The cultural differences between the United States and Norway are relatively small, but the cultures differ significantly from one another in a few very relevant areas.
Norwegians tend to be more sexually liberal than Americans. Americans are clearly more religious than Norwegians.
However, despite these differences, Norwegians and Americans regret casual sex about equally. They also regret missing an opportunity for casual sex to about the same degree.
"We find only small differences between the two nations when it comes to sexual regret," says Professor Leif Edward Ottesen Kennair at NTNU - the Norwegian University of Science and Technology's Department of Psychology.
"There is a difference between religious and non-religious individuals, but this difference is quite small," says Mons Bendixen, Associate Professor in the same department.
Norwegian psychologists Bendixen and Kennair have collaborated on a recent study with psychologists David Buss, Kelly Asao and Joy Wyckoff at the University of Texas at Austin.
Big gender difference the most important
Bendixen and Kennair have looked at sexual regret previously, and their new findings replicate the main findings from their previous study.
Women regret their most recent one-night stand much more at than men do. Men regret a missed opportunity much more than women do. Hardly any women regret passing up a sexual encounter.
This big gender difference strongly dominates the results, regardless of how religious or sexually liberal people consider themselves to be.
"The fact that we find this gender difference in both Norway and the United States suggest there is more to the gender difference in sexual behavior than cultural norms and gender roles," says Wyckoff.
Religious or liberal
Study participants were asked if they considered themselves to be religious. They were also asked how important they believed it was to follow their religious doctrines. Here the two countries differ significantly.
On average, Americans find it far more important to live by the precepts of their faith.
Most Norwegians, on the other hand, have more sexually liberal attitudes, and on average have moderately more casual sex than Americans do.
"Although the differences between the cultures may be small from an anthropological perspective, the differences we have measured are relevant to sexual regret," Professor Kennair said.
Americans fantasize more about sex
However, national differences in religiosity or sexual liberalism are small compared to the gender differences related to men's and women's contrasting levels of regret.
But refraining from having casual sex does not mean you do not want to have it. On the contrary, although "Americans have less casual sex, they fantasize more than Norwegians do about having sex with people they meet," says Bendixen.
Evolutionary sexual psychology
Why does the gender difference so completely overshadow religion when it comes to regret? This question leads directly into the discussion of whether culture or biology dominates our behaviour.
"Nature versus nurture is a false dichotomy," Asao points out, "Sexual regret is the result of the complex interaction between reproductive biology and cultural moral standards."
"Sexual regret involves counterfactual thinking and emotions, and it is rooted in the human mind just like our sexual psychology," says Kennair.
The cost of casual sex that can result in children differs markedly for women and for men.
High cost for women
For our foremothers, the cost of casual, unrestricted sex was potentially very high in terms of pregnancy, birth and breastfeeding. Without the investment of a committed partner, the offspring's chances of survival were greatly reduced.
"When this happens over hundreds of generations, we get a selection of women who are less likely to have casual sex and who experience this as less positive when it happens," Bendixen says.
"Women did not only face greater costs from poor casual sex choices, but also received fewer benefits from increased casual sex because women's reproductive success is limited by reproductive biology, whereas men's success is limited by access to fertile women," says Kelly Asao.
Low cost for men
For our forefathers, there were few costs of having casual sex, since it did not entail any investment.
The costs for men were instead linked to passing up opportunities for sex, because men can potentially increase their reproductive fitness by impregnating many women. That is, they can ensure that they pass on a larger proportion of their genes to the next generation.
"None of this is consciously articulated, of course," says David Buss. "Rather, male and female sexual psychology is the end product of a long prior history in which men and women have faced different adaptive problems in the context of selecting or foregoing sex partners."
All of which then means that we are more likely to be descendants of men who did not pass up the chance.
The researchers believe that the sexual psychology of women and men has become relatively differentiated through this selection process, resulting in clear gender-specific patterns of thinking and feeling after individuals choose either to have casual sex or to pass up the opportunity.
These attitudes persist today despite easy access to contraceptives and good social support schemes for mothers without partners, and despite cultural conditions such as religiosity and sexual liberalism.
Explore further: One night stand regrets
Mons Bendixen et al, Sexual regret in US and Norway: Effects of culture and individual differences in religiosity and mating strategy, Personality and Individual Differences (2017). DOI: 10.1016/j.paid.2017.04.054