Young women regret casual sex less if they take the initiative and the sex was good, according to a new study.
Previous research has found that in general women regret one-night stands more than men.
But researchers interviewed 547 Norwegian and 216 American university students, all of them heterosexual.
The answers suggested that the "clearest gender-differentiating factor" for regret after casual sex is who made the first move.
They also found that women feel less regret if the "partner was skilled and they felt sexually satisfied".
All the participants in the the study undertaken by researchers at the Norwegian University of Science and Technology (NTNU) and the University of Texas were aged under 30 years.
Previous research has found that men in general regret casual sex much less than women and the researchers found this was not affected by whether the men took the initiative.
"Women who initiate sex are likely to have at least two distinguishing qualities," says Prof David Buss from the University of Texas.
"First, they are likely to have a healthy sexual psychology, being maximally comfortable with their own sexuality.
"Second, women who initiate have maximum choice of precisely who they want to have sex with. Consequently, they have less reason to feel regret, since they've made their own choice," he adds.
The results are "another reminder of the importance of women's ability to make autonomous decisions regarding their sexual behaviours," says Joy P. Wyckoff from the University of Texas.
"Regret is a highly unpleasant emotion and our findings suggest that having control over their decision to engage in sex buffered women from experiencing regret," she says.
The study also found that sexual competence of the partner played a role in whether women have regrets.
"Women have less regret if the sex was good. For men, this also plays a less important role. The underlying causes are biological," says Mons Bendixen, an associate professor in the Department of Psychology at the NTNU.
Biologically women have a higher investment in the repercussions of mating decisions due to the possibility of getting pregnant.
"For women, sexual skill might be a cue to high male quality," says Kelly Asao, who worked on the study, so women may profit more from high quality in their sexual partners than men do.
The researchers found that feelings of "disgust" or "revulsion" were the biggest reason why both men and women regret a brief sexual encounter.
They included under this area moral regret, but also if it was unhygienic or if they perceived the sex as "gross".
But they say this has a purpose.
"Sexual disgust is an important adaptive emotion," says Prof Buss.
"It functions to help people avoid, now or in the future, potential sex partners who are either low in mate value or who carry some risk of sexually transmitted infections."
The study also found that while more of the Norwegian students had casual sex than the Americans, there was little difference between their reasons for regret.