Men harbor regrets about not having sex with more women, while women had doubts if they had sex with the right partner.
Researchers from the University of Texas said that these regretful thoughts are part the evolutionary process when it comes to reproduction.
"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," said Martie Haselton, a UCLA social psychology professor who was part of the study.
Some of the topmost disappointments men experience are lack of persuasiveness with a potential partner, and not being sexually adventurous, especially when they were single.
Women on the contrary regret having lost their virginity to a wrong partner or not being loyal to their partner. They also regretted having sex with a physically unattractive partner.
"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," researchers said.