"We're not saying that there aren't men who regret casual sex," added Kennair. "But it is far more common for women to regret saying yes. They are also less unequivocally happy about the experience.
"Women regretted having a one-night stand the most, but they weren't sorry about saying no at all."
The team said a number of reasons make women more unhappy about casual sexual encounters, including the fact that women worry more in general about making spontaneous decisions, and mostly do not engage in actions that put them in danger. Women also worry about becoming pregnant, STIs and getting a bad reputation.
Men in the study were also found to enjoy the actual sex more, with more men saying they had achieved orgasm than women.
However the researchers believe that evolutionary psychology is largely to blame for the gender difference, because for thousands of years men and women have adopted opposing sexual strategies because women have to carry and care for children.
"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," the authors conclude in the study, published in the journal Evolutionary Psychology.
Men can theoretically father thousands of children and are only limited by the supply of willing, fertile women. In the past those who could reproduce freely could have so many children that it would not matter if some did not survive.
The "scatter-gun" strategy means that the quality of a sexual partner for men does not have to be as high as for women, the study suggests. Men who moved from woman to woman and got them pregnant would have scored best in the evolutionary race.
"Women and men differ fundamentally in their sexual psychology," says Professor David Buss, of the University of Texas, who collaborated on the study.
"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."
However for women, partner quality is far more important and adding additional sexual partners does not increase their chance or reproductive success.
For most women through the generations, their goal is 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.
"Female choice - deciding when, where, and with whom to have sex - is perhaps the most fundamental principle of women's sexual psychology," added Buss.
"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."
The researchers conclude that cultural changes since the 1960s have not altered underlying gender differences in how men and women view sex.
- Originally published in Telegraph UK
