycle just as estrogen levels do. How can we neglect testosterone when men have so much of it and men love sex so madly, don't they? Many textbooks on human sexuality declare flatly that testosterone is the source of all lust, and some women have added testosterone to their hormone replacement regimens in an effort to shore up their ebbing libido. But if testosterone is relevant to female lust, evidence suggests that it is as a handmaiden to estrogen rather than as Eros descended. As it happens, some proteins in the blood will cling to both testosterone and estrogen and in so clinging prevent the hormones from penetrating the barrier between blood and brain. Estrogen accelerates the production of these binding proteins, but the proteins have a slight preference for testosterone. Hence, as the levels of sex hormones and binding proteins climb with the menstrual cycle, the binding proteins seek out testosterone prejudicially, defusing it in the blood below before it can accomplish much of psychodynamic interest above. The testosterone proves useful indirectly, though: by occupying the binding proteins, it frees estrogen to reach the brain unimpeded. This power of distraction could explain why testosterone therapy works for some women with low libidos: it keeps the blood proteins busy and lets estrogen breaststroke straight to the brain.
But to view estrogen as the hormone of libido is to overstate it and underrate it. If estrogen is the messenger of the egg, we should expect the brain to pay attention, but not in any simple, linear fashion. Just as the mechanics of our genitals have been released from the hormonal chokehold, so have our motives and behaviors. We would not appreciate a hormonal signal that is a blind nymphomaniac, an egg groupie, telling us we're horny and must fornicate. We do not want to indulge an egg just because it is there. We live in the world, and we have constraints and desires of our own. What we might like, though, is a pair of well-appointed glasses, to read the fine print better. Estrogen's basic behavioral strategy is to hone the senses. It pinches us and says, Pay attention. A number of studies have suggested that a woman's vision and sense of smell are heightened at ovulation. So too do the senses shine at other times of high estrogenicity, such as right before menstruation, when your progesterone levels have dropped way down and left estrogen to act unopposed. During pregnancy, you can smell a dirty cat box from two flights away, and you can see dim stars and the pores on every face you meet. It must be emphasized that we don't need estrogen to pay attention or to smell a thing, but there it is, coursing from blood to brain and lending the brain a mild buzz, just as it does the bones and heart and breast and little gray basket.
If estrogen is to help at all, it should help us best when our minds must be wonderfully concentrated. Ovulation is a time of danger and of possibility. Estrogen is like hunting magic, the hallucinogenic drug that Amazonian Indians extract from the skin of the poison-dart frog to lend them the sensorial strength of heroes. The more we are of the world, the greater are our chances of meeting others who suit us, but the more incumbent it is on us to notice and assess those around us. If there is such a thing as feminine intuition, it may lie in the occasional gift of a really sweet estrogen high, the great emulsifier, bringing together disparate observations. But estrogen is also at the behest of history and current affairs. If you are in a sour, reclusive mood to begin with, the hump of estrogen at ovulation, or its unopposed premenstrual energy, may make you feel more rather than less reclusive. Estrogen is a promoter, not an initiator. We can understand this by considering how estrogen contributes to breast cancer. The hormone is not, strictly speaking, a carcinogen. It does not crack or destabilize the genetic material of breast cells, in the way radioactivity or toxins such as benzene can. Yet if an abnormal cell exists, estrogen may stoke and stimulate it, abetting its growth until a minor aberration that might otherwise regress or be cleaned up by the immune system survives and expands to malignant dimensions.
The strength of estrogen lies in its being context-dependent. It does not make us do anything, but it may make us notice certain things we might otherwise neglect. Estrogen may enhance sensory perception, giving us a slight and fluctuating advantage overlaid on the background of the self. If we are good, we may have our moments of being very, very good, and if we are mediocre, well, we can blame it on our hormones. They are there to be used.
