A study of almost 800 people, to determine why women regret casual sex more than men, found feeling pressured into it is a key reason. Women feel much less regret if they initiate the encounter, while men care less about who makes the first move.
Lead author Dr Leif Edward Ottesen Kennair, from the Norwegian University of Science and Technology, said women's greater reluctance to have no-strings sex may be due to humans' evolutionary past.
He said: "Women in the past had a lot more to lose from having sex with a non-committed partner, as if they got pregnant the man could walk away at no cost to anything but his reputation while she would be left to bring up the child.
"We know today that women are more worried than men about pregnancy, sexually transmitted infections and their reputation."
Women may also regret a one night stand more because they are less likely to climax, according to research. Put simply, men tend to have a better time during sex, which cancels out some of the more unpleasant feelings such as shame and guilt.
The psychologists interviewed 763 people about their last no-strings sexual encounter. Around half of women said they regretted it – compared with about a third of men. To find out why, they asked participants how much they enjoyed the sex, whether they had an orgasm, and how much they worried, felt immoral or felt pressured.
The study group were also asked to rate their agreement with the statement: 'I was the one to take the initiative.'
The results showed men and women regret having sex more if the sex is bad, the other person is "sexually incompetent" or they experience worry or disgust. Each factor is more important for women than for men.
Women felt less regretful if they made the first move, but this had no impact on whether men felt regret or not.
Study co-author Professor David Buss, from the University of Texas, said: "Women who initiate sex are likely to have at least two distinguishing qualities.
"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 have sex with."
Dr Kennair added: "It may mean they choose a better-looking man to sleep with."
The study, published in the journal Personality and Individual Differences, found disgust was the biggest cause of regret over a one night stand.
This could be disgust over the type of sex, someone's hygiene or the act of having sex outside a relationship.
The authors conclude: "Women's greater worry and lower levels of sexual gratification partially explain why women regret casual sex."