Woman: An Intimate Geography (36 page)

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Authors: Natalie Angier

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BOOK: Woman: An Intimate Geography
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she sidles up to a male and begins engaging in a bit of heavy petting, other group members strive to intervene, raucously and snappishly. A female rhesus doesn't often bother defying convention. What does she look like, a bonobo?
Hormones change everything. They tint her judgment and sweep her from Kansas to Oz. When she is ovulating and her estrogen levels soar, her craving overcomes her political instincts and she will mate madly and profligately, all the while outsnarling those who would dare to interfere.
When we think about motivation, desire, and behavior, we accord the neocortex and the thinking brain the greater share of credit. We believe in free will, and we must. Free will, of a sort, is a hallmark of human nature. This is not to say that we start each morning afresh, with an infinity of possible selves awaiting us that is a figment, alas, and a durable one. Nevertheless, we have what Roy Baumeister, of Case Western Reserve University, calls an "executive function," the dimension of the self that exercises volition, choice, self-control. The human capacity for self-control must be counted among our species' great strengths, the source of our adaptability and suppleness. Very little of our conduct is genuinely automatic. Even when we think we're operating on automatic pilot, the executive function keeps an eye out, checks, edits, corrects the course. If you know how to touch-type, you know that the executive brain is never far removed from the drone brain. When all is well, you type along automatically, your fingers so familiar with the keys that it's as though each digit has a RAM chip embedded in its tip. But the moment you make a mistake, the automaton stops and the executive function kicks in, even before you're quite aware of what went wrong. With its guidance, your finger reaches for the backspace key to correct the error, and you see what happened and you fix things, and a moment later your hands have returned to robot mode. Athletes, surgeons, and musicians perform similar exchanges between intentional and programmatic behaviors hundreds of times a minute; such commerce is the soul of mastery. The human capacity for self-control is limited, and we get into trouble when we overestimate it and embrace the caustic ethos of perfectionism, but volition still deserves our gratitude.

 

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At the same time, we know that there's a macaque darting about in the genomic background and that we feel like monkeys and can act like them too. The moment a young girl enters adolescence, she begins dwelling on sex, consciously, unconsciously, in her dreams, alone in the bath however or wherever it happens, it happens. Her desire is aroused. The changes of puberty are largely hormonal changes. The shifting of the chemical setting stirs desire. Intellectually, we accept the idea that sexuality is a hormonally inflected experience, but we still resent the connection. If hormones count, we worry that they count too much and that therefore we have no free will, and so we deny that they count, all the while knowing that they count, because we see it in our teenage children and we remember, please goddess, our teenage greed.
Rather than denying the obvious, we should try to appreciate the ways in which estrogen and other hormones affect behavior. Granted, our knowledge of neurobiology is primitive, presimian. We don't understand how estrogen or any other substance works on the brain to elicit desire, or feed a fantasy, or muffle an impulse. But there are enough indirect strands of evidence to knit a serviceable thinking cap with which to mull over estrogen's meaning.
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

 

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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

 

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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 sub-

 

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liminal 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 unerotic.
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 nonhormonal 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.

 

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The strongest 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

 

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