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An important starting point for understanding sexual desire is to appreciate that hormones play a key role in motivating our sexual appetites and behaviors. In fact the operation of our sexual systems depends on a cascade of hormones that affect us at two critical points during our life cycleinitially during the course of embryonic development (these are called organizing effects, which establish the architecture of the brain/body) and later on, when we hit puberty (these are the activating effects, which turn on the wired-in systems).

Most people dont know that the masculinization of the brain and body happen during embryonic development at two distinct and separate points, facilitated by two different hormones, which has huge potential in clarifying some issues involving sexual orientation, gender identification, and transgenderism (in which the sex of the brain does not apparently match that of the body). As far as the activating effects of the hormones, we know that the hormone testosterone is responsible for the sex drive in both male and female mammals and people.

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What is not usually discussed (but has been well established) is that testosterone has more impact on the male mammalian brain. While it might be considered politically incorrect to call attention to fundamental differences in how male and female brains are wired, modern neuroscience has provided tons of evidence to support that there are indeed significant differences. Although it is true that the brain of each sex has some feminine and some masculine networks or circuits, if all goes according to plan in the process of embryological development, male brains simply have more places for the testosterone (and another hormone related to testosterone, vasopressin) to workby connecting with structures called receptors activated by the hormones. The receptive fields or places for testosterone to have influence are plentiful in the male brain. This feature explains the general finding that males tend to report having sex on the mind more frequently than females.

On the other hand, if all goes well during embryonic development, the female brain comes equipped with sufficient receptors sensitive to the hormone oxytocinthe hormone that is associated with decreased anxiety, increased trust, and social bonding. Oxytocin affects male brains similarly but there are substantially fewer oxytocin circuits in the male brain. Interestingly enough, the female sex hormone estrogen turns up the activity of the oxytocin circuits in the brain, while testosterone in the male brain fires up the vasopressin circuits, fueling competition and sexual interest.

Another fascinating aspect of female sexuality is that womens interest in the erotic can wax and wane in response to cyclical changes in brain chemistry. At peak fertility, when estrogen and progesterone levels are high, some women report more sexual thoughts and fantasies. Studies have also shown that women engage in more sexual behavior during their fertile periods (with rates of intercourse rising by 24 percent during the six days flanking ovulation). But way more than biology affects female sexuality. One need only read the fascinating research done by Dr. Meredith Chivers, a colleague from Queens University in Ontario, who has demonstrated that, in women, arousal of the genitals (as measured by blood flow in response to audiovisual erotic stimuli) simply doesnt translate into subjective sexual arousal or feeling turned on. In other words, blood flow to female genitals tends to be a nonspecific response to all sorts of erotic stimuli, regardless of the female participants sexual orientation, which doesnt necessarily correlate with feeling turned on. Blood flow to the womens genitals increases when the participants watched males with females, males with males, females with females, and even bonobos (those randy pygmy chimpanzees) getting it on.

This type of arousal is keenly different from how male arousal works: mens genitals only rise to the occasion, so to speak, in response to stimuli matching their own sexual orientation. And for men, increased blood flow to the penis in this type of study usually translates into increased subjective turn on. This explains why Viagra-type drugs dont work for women. With these drugs, you can indeed increase blood flow to the female genitals, but it doesnt do much for the ladies in terms of either subjective (experienced) arousal or desire. Suffice it to say that sexuality for women appears to have more complex underpinnings, which are not as well understood.

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Men and women also experience some aspects of sex differently. At first, in the early days, sexuality research by Masters and Johnson (1966), Kaplan (1979), and Lief (1977) described the sexual response cycle as a linear process that begins with desire/arousal, moves to a plateau or middle stage of intensified arousal or excitement, and then on to a third stage of orgasm and/or ejaculation. We have moved beyond this model not only because women dont fit neatly into it but also because we know so much more about the varying ways that humans in general become aroused and experience desire.

Several sexologists, including Beverly Whipple and K. B. Brash-McGeer and Rosemary Basson, distinguish the female sexual response cycle as being more circular than linear because there are so many more dimensions to what drives female desire and arousal. Basson shifted the nonlinear model farther by emphasizing that women are not necessarily motivated by sex for the release of orgasm but rather personal satisfaction, which may come through the emotional experience of intimacy with a partner. Essentially, for women, as compared with men, sexual desire might not be as driven by physically experienced horninessbut rather more motivated and accessed by and through the warm, intimate, and fuzzy partnership pathway.

Why are these models helpful or significant? Because they underscore that the pleasure of sex comes at different stages and in different forms: in the turn-on level of stimuli; in the predicted expectation that sex is going to happen; in the body-focused build-up of increased blood flow and muscle tension of the excitement and plateau stages; and ultimately in the release of the orgasm. If pleasure is experienced all along the way and is naturally variable, and the brains involvement is paramount, then how we think about solving our sexual problems needs to consider these realities.

Nan Wise, Ph.D., is AASECT certified sex therapist, neuroscientist, certified relationship expert. This piece is an excerpt from her new book, Why Good Sex Matters: Understanding the Neuroscience of Pleasure for a Smarter, Happier, and More Purpose-Filled Life, out Jan. 28, 2020. Follow her @AskDoctorNan.

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