Half a century ago, the birth-control pill gave women the ability to switch off ovulation, to separate sex from reproduction.
As we speak, researchers are working on a pill to give women the power to switch on lust, to free sexual desire from its various shackles -- including, perhaps, long-term monogamy.
With Dutch inventor Adriaan Tuiten just months away from presenting the drug, Lybrido, to the United States' Food and Drug Administration for approval -- if all goes well it could hit the market by 2016 -- out comes a book that turns everything we think we know about female sexuality on its head.
In What Do Women Want? Adventures in the Science of Female Desire, American journalist Daniel Bergner posits that the reason women in long-term relationships lose sexual interest in their partners more often -- and more quickly -- than men is because they're not as turned on by emotional intimacy and stability as we've been led to believe.
"Studies conducted recently are beginning to hint that female eros isn't in the least programmed for fidelity. These range from close focus on the sexual habits of our primate ancestors to research exploring women's wish for casual sex," he writes in a recent New York Times magazine cover story, adapted from his book.
Bergner cites research that suggests flagging female libido often stems from psychological as much as physical factors. He makes the case that women are socially, not biologically, discouraged from initiating and enjoying sex. In the process, he debunks the conventional wisdom of evolutionary biology that men are hard-wired to spread their seed far and wide and women seek a resource-rich provider.
Drugs like Lybrido (and a similar medication called Libridos), may mark the next phase of the pharmaceutical industry's obsessive, better-loving-through-chemistry quest, but Bergner says it's misleading to refer to it as "female Viagra."
Although Viagra has been found to have some influence on men's mental state of desire, he says, the blue diamond and its competitors only address the "simple hydraulics of impotence."
Redirecting blood flow to a woman's genitals, research has shown, isn't enough to light her fire.
"A female-desire drug would be something else," Bergner writes. "It would adjust the primal and executive regions of the brain. It would reach into the psyche."
Hypoactive sexual-desire (HSDD) is the clinical term for lack of lust when it creates personal distress and/or interpersonal difficulty. Researchers estimate its prevalence among women ages 20 to 60 to be as high as 30 per cent.
Bergner cites a German researcher who found "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."
Evolutionary psychologists argue that women have HSDD much more than men because of innate biology; the male sex drive is simply stronger. But Bergner points to mounting evidence that the statistics those researchers use to prove innate differences are heavily influenced by culture.
"There's no slut-shaming in the animal kingdom," columnist Ann Friedman recently tweeted in response to Bergner's article.
No one knows that better than Jim Pfaus, a professor of psychology and neuroscience at Montreal's Concordia University whom the piece referred to as "one of the world's masters of rat lust."
Pfaus is a consultant to drug companies, including Emotional Brain, which is conducting clinical trials on Lybrido. (Tuiten has described the results thus far as "very, very promising.")
By dissecting and studying the brains of copulating rats, Pfaus has determined that sexual desire for both men and women seems to originate in two low-lying brain zones. From there, it's a complicated biochemical dance between the hormone testosterone and the neurotransmitters dopamine (the "molecular essence of desire") and serotonin, which is all about inhibition and self-control.
Lybrido will tamper with the interplay between serotonin and dopamine, giving the latter a libido-boosting edge.
The old adage about the brain being the biggest sex organ is true, Pfaus says, and it's a plastic one at that.
Experience and expectations can alter neural networks, he says. So if men, having been socialized to associate sex with masculinity, think about sex often as a result, then the neural pathways associated with desire will become stronger over time. Whereas those same neural networks in women, who have been more likely to receive less positive messages about sexual desire and expression, will have gotten less of a workout and be comparatively weak.
"So if you expect to spread your legs and think of England, you will," Pfaus says during a phone interview.
Contrary to the canard of evolutionary biology, which downplays the importance of sex to women, we're hard-wired to be flexible, Pfaus says. "We're hard-wired to learn, to say, 'Wow, that was fantastic, I want to do that again with him, or her.'"
Just as Viagra isn't for every man, drugs like Lybrido are not going to be a panacea for every woman who has lost or lacks the urge, he says.
"We're seeing premenopausal women describing a lack of desire. Now, how much of that is their partners not giving them what they need? Some of it is, and those individuals may be better off with traditional sex therapy."
The medication should be prescribed in the context of therapy, Pfaus says, and specifically for individuals who have been diagnosed with HSDD -- and who want to treat it.
"There are going to be plenty of women who say, 'I'm 70, I'm done. There will be others who will say, 'I'm 70 and I'm just starting.'"
But beyond priming a woman's brain so she wants to get busy, Pfaus says, a desire drug will not let the object of a woman's desire off the hook. Ideally it will be used "in the presence of somebody who is a competent sexual stimulus."
Drugs like Lybrido seem like the next logical step in a profit-driven pharmaceutical industry, says Shawna Ferris, an assistant professor in women's and gender studies at the University of Manitoba. It's telling, she says, that a female sex drug is being developed after the success of the male one, and that there's so much cultural controversy surrounding it.
It'll be interesting, Ferris says, to see whether a drug that enhances -- or even produces -- female sexual desire will sustain or threaten the socio-sexual order.
"Women's sexual desire, it seems, is a very powerful thing," she writes in an email. "For more than 2,000 years, dominant Western culture has alternated between fear and loathing and denial and repression of sexual desire in women."
A society that takes male sexual desire as a given but can barely admit to the existence, let alone the importance, of female desire, is inherently dysfunctional, Ferris says.
"Many women have unsatisfying or bad sexual experiences in otherwise consensual interactions, in part because they are not taught to recognize or express their sexual needs; moreover, neither they nor their partners -- male or female -- are taught to value and give sexual pleasure to women."
Interestingly, Pfaus says even drug trial subjects in placebo groups have reported a slight increase in sexual desire.
"Why does that happen? Probably because it's the first time many of these people have been in a sex-positive environment, where they've actually talked openly about sex."