It's no secret that men are more likely than women to jump into the sack. But a new study adds some twists to the rules of such casual sex.

The research suggests men are far less choosy about the attractiveness of a potential one-night stand. For women to be tempted into considering casual sex, the guy better be a hottie.

These results, based not on real-life encounters but rather on interviews, match with past research showing that men lower their standards when it comes to one-night stands. And it turns out, from the new study, women raise their standards.

Achim Schützwohl of Brunel University in London and his colleagues surveyed nearly 900 students (split about evenly between guys and gals) from the United States, Germany and Italy.

Participants were asked to imagine being approached by a member of the opposite sex, described as either "slightly unattractive," "moderately attractive," or "exceptionally attractive," who asked the student to go out, go to their apartment, or go to bed. Students rated on a scale from 0 (definitely not) to 100 (definitely yes) how likely they would be to accept each request.

Regardless of the opposite sex's attractiveness, men were more likely than women to accept all three offers. For instance, in response to the sex request, men indicated an average likelihood of 46, a whopping figure compared with women's average rating of just 4.

Men were also more likely to accept any of the three offers when the hypothetical woman was moderately or exceptionally attractive than when she was slightly unattractive.

Women were more likely to accept the apartment and bed requests from an exceptionally attractive man than from either a moderately attractive or slightly unattractive man. For instance, female students reported an average rating of about 2 for a slightly unattractive guy, compared with an 8 for an exceptionally attractive suitor.

Such choosiness for short-term sex could be explained by the so-called good genes hypothesis, in which women, for instance, prefer guys who show some feature indicating strong genes that can be passed down to offspring, say the researchers.

While the current study considered general attractiveness, past research has revealed women look for masculine facial features, such as square jaws, when choosing a fling and more feminine facial traits - perhaps because it indicates less testosterone and a more faithful father - in a long-term mate. (Masculine features can indicate high testosterone, which could mean the guy is strong and healthy - traits women would want to pass on to progeny - but such a guy could also be more likely to cheat, research shows.)

The new results, which will be published online this week in the journal Human Nature, also revealed cultural differences. Italian men were more likely than Americans and Germans to say they'd go to bed with the requestor. German men were also less likely than the other men to accept the apartment or date offers.