Biology: The Chemistry of Desire

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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.

--FUELS FOR LUST

If there's one substance that ultimately makes it possible to get turned on in the first place, testosterone is probably it. "When testosterone is gone," says UCLA's Berman, "for whatever reason--aging, medication--men experience erection and libido problems." Restore the testosterone, and you usually fix those problems.

Women too seem to have problems getting interested in sex when their testosterone levels are too low, which is why Procter & Gamble is experimenting with testosterone patches. Says Altman: "When women are having normal menstrual cycles in their prime reproductive ages, their ovaries make two times more testosterone than estrogen." A few days before ovulation, triggered by surging levels of testosterone--along with other hormones including progesterone and estrogen--sexual desire peaks, according to new research by Martha McClintock of the University of Chicago that dispels a long-held theory that fertility precedes desire.

But for women, at least, estrogen may also be crucial. "Give estrogen to women with decreased desire," says Pfaus, "and you don't restore desire. Give them testosterone alone, and you get a little increase in desire. Give them estrogen and testosterone together, and you get a whopping increase." Why? Some research suggests that testosterone's role in women is diversionary: it attaches to so-called steroid-binding globulins in the blood that would otherwise latch onto estrogen molecules and render them inert. The testosterone is taken away to the liver, while the estrogen is free to make a lust-inducing dash for the brain.

Pfaus argues further that estrogen may be the ultimate love hormone for men as well. "A lot of studies on rats and birds," he says, "show that brains are like giant ovaries, in the sense that testosterone and other androgens are converted into estrogens in the hypothalamus. And this conversion appears to be critical to the expression of male sexual behavior."

--THE FEEL-GOOD CHEMICAL