Women are shattering political glass ceilings, surpassing men in the workforce, and even winning Indy-car races. But there’s one area where the gender gap has proved particularly stubborn.

“The orgasm gap is an inequity that’s as serious as the pay gap, and it’s producing a rampant culture of sexual asymmetry,” says Paula England, a professor of sociology at Stanford University.

New academic research conducted by England and others is shedding light on one of the world's most familiar bedroom problems. In a study to be published later this year by W.W. Norton in the book Families as They Really Are, researchers found that college women have orgasms half as often as men on repeat hookups (meaning hooking up more than twice) and only a third of the time in first-time hookups. And they concluded that a lack of sexual reciprocity could be a key reason for this orgasm gap. The study was conducted by a team of researchers from Stanford and Indiana University.

The male psychology on women's orgasms is comparable to their psychology on housework: Men don’t pull their weight on either front because no one makes them.

Their research confirms that the orgasm gap is widespread among young people in both casual hookups and relationships. Surveying 12,925 undergraduates from 17 universities, researchers examined four sexual contexts—a first hookup, one to two previous hookups, three or more previous hookups, and a relationship—and found that in all cases, men were twice as likely to orgasm. That gap is far wider in hookup situations than in relationships. In the context of relationships, women orgasm about 80% as often as men.

It’s not just sexual neophytes on college campuses that are having trouble. After looking at 32 studies that included mostly married women and spanned the past 90 years, Dr. Elisabeth Lloyd, author of The Case of the Female Orgasm: Bias in the Science of Evolution and a professor of history and philosophy of science at Indiana University, found that a third of women never had an orgasm during intercourse.

Where does this orgasm gap come from, and why is it so much more pronounced in instances of casual sex than in relationships? Analyzing data from the survey, the researchers found a few possible explanations, one of which has to do with the amount of effort expended in bed—and who’s expending it.

England’s study found that women give oral sex to their male partners in all contexts—from casual hookups to relationships—at higher rates than men do, sometimes dramatically higher. The study’s anecdotal evidence backs this up. “The ratio of oral sex was 4-to-1 in his favor,” says writer Kimberlee Auerbach, 36, of her last long-term relationship. (She adds that despite the lopsided ration, he was still invested in her orgasm.)

In casual hookups it’s much worse—during the first few times they hook up with a man, women are far more likely to give him oral sex than to receive it. Men receive oral sex about 80% of the time in first-time hookups, whereas women receive it less than half the time. This discrepancy goes a long way toward explaining the orgasm gap, according to the study’s authors. In 1976, the Hite Report on Female Sexuality empirically established a fact that’s been confirmed by subsequent studies in the years since: Many women need oral sex, along with intercourse, to reach orgasm.

Why aren’t they getting it? Michael Kimmel, author of Guyland and a leading writer on men and masculinity, sees the male psychology on orgasms as comparable to housework: “Men don’t pull their weight on either front because no one makes them.” But he also sees sexual asymmetry as an impoverished view of sex. “The grown-up version [of sex] is certainly not thinking about giving oral sex as akin to some kind of community service, but that it gives me pleasure to give you pleasure. I know that’s certainly true for me personally.”

Another possible—and possibly related—reason for the orgasm gap is that while many men are happy to be passive recipients when it comes to oral sex, they want to be the proactive one when it’s time for intercourse. Courtney Martin, 29, author of Perfect Girls, Starving Daughters, says she believes passivity in intercourse among some women prevents them from sculpting the sexual experience to their preference.

“A lot of these girls end up forgoing asserting themselves in order to avoid the awkwardness of doing clitoris 101,” says Ms. Martin. “One girl at a small-liberal arts school in the East told me that being with most guys felt like ‘they are masturbating into you.’”

Natalie Angier, a science columnist at the New York Times, say that clumsy sexual behavior should not be misinterpreted as male disinterest in the female climax. “He may never call her again, but he wants her to come,” says Angier.

But that may be optimistic, at least in certain scenarios. The research doesn’t bode well for the late-night booty text, one-night stand, or random fornication in the fraternity house as pathways to an orgasm. In the one-on-one interviews included in the study, one man explained that with his girlfriend, “definitely oral is really important [for her to orgasm],” but that with a casual hookup, “I don’t give a shit.”

“Women and men are more ambivalent about the importance of women’s sexual pleasure [outside] of relationships,” says England. “Our findings suggest that both women and men have absorbed a notion that women are entitled to sexual pleasure in a relationship, but not necessarily in casual scenarios.”

Another male college student interviewed for England’s study put it this way: “Now that I’m in a relationship, I think [her orgasm is] actually pretty important. More important than [in a] hookup. Because you have more invested in that person…When it’s a hookup you feel less investment.”

The bright spot of the study is this: Even though folklore has it that women don’t achieve orgasms with strangers because they need emotional attachment to feel that sort of pleasure, the truth is that women’s orgasms are not usually the result of emotional attachments, but simple physicality. Which is why Angier says she believes the sexual paradigm of women as passive receiver and man as sexual agent needs to be stamped out if women are going to get serious about their orgasms.

“The woman really has to be the boss of the sexual experience, because it’s harder for women to have an orgasm through a straightforward sexual position. Women need to start understanding how their clitoral nerves are positioned,” says Angier. Speaking from her own experience, she says multi-orgasmic women take responsibility for their own pleasure. “Personally, I made that my pet project.”

Although Sigmund Freud argued that a clitoral orgasm was adolescent and that the vagina was the fountain of the more “mature” orgasm, there’s evidence that theory is not only misguided, but is also fueling the orgasm gap.

“Stimulation of the clitoris is what gives a woman an orgasm. It’s the center of orgasmic function,” says Dr. Lloyd. “The clitoris is the homologue of the penis—they have the same tissue. In embryos, the same organ that turns into the penis, turns into a clitoris."

Clitoral stimulation, however, is not an image that gets a lot of airtime.

Laura Sessions Stepp, the author of Unhooked, a book about hooking up on college campuses, points to the popular television series Gossip Girl.

“Watching that show, you’d think all you have to do is poke it in her and she’ll have an orgasm.”

Hannah Seligson is a journalist and the author of New Girl on the Job: Advice from the Trenches, a career guide for young women. Her second book, A Little Bit Married: How to Know When to Walk Down the Aisle or Out the Door, will be published by De Capo this year. Her website is www.hannahseligson.com