January 19, 2004 Health

	photo essay
	Animal Attraction There's more than one way to make hay, as birds, bees and bonobos know
	graphic
	Where Our Sex Drive Comes From Mapping the origins of sex drive on the human body
	remedies
	Love Potions A guide to some of the medical treatments available for what ails our libidos
	self-test
	The Passionate Love Scale Determine just how you feel about that special (or ex-special) someone

Nevertheless, scientists are light-years ahead of where they were in the 1920s and '30s, when estrogen and testosterone were first identified, and they know a great deal more than they did in the 1940s, when Alfred Kinsey, followed by the research team of William Masters and Virginia Johnson in the 1960s, published some of the first scholarly studies of human sexuality. Those studies concluded that sexual response proceeds in distinct stages, beginning with excitement—erection in men, engorgement of vaginal and clitoral tissue in women—proceeding to orgasm and finally to "resolution," in which tissues return to their normal state.

They didn't delve into biochemistry, though, and it turns out they probably didn't get the stages right either. In the 1970s psychiatrist Helen Singer Kaplan, who founded the Human Sexuality Program at New York Weill Cornell Medical Center, pointed out that before you get physically aroused, you have to feel sexual desire—a statement that seems pretty obvious. It's also pretty obvious to anyone who has been in a heterosexual relationship that men and women tend to experience sexuality somewhat differently. So where Masters and Johnson saw sexual arousal as a linear progression toward orgasm, researchers like Dr. Rosemary Basson of the University of British Columbia argued in 1999 that women, at least, operate in a more circular pattern. Desire can precede stimulation or be triggered by it. Satisfaction is possible at any of the stages. And orgasm isn't necessarily the ultimate goal.

Stimulation, moreover, can take all sorts of forms. Says Dr. Jennifer Berman, a urologist and director of the Female Sexual Medicine Center at UCLA: "Women experience desire as a result of context—how they feel about themselves and their partner, how safe they feel, their closeness and their attachment." Men, says Berman, "tend to be more visually directed and stimulated than women are." Thus Playboy and Hooters and the estimated $10 billion-a-year mainly male-oriented pornography industry.

But the reasons for that difference may be as much cultural as they are physiological. Dr. Julia Heiman, a psychologist and director of the Reproductive and Sexual Medicine Clinic at the University of Washington Medical School, is one of a growing number of researchers who think it's misguided to see men as simple and linear and women as complex and circular. "I don't think we've taken the time to talk to men about what desire is," she says. "If they are emotional about their sexuality, they don't feel in step with other men."

Women who don't fit stereotypes don't fare much better, says Jim Pfaus, a psychologist at Concordia University in Montreal who studies behavioral neurobiology. "What is a woman who expresses arousal in response to blatantly visual sexual cues? I hope we've moved beyond calling her a slut while calling a man who does the same a stud." But the cultural prejudice behind those labels persists, he says.

Research by Meredith Chivers at the Center for Addiction and Mental Health, affiliated with the University of Toronto, shows that women do respond to sexy visual stimuli. In fact, in a study recently presented at a Kinsey Institute conference on female sexuality, Chivers found that women show physical signs of arousal in response to a wider variety of images (including films of bonobo chimps mating) than men do. But unlike in men, this physical arousal is not closely paired with a subjective feeling of being turned on. In short, physical arousal for women can come before or even in the absence of conscious desire—doubtless a source of much confusion between the sexes. Arousal and desire can also happen at once.

But while arousal and desire are intimately intertwined and probably involve all sorts of feedback between brain and genitalia that have yet to be untangled, at least some of the underlying biochemistry is becoming clear. Here is a catalog of some of the key chemicals of love:

LETTING IT FLOW

Desire is complicated. Arousal, by contrast, is pretty straightforward: fill the penile arteries with blood or divert blood to the vagina and clitoris, and you're there. "Once the brain gets turned on—however it gets turned on—it's a relatively simple concept to increase blood flow," says Dr. Alan Altman, a specialist in menopause and sexuality at Harvard Medical School. In men, a chemical that facilitates the flow is vasoactive intestinal polypeptide, a hormone that also directs the expansion and contraction of smooth muscles in the gastrointestinal tract.

But the primary chemical in charge of that function is nitric oxide. It's a vascular traffic cop, activating the muscles that control the expansion and contraction of blood vessels. If the mind is in the mood—or when you pop a nitric-oxide-boosting drug such as Viagra or Levitra—the body responds. Men tend to be more focused on genital stimulation than women, so they are more likely to perceive an increased blood flow to the genitals as arousal, while women may be unaware of it. That may be one reason why trials of Viagra on women have been disappointing.

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