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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.