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Natasha Gadinsky, 23, says she doesn’t have any regrets from her years in college. But the time she hooked up with a guy at Brown University does come close.

After his own orgasm that night, she said, he showed no interest in her satisfaction. The next time they got together, it happened again. He “didn’t even care,” said Ms. Gadinsky, a health care case manager in New York City. “I don’t think he tried at all.” He fell asleep immediately, leaving her staring at the ceiling. “I was really frustrated,” she said.

Like generations before them, many young women like Ms. Gadinsky are finding that casual sex does not bring the physical pleasure that men more often experience. New research suggests why: Women are less likely to have orgasms during uncommitted sexual encounters than in serious relationships.

At the same time, researchers say that young women are becoming equal partners in the hookup culture, often just as willing as young men to venture into sexual relationships without emotional ties.

“The notion of sexual liberation, where men and women both had equal access to casual sex, assumed a comparable likelihood of that sex being pleasurable,” said Kim Wallen, a professor of neuroendocrinology at Emory University who studies female desire. “But that part of the playing field isn’t level.”

Research involving 600 college students led by Justin R. Garcia, an evolutionary biologist at the Kinsey Institute at Indiana University, and researchers at Binghamton University found that women were twice as likely to reach orgasm from intercourse or oral sex in serious relationships as in hookups. The paper was presented at the annual meeting of the International Academy of Sex Research and at the Annual Convention for Psychological Science this year.

Similarly, a study of 24,000 students at 21 colleges over five years found that about 40 percent of women had an orgasm during their last hookup involving intercourse, while 80 percent of men did. The research was led by Paula England, a sociologist at New York University who studies the dynamics of casual sex.

By contrast, roughly three quarters of women in the survey said they had an orgasm the last time they had sex in a committed relationship.

“We attribute that to practice with a partner, which yields better success at orgasm, and we also think the guys care more in a relationship,” Dr. England said.

Indeed, young men surveyed in Dr. England’s study often admitted that they are less focused on sexually pleasing a woman they are seeing casually than one they are dating.

Duvan Giraldo, 26, a software technician in Elmhurst, Queens, said that satisfying a partner “is always my mission,” but added, “I’m not going to try as hard as when I’m with someone I really care about.” And with women he’s just met, he said, it can be awkward to talk about specific needs in the bedroom.

“You’re practically just strangers at that point,” he said.

The lack of guidance is common, Dr. England said. “Women are not feeling very free in these casual contexts to say what they want and need,” she said. Part of the problem, she added, is that women still may be stigmatized for having casual sex.

Dr. Garcia said, “We’ve been sold this bill of goods that we’re in an era where people can be sexually free and participate equally in the hookup culture. The fact is that not everyone’s having a good time.”

What women need to achieve orgasm can be very different from what they find in casual sex. Roughly one-quarter of women reliably experience orgasm through intercourse alone, according to a review of 32 studies conducted by Elisabeth Lloyd, a professor of the history and philosophy of science at Indiana University, in her 2005 book “The Case of the Female Orgasm: Bias in the Science of Evolution.” Another third of women rarely or never have orgasms from intercourse.

Vanessa Martini, 23, from Marin County, Calif., learned early on that most men she slept with casually would not intuit her needs.

“I haven’t hooked up with anybody who was so cavalier as to just, like, not even care,” she said. “But I think most of them were somewhat baffled that it would require more than just them thrusting.”

Ms. Martini said she was never taught how to have good sex, let alone how to ask for what she needs. The education she received in school was aimed at stopping teenagers from having sex at all; there wasn’t much discussion of arousal. Ms. Martini said most cultural representations of sex left out the messy details.

“The way we view sex in porn and in movies and in books, people aren’t talking to each other like, ‘Oh, my foot’s falling asleep, we need to move,’” she said.

Communicating about those particulars is especially tricky in hookups. When one awkward exchange or misread text message could end the arrangement altogether, there’s a certain amount of pressure to tread softly, Ms. Martini said.

“You have to balance a lot of things in your brain, like what’s more important to me — just getting off, or do I actually want to have a connection with this person?”

Debra Herbenick, a research scientist at Indiana University, said that for women, casual sex is exciting precisely because it is spontaneous. She compared a hookup with having dinner at a friend’s house. “You wouldn’t be like, ‘This is what I want and this is how I want you to make it, and I want you to use only this amount of basil,’” she said.

Some women, confronted with these roadblocks, are redefining casual sex and the physical pleasure that they expect from it. Sex without strings has carnal and emotional benefits that don’t depend on reaching orgasm, they say.

“Something we don’t talk about is why having an orgasm is the main goal or the only goal” of sex, Dr. Herbenick said. “Who are we to say women should be having orgasms?”

Casey Romaine, a 22-year-old Bard College graduate living in Nashville, Tenn., said that more than sex, hookups are often much more about two people giving each other the sense of intimacy, however brief, they need to get through the week.

“It’s just sort of like having the experience, and having somebody that you can call or you can like, whose house you can spend the night in if you don’t feel like you want to be going home alone,” she said. “I think a lot of the time it almost is weirdly irrelevant whether or not the sex is actually good.”

For Kim Huynh, a 29-year-old filmmaker in San Francisco, sacrificing a reliable orgasm for sex without the burden of commitment was a conscious decision. After a couple of relationships in college, Ms. Huynh spent about five years without a serious boyfriend and many on-again, off-again flings.

“As far as my ability to climax consistently, that’s something I was able to have in my monogamous relationships that I never had” in less committed circumstances, she said.

Yet mediocre sex was a small price to pay “for the freedom to be able to enjoy it all.” The physical aspect of a tryst with a relative stranger was gratifying, she said, even if her chances of reaching orgasm were limited. When her partner’s performance was lackluster, she still took pride in her own sexual prowess.

“To sort of know yourself to be sort of skilled in a way or to be able to see someone else’s pleasure that was your own doing, I think there’s definitely something very empowering about that,” she said.