Desire. When you have it, nobody questions it. When it is absent, it can be tricky to talk about. After all, the subject is delicate, and what is the point? You probably have little clue what is going on anyway. Luckily, scientists are looking out for you—because it is not even close to being just you. An astounding 40 percent of American women between the ages of 20 and 70 have problems with low sexual desire. Men have issues, too, but the numbers are shaky, because, as difficult as this is for women to talk about, men won’t touch the subject. (Even in a doctor’s office, their willingness to address an absence of sexual wants begins and ends with the mechanical issue.)

Plenty of folks have considered the usual suspects in the sapping of sexual desire. A recent study of young women shows the most common factor in females is stress and fatigue, followed by poor self-image and then sexual difficulties such as the inability to reach orgasm. But a lack of desire can stem from conceptual problems involving the way you view sexual experiences, says Syracuse University neuroscientist Stephanie Ortigue. “Desire is more than an emotion,” says Ortigue. “It involves brain areas involved in thinking intellectual things. That is why it’s so personal, so subjective, and so common.”

And although endocrine, genetic and psychological factors can all contribute to desire, or the lack of it, Ortigue’s perspective comes largely from looking at brain circuits.

Several years ago, Ortigue’s team and two others sketched a network in the brain that is always activated when we experience sexual desire. The three research teams independently landed on the same brain regions. These included areas governing emotion, motivation, body image—and, notably, memories associated with life experiences. Memories can affect desire in subtle, subconscious ways. For example, if a person has a feature or personality that reminds you of something positive in your past, a subconscious association between that person and pleasant reveries may trigger desire, Ortigue says. In this way, desire emerges from a collaboration of emotional, motivational and intellectual parts of the brain, she says.

Ortigue and her team recently took a look at these brain regions in 13 women between 26 and 47 who qualified as having hypoactive sexual desire disorder. These women either had no feelings of sexual interest or those feelings had plummeted to a low level of late; many had no sexual thoughts or fantasies. They lacked any impulse to even try to become aroused. And they said the absence of these feelings and thoughts distressed them.

The researchers asked these women—as well as 15 women with no lack of desire--to look at both pictures of male models and nonerotic photos while their brains were being scanned. As expected, the women who lacked desire showed abnormally low activity in the brain network previously linked with that feeling. More surprising, however, was that these same women also showed more activity in prefrontal brain regions involved in inhibition of action, attention to and judgment of the self, and interpreting the actions of other people. That is, that proverbial headache probably has its roots in this decision-making, self-control, theory-of-mind part of the brain.

The increased activity there suggests that people with depleted desire have two problems. One is that they are spending time trying to interpret the intentions of the other person—and probably coming to incorrect conclusions, says Ortigue. The second is that, they are monitoring or evaluating their own responses to erotic stimuli. They are not “living in the moment,” Ortigue says. Such analysis can interfere with the erotic experience, perhaps in the same way that explaining a joke can sap it of its humor.

The work dovetails with decades old studies by William H. Masters and Virginia E. Johnson, who coined a behavior called “spectatoring” they found common among people with sexual disorders. In spectatoring individuals become spectators of themselves during intimate encounters, viewing themselves from above and judging the situation.

“People really suffer from this,” says Ortigue, referring to hypoactive sexual desire. And knowing that the cause is less chemical in nature than psychological and intellectual may offer hope. Instead of fiddling with hormones, says Ortigue, teach people to stop judging themselves and others so much.

Postscript: Stephanie Ortigue is at the forefront of a field called social neuroscience, which blends chemistry, neuroscience and social psychology to improve our understanding of human relationships and develop treatments for social disorders such as autism. The second annual meeting of The Society for Social Neuroscience takes place November 10 and 11, 2011 in Washington, DC.