Have you ever ended up in bed with someone without it turning into anything more? You're far from alone.
The numbers vary a lot with sources and countries, but a rather safe bet is that around half of people in Western Europe and the USA will have at least one one-night stand. In some countries maybe as many as 7 out of 10.
But how men and women experience the "morning after" varies greatly between the sexes.
"Women regret that they agreed to a one-night stand more often than men. Men regret passing up the chance more than women," says Professor Leif Edward Ottesen Kennair at the Norwegian University of Science and Technology's (NTNU) Department of Psychology.
These are the results of a previous American study. Kennair and Associate Professor Mons Bendixen wanted to see whether this is also true in Norway, which is supposedly a more sexually liberal and egalitarian country. They also wanted to try and find out why the results varied so much by gender. The Norwegian researchers collaborated with evolutionary psychologist David Buss of the University of Texas at Austin.
Same differences in Norway
Kennair, Bendixen and Buss found exactly the same pattern in Norway as in the U.S.
A larger proportion of women than men regretted the last time they had casual sex. Around 35 per cent of women and only 20 per cent of men regretted the experience to some degree.
"So we're not saying that there aren't men who regret casual sex," says Kennair.
But it is far more common for women to regret saying yes. They are also less unequivocally happy about the experience.
About 30 per cent of women in Norway were happy about their most recent casual sex experience, as were over 50 per cent of the men, according to Bendixen.
Only men regret saying no
At the same time, nearly 80 per cent of women were happy that they said no to casual sex last time. Only 43 per cent of men were totally happy that they passed it up.
"Women regretted having a one-night stand the most, but they weren't sorry about saying no at all, says Kennair.
But that's not the biggest difference. Very few women regretted saying no. But nearly 30 per cent of the men regretted not having casual sex, according to Bendixen.
Several possible explanations
Women worry more than men generally, and they worry about more than just casual sex. As Kennair sums it up, men do stupid things that they die from. Fewer women do these stupid things.
But this basic phenomenon does not change the study's main conclusion that women and men react so differently to one-night stands.
So why such dramatic gender differences in regret?
The researchers found no difference in regret between those who were single and those who were in a relationship, so they did not take partner status into account in their further analyses.
However, they did examine several possible reasons for regret, such as pregnancy concerns, STD infections and getting a bad reputation.
Across the board, women worry more about all these factors. But this doesn't explain why Norwegian women regretted casual sex so much more than men did, though, said Bendixen and Kennair.
And worry was certainly not the reason why virtually only men regretted saying no.
Less sexual gratification for women?
It's conceivable that women feel more regretful because they do not get as much sexual pleasure out of a one-night stand as men do. Maybe they regret it more because they don't have an orgasm?
This idea led the researchers to also ask whether participants achieved an orgasm or not.
Not surprisingly, men had orgasms during sex to a far greater extent than women the last time they had casual sex. But at the same time, the results show that far fewer women than men think orgasm is particularly important.
Both women and men are more regretful about one-night stands that did not end in orgasm for them. But overall, lack of sexual satisfaction does not seem to play into why women regret the experience more often than men.
"Men enjoy casual sex considerably more, but this doesn't explain the gender difference in regret, because gender is the most important influencing factor for both orgasm probability and sexual regret after casual sex," says Professor Kennair.
So what can be the reason?
An overall explanation presumably lies in the fundamental differences between men and women.
The study results support theories of parental investment and sexual strategy: men and women have throughout generations invested differently in their relationships and any children that resulted.
We're talking evolution psychology here.
All of us are descendants of individuals who managed to reproduce. Passing our genes on to the next generation as effectively as possible is the ultimate biological goal for everyone. If you don't, your genes die out eventually.
Through evolution, nature weeds out what is not working. But when it comes to short-term sex, the best strategy is fundamentally different for men and women.
"Due to selective pressure from the big difference in parental investment, one would expect men and women to regret different aspects of casual sex decisions - having casual sex with the wrong partner versus missing a casual sexual opportunity," notes the article about the study published in Evolutionary Psychology.
This might require an explanation.
Men are limited by access
Men can theoretically father thousands of children and are primarily limited, at least in theory for most of us, by the supply of willing, fertile women. Men who could reproduce freely would be able to afford having some of the kids fail to multiply as long as most functioned serviceably.
"Women and men differ fundamentally in their sexual psychology," says Professor Buss. "A key limitation on men's reproductive success, historically, has been sexual access to fertile women. These evolutionary selection pressures have created a male sexual mind that is attentive to sexual opportunities."
The quality of one's sexual partner in short-term relationships plays a lesser role biologically for men. Assuming women did not avoid having sex with them, men who ran from woman to woman and got them pregnant would have scored best in the evolutionary race.
"The winner takes them all," says Kennair with a smile.
Few men have such unlimited access to the other sex, but quantity over quality has been the main strategy for men in general.
Consider this example. A married man with two children, historically, could have increased his reproductive success by a full 50 percent by impregnating one other woman.
Men do not think about these things consciously, of course. Rather, men's sexual psychology is highly attuned to sexual opportunities and they experience regret at missed sexual opportunities.
Women see it differently. Partner quality has been far more important to them.
Women have the most to lose
Our ancestral mothers rarely could have increased their reproductive success by adding additional sex partners.
Women can seldom have more than 10-15 children during their lifetime, no matter how much they try. Obviously most women today bear far fewer children than that. The quality of the children, and thus the quality of the sexual partner who contributes to the children's genes, is far more important for women than for men.
"Female choice--deciding when, where, and with whom to have sex-- is perhaps the most fundamental principle of women's sexual psychology," says Dr. Buss.
For a woman, an ideal partner helps raise their children in order to give the next generation the best possible conditions to reproduce.
For most women through the generations, it has been important to secure a partner of high quality who was willing to invest more in their children together, and who did not waste resources by getting involved with other women and their potential children.
Thus it is quite natural that women regret casual sex much more with a man who is not an ideal partner. Women have for generations had much more to lose.
Culture does not change biology
Of course, most Norwegian women manage much better on their own today than they did even a few generations ago.
Many of the ancestral conditions that created these fundamentally different male and female sexual psychologies are no longer with us, especially in Norway.
Men are less important than they were in the past for providing resources to children. Women earn their own money, society offers various support schemes, and women are largely able to raise children without appreciably involving a man beyond the actual fertilization.
In sexually liberal countries one's reputation is not necessarily harmed in the same way as it once might have been by having casual sex. Effective contraceptives that women have control over also reduce the risk of getting pregnant with a partner who is less than ideal. But the fear of getting pregnant with a less suitable partner can still hang on, consciously or not. So can the fear of a bad reputation.
"Many social scientists expect that in sexually egalitarian cultures such as Norway, these sex differences would disappear. They do not. This fact makes the findings on sex differences in sexual regret in modern Norwegian people so fascinating scientifically," says Dr. Buss.
New cultural changes, it turns out, do not alter our biological foundation. Evolution doesn't work that way. The underlying gender differences in parental investment in offspring and the way we act do not change within a few generations.
Our evolved sexual psychology is the only one we have. It operates in the modern world as much as our evolved food preferences, whether or not it is currently adaptive.
The study participants consisted of 263 students aged 19 to 37 years. All had at least one one-night stand behind them.