Half a century ago, the birth-control pill offered women the ability to switch off ovulation, to separate sex from reproduction. It played a part, as the ‘60s got under way, in propelling a host of profound changes, cultural as well as reproductive, societal as well as intimate — in how women saw themselves and lived their lives, starting with the notion of women being above all baby makers and mothers. The promise of Lybrido and of a similar medication called Lybridos, which Tuiten also has in trials, or of whatever chemical finally wins the race for F.D.A. approval, is that it will be possible to take a next step, to give women the power to switch on lust, to free desire from the obstacles that get in its way. “Female Viagra” is the way drugs like Lybrido and Lybridos tend to be discussed. But this is a misconception. Viagra meddles with the arteries; it causes physical shifts that allow the penis to rise. A female-desire drug would be something else. It would adjust the primal and executive regions of the brain. It would reach into the psyche.

Beckoned by ads on the radio and in newspapers and on Craigslist, in the fall of 2011 women across America began applying to be among the 420 subjects in the Lybrido and Lybridos studies. Plenty were turned away when the trials filled. Lack of lust, when it creates emotional distress, meets the psychiatric profession’s clinical criteria for H.S.D.D., or hypoactive sexual-desire disorder. Researchers have set its prevalence among women between the ages of about 20 and 60 at between 10 and 15 percent. When you count the women who don’t quite meet the elaborate clinical threshold, the rate rises to around 30 percent. For a minor fraction of all the sexually indifferent (or repelled), the condition has been lifelong, regardless of whom they’re with or how long they’ve been with them. For middle-aged or older women, menopause and its aftermath may play a role, though its importance is much debated. For a sizable segment of the undesiring, the most common antidepressants, the selective serotonin reuptake inhibitors, can be the culprit. Millions of American women are on S.S.R.I.'s, and many of them would have good use for a pill to revive the libido that has been chemically dulled as a side effect of the pill they take to buoy their mood.

But for many women, the cause of their sexual malaise appears to be monogamy itself. It is women much more than men who have H.S.D.D., who don’t feel heat for their steady partners. Evolutionary psychologists argue that this comes down to innate biology, that men are just made with stronger sex drives — so men will settle for the woman who’s always near. But the evidence for an inborn disparity in sexual motivation is debatable. A meta-analysis done by the psychologists Janet Hyde and Jennifer L. Petersen at the University of Wisconsin, Madison, incorporates more than 800 studies conducted between 1993 and 2007. It suggests that the very statistics evolutionary psychologists use to prove innate difference — like number of sexual partners or rates of masturbation — are heavily influenced by culture. All scientists really know is that the disparity in desire exists, at least after a relationship has lasted a while.

Dietrich Klusmann, a psychologist at the University of Hamburg-Eppendorf in Germany, has provided a glimpse into the bedrooms of longtime couples. His surveys, involving a total of almost 2,500 subjects, comprise one of the few systematic comparisons of female and male desire at progressive stages of committed relationships. He shows women and men in new relationships reporting, on average, more or less equal lust for each other. But for women who’ve been with their partners between one and four years, a dive begins — and continues, leaving male desire far higher. (Within this plunge, there is a notable pattern: over time, women who don’t live with their partners retain their desire much more than women who do.)

Read it at NY Times.