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No matter how lust is triggered, though, sex, like eating or sleeping, is ultimately biochemical, governed by hormones, neurotransmitters and other substances that interact in complicated ways to create the familiar sensations of desire, arousal, orgasm. By understanding how that happens, scientists should in principle be able to help people like Washington for whom sex just isn't working. And indeed, over the past decade or two, scientists have identified many of the pieces of this complex puzzle. It clearly involves testosterone, along with other hormones, including estrogen and oxytocin, and brain chemicals such as dopamine, serotonin and norepinephrine. And there are numerous other bodily chemicals that turn us on, ranging from the commonplace, nitric oxide, to the obscure, vasoactive intestinal polypeptide.
Scientists have also learned that the old notion that 90% of sex is in the mind is literally true: the parts of the brain involved in sexual response include, at the very least, the sensory vagus nerves, the midbrain reticular formation, the basal ganglia, the anterior insula cortex, the amygdala, the cerebellum and the hypothalamus.
If all this sounds complicated, it is. Researchers are still struggling to understand how these pieces fit together and how they might be different for men and women. It's not clear which chemicals of desire are unleashed and under which circumstances, because setting and mood, as women know better than men, can make all the difference between arousal and annoyance.
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.