As a lubricant for learning, estrogen is of greatest benefit in young women, who are sorting themselves out and gathering cues and experiences. Young women may reap advantages from intuition for lack of anything better to draw on as they assess the motives and character of another. But we can become too enamored of our intuitive prowess, our insight into others, and believe too unshakably in the correctness of our snap judgments. The older we get, the softer the peaks and valleys of our estrogen cycles are, and the less we need them and their psychotogglings. Experience, after all, is a trustworthier friend than intuition. How many times do you have to encounter a man who reminds you of your cold, aloof, angry, hypercritical, and infinitely alluring father before you can recognize the phenotype in your sleep and know enough to keep your eyes and nose and hormones far, far away?
Each of us is a privately held chemistry lab, and we can play with ourselves if we want. You may find your ovarian cycle too boring to dwell on or you may try to explore its offerings, and you may be disappointed or you may not. It took me many years to realize that my orgasms were very strong at midcycle. I always knew that they were good right before menstruation, but I thought that had to do with mechanics, the congesting of the pelvis with premenstrual fluid, and I didn't attend to the other side of the equation, because I didn't believe in it. When I started to investigate the link between rising estradiol and the quality of climax, I found a wonderful connection. The midway orgasms are deep and resounding, accentuated, maybe by estrogen, maybe by decoy testosterone, maybe by autohypnosis. I could be experiencing a placebo aphrodisiac. It doesn't matter. As a chemist, I'm an amateur, and I can't do a controlled experiment with myself. Nevertheless, on matters that count I'm a quick study, and I've learned to find my way home to ecstasy whatever the moon, month, menses, may be doing.
We each of us have but one chemistry set and brain to explore, and the effects of estrogen will vary from head to head. Yet if there is a principle to be drawn from the general recognition that hormones can stimulate and emulsify the brain and sensitize it to experience and input, it is this: puberty counts. Under the influence of steroid hormones, the brain in early adolescence is a brain expanding, a Japanese flower dropped in water. It is also vulnerable to the deposition of dreck and pain, which can take a lifetime to dump back out again. The plasticity of the pubertal mind is grievously underestimated. We've obsessed over the brain of early childhood and the brain of the fetus, and though those brains matter deeply to the development of all-round intelligence, character, and skill, the adolescent brain counts in another way.
As the brain stumbles toward maturity, and as it is buffeted by the output of the adrenal glands at age ten and of the gonads a year or two later, it seeks to define itself sexually and socially. The brain of a prepubertal girl is primed to absorb the definitions of womanness, of what counts and what doesn't, of what power is and how she can get it or how she will never get it. We've all heard about the crisis of self-confidence that supposedly strikes girls as they leave childhood and climb the Bunker Hill of junior high, but what has been less recognized is the correspondence between this period of frailty, this tendency for the personality to mutate beyond recognition, and the hormonal squall in the head. The pubertal brain is so aware of the world that it throbs, it aches, it wants to find the paths to calm it down and make sense of the world. It is an exposed brain, as tender as a molted crab, and it can be seared deeply. Who can forget adolescence? And who has ever recovered from it?
At the same time that hormones challenge the pubertal brain, they change the body. A girl's high estrogen content helps in the deposition of body fat on the breasts, hips, thighs, and buttocks, subcutaneously, everywhere. Because of estrogen and auxiliary hormones, women have more body fat than men. The body of the average woman is 27 percent fat, that of the average man 15 percent fat. The leanest elite female athletes may get their body fat down to 11 or 12 percent, but that is nearly double the percentage of body fat found on the elite male athlete, who is as spare as a pronghorn antelope. We can look at the deposition of body fat that comes with womanhood and say it's natural for girls to fatten up when they mature, but what natural means is subject to cultural definition, and our culture still hasn't figured out how to handle fat. On the one hand, we're getting fatter by the year, we westerners generally and North Americans particularly, and why should we expect otherwise? We are stapled to our desks; food is never far from our hands and mouths, and that food tends to be starchy and fatty and overrich; and we get exercise only if we exert willpower, not because sustained body movement is an integrated feature of work, social life, or travel. On the other hand, we are intolerant of fatness, we are repulsed by it, and we see it as a sign of weak character and sloth. Contradictory messages assail us from all sides: we must work all the time, the world is a competitive place, and technology requires that our work be sedentary, cerebral, but we must not get too fat, because fat is unhealthy and looks self-indulgent. So we must exercise and control our bodies, because our natural lives won't do it for us.
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