MODERN mating without dating is changing sexual behaviour in America. From teenagers to adults, hooking up has become part of the vocabulary of sex.

Encompassing a range of conduct — from heavy petting to oral sex and intercourse — it is becoming standard practice in schools, on campuses and among adults, with damaging consequences to self-esteem, according to new research.

Participants often call their hook-ups “friends with benefits”. They are a bit more than casual acquaintances but a lot less than a boyfriend or a girlfriend. You “hook up” with them, nothing more. You are free to string them along or hook up with somebody else at the same time. It is important not to “catch feelings” or fool yourself that you are in love.

A book on the phenomenon — The Happy Hook-Up, a Single Girl’s Guide to Casual Sex — is to be published this month. Alexa Joy Sherman, one of the co-authors, questioned 3,000 women and found that 25% agreed that they could have a one-night stand “without becoming even somewhat attached”. But 80% of such women regretted their conduct, which sometimes made them feel like “sluts”.

Sherman, 34, who had her share of hook-ups while working in the music business before getting married last year, believes that casual sex helped her to focus on her career rather than her private life.

“I was acting like a man,” she said, “but it was a great way to explore my sexuality and I enjoyed it while it lasted. I didn’t have any emotional issues about keeping things casual.”

She suggests that Angelina Jolie, the film star and mother of an adopted boy, is typical of the way the hook-up generation is adapting sex to suit its needs.

Jolie said earlier this year: “Meeting a man in a hotel room for a few hours and then going back and putting my son to bed and not seeing that man again for a few months is about what I can handle now. I can feel like a woman and get close to a man, but it’s not a relationship that interferes with my family.”

Hooking up is becoming the norm for teenagers and students. The ubiquity and persistence of the trend is troubling academic researchers.

Elizabeth Paul, head of psychology at the College of New Jersey, published the first study of it in 2000. She surveyed 555 students and found that 78% had “hooked up”. They accumulated an average of 10.8 sexual partners during their college years.

Paul believes that hooking up is different to the casual sex of a decade or more ago. It is a more cynical endeavour for both sexes, who have little or no expectation of forming a relationship. “I ask my students, ‘Do you plan hook-ups?’ and they say, ‘Not necessarily with a specific person, but yeah, we want to hook up on Friday night’.”

Students are blunt about why they would rather hook up than date. Laura, 18, a pretty first-year student at the City University of New York, said: “There’s no stress about it. You don’t have to worry about whether he’s going to call or if he’s seeing someone else, because you know he is and so are you.”

From sink schools to Ivy League campuses, the gross behaviour of teens trying to “get laid” in American Pie, the 1999 film, has become commonplace. Boys are called “players” and girls are “sluts”.

Dan, 22, a recent graduate of Harvard University, said: “Hooking up is my primary MO (modus operandi). There is nothing remotely scandalous or shameful about it. It’s fairly egalitarian. Whatever goes for me goes for her.”

Dan considers himself to have been single for the past two years. “I tend to have one main but non-committed relationship, which can overlap with others,” he said. “It’s a good idea after hooking up a few times to have ‘the talk’, where you set a few ground rules and mention words like ‘open relationship’.”

Fear of Aids and a rising incidence of sexually transmitted diseases — more than 15m new infections are diagnosed each year in America, a quarter of them among teenagers — have increased the popularity of sex that stops just short of intercourse.

Allison Caruthers, 29, a PhD student who is tracking a group of 25 women as part of a broader Michigan University study of students’ sexual behaviour, said that condoms were used more in hook-ups than in relationships.

More than 60% per cent of students reported hooking up. “From my interviews, the women having these hook-ups are not satisfied with how they are being treated,” Caruthers said.

At the College of New Jersey, Paul is disturbed by her continuing research into the long-term psychological effects on students. “People go into hooking up with goals and expectations that are very different to the feelings they come out of it with,” she said.

“They think it is going to be a normal fun thing to do and then discover they have complicated feelings. They might feel lonely, angry, regretful or pressured. It catches them unaware and they start to think, ‘Oh, I must have done something wrong’.”

Girls are particularly affected, Paul added: “Men and women may be hooking up at the same rate but it’s still a lot worse to be a ‘slut’ than a ‘player’.”

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