By some estimates, 30 percent of women fall into this category, though plenty of sexologists argue that pharmaceutical companies have managed to drive up the figures as a way of generating awareness and demand. It’s a demand, in any event, that hasn’t been met. In men who have trouble getting erect, the genital engorgement aided by Viagra and its rivals is often all that’s needed. The pills target genital capillaries; they don’t aim at the mind. The medications may enhance male desire somewhat by granting men a feeling of power and control, but they don’t, for the most part, manufacture wanting. And for men, they don’t need to. Desire, it seems, is usually in steady supply. In women, though, the main difficulty appears to be in the mind, not the body, so the physiological effects of the drugs have proved irrelevant. The pills can promote blood flow and lubrication, but this doesn’t do much to create a conscious sense of desire.

Chivers isn’t especially interested at this point, she said, in pharmaceutical efforts in her field, though she has done a bit of consulting for Boehringer Ingelheim, a German company in the late stages of testing a female-desire drug named Flibanserin. She can’t, contractually, discuss what she describes as her negligible involvement in the development of the drug, and the company isn’t prepared to say much about the workings of its chemical, which it says it hopes to have approved by the Food and Drug Administration next year. The medication was originally meant to treat depression — it singles out the brain’s receptors for the neurotransmitter serotonin. As with other such drugs, one worry was that it would dull the libido. Yet in early trials, while it showed little promise for relieving depression, it left female — but not male — subjects feeling increased lust. In a way that Boehringer Ingelheim either doesn’t understand or doesn’t yet want to explain, the chemical, which the company is currently trying out in 5,000 North American and European women, may catalyze sources of desire in the female brain.

Testosterone, so vital to male libido, appears crucial to females as well, and in drug trials involving postmenopausal women, testosterone patches have increased sexual activity. But worries about a possibly heightened risk of cancer, along with uncertainty about the extent of the treatment’s advantages, have been among the reasons that the approach hasn’t yet been sanctioned by the F.D.A.

Thinking not of the search for chemical aphrodisiacs but of her own quest for comprehension, Chivers said that she hopes her research and thinking will eventually have some benefit for women’s sexuality. “I wanted everybody to have great sex,” she told me, recalling one of her reasons for choosing her career, and laughing as she did when she recounted the lessons she once gave on the position of the clitoris. But mostly it’s the aim of understanding in itself that compels her. For the discord, in women, between the body and the mind, she has deliberated over all sorts of explanations, the simplest being anatomy. The penis is external, its reactions more readily perceived and pressing upon consciousness. Women might more likely have grown up, for reasons of both bodily architecture and culture — and here was culture again, undercutting clarity — with a dimmer awareness of the erotic messages of their genitals. Chivers said she has considered, too, research suggesting that men are better able than women to perceive increases in heart rate at moments of heightened stress and that men may rely more on such physiological signals to define their emotional states, while women depend more on situational cues. So there are hints, she told me, that the disparity between the objective and the subjective might exist, for women, in areas other than sex. And this disconnection, according to yet another study she mentioned, is accentuated in women with acutely negative feelings about their own bodies.

Ultimately, though, Chivers spoke — always with a scientist’s caution, a scientist’s uncertainty and acknowledgment of conjecture — about female sexuality as divided between two truly separate, if inscrutably overlapping, systems, the physiological and the subjective. Lust, in this formulation, resides in the subjective, the cognitive; physiological arousal reveals little about desire. Otherwise, she said, half joking, “I would have to believe that women want to have sex with bonobos.”

Besides the bonobos, a body of evidence involving rape has influenced her construction of separate systems. She has confronted clinical research reporting not only genital arousal but also the occasional occurrence of orgasm during sexual assault. And she has recalled her own experience as a therapist with victims who recounted these physical responses. She is familiar, as well, with the preliminary results of a laboratory study showing surges of vaginal blood flow as subjects listen to descriptions of rape scenes. So, in an attempt to understand arousal in the context of unwanted sex, Chivers, like a handful of other sexologists, has arrived at an evolutionary hypothesis that stresses the difference between reflexive sexual readiness and desire. Genital lubrication, she writes in her upcoming paper in Archives of Sexual Behavior, is necessary “to reduce discomfort, and the possibility of injury, during vaginal penetration. . . . Ancestral women who did not show an automatic vaginal response to sexual cues may have been more likely to experience injuries during unwanted vaginal penetration that resulted in illness, infertility or even death, and thus would be less likely to have passed on this trait to their offspring.”

Evolution’s legacy, according to this theory, is that women are prone to lubricate, if only protectively, to hints of sex in their surroundings. Thinking of her own data, Chivers speculated that bonobo coupling, or perhaps simply the sight of a male ape’s erection, stimulated this reaction because apes bear a resemblance to humans — she joked about including, for comparison, a movie of mating chickens in a future study. And she wondered if the theory explained why heterosexual women responded genitally more to the exercising woman than to the ambling man. Possibly, she said, the exposure and tilt of the woman’s vulva during her calisthenics was proc­essed as a sexual signal while the man’s unerect penis registered in the opposite way.