Researchers for the peer-reviewed study included University of Texas at Austin evolutionary psychologist David Buss. The study was led by Andrew Galperin, a former social psychology doctoral student at the University of California-Los Angeles; and Martie Haselton, a UCLA social psychology professor. It is published in the current issue of Archives of Sexual Behavior.

The findings show how human emotions such as regret can play an important role in survival and reproduction. They suggest that men are more likely to regret not taking action on a potential liaison, and women are more remorseful for engaging in one-time liaisons.

"Prior sex researchers have focused primarily on the emotion of sexual attraction in sexual decisions," Buss says. "These studies point to the importance of a neglected mating emotion -- sexual regret -- which feels experientially negative but in fact can be highly functional in guiding adaptive sexual decisions."

Evolutionary pressures probably explain the gender difference in sexual regret, says Haselton, who earned her Ph.D. in psychology at UT Austin.

"For men throughout evolutionary history, every missed opportunity to have sex with a new partner is potentially a missed reproduce opportunity -- a costly loss from an evolutionary perspective." Haselton says. "But for women, reproduction required much more investment in each offspring, including nine months of pregnancy and potentially two additional years of breastfeeding. The consequences of casual sex were so much higher for women than for men, and this is likely to have shaped emotional reactions to sexual liaisons even today."

In three studies the researchers asked participants about their sexual regrets. In the first study, 200 respondents evaluated hypothetical scenarios in which someone regretted pursuing or failing to pursue an opportunity to have sex. They were then asked to rate their remorse on a five-point scale. In the second study, 395 participants were given a list of common sexual regrets and were asked to indicate which ones they have personally experienced. The last study replicated the second one with a larger sample of 24,230 individuals that included gay, lesbian and bisexual respondents.

According to the findings:

• The top three most common regrets for women are: losing virginity to the wrong partner (24 percent), cheating on a present or past partner (23 percent) and moving too fast sexually (20 percent).
• For men, the top three regrets are: being too shy to make a move on a prospective sexual partner (27 percent), not being more sexually adventurous when young (23 percent) and not being more sexually adventurous during their single days (19 percent).
• More women (17 percent) than men (10 percent) included "having sex with a physically unattractive partner" as a top regret.
• Although rates of actually engaging in casual sex were similar overall among participants (56 percent), women reported more frequent and more intense regrets about it.
• Comparing gay men and lesbian women, and bisexual men and bisexual women, a similar pattern held -- women tended to regret casual sexual activity more than men did.

Regret comes after the fact, so it's not protective, Haselton notes. But it might help women avoid a potentially costly action again.

"One thing that is fascinating about these emotional reactions in the present is that they might be far removed from the reproductive consequences of the ancestral past," Haselton says. "For example, we have reliable methods of contraception. But that doesn't seem to have erased the sex differences in women's and men's responses, which might have a deep evolutionary history."