Why the difference?
First, respondents who reported very high numbers of sexual partners skewed the averages, and this effect was stronger among men than among women, the researchers found.
Men and women at the top end reported 110 and 50 or more partners, respectively. Excluding these men and women reduced some of the gap between genders, the study authors explained.
Secondly, the researchers discovered that men were more likely than women to estimate, rather than actually count, their lifetime sexual partners. For example, among survey respondents who reported having five to nine partners, 24% of men estimated, compared with 15% of women.
Attitudes surrounding casual sex
Third, attitudes about casual sex also played a role. Women were less likely than men to view one-night stands as "not wrong at all" (9% versus 18%, respectively). In addition, women were more likely than men to consider a married person having sex with someone other than his or her spouse as "always wrong" (65% versus 57%, respectively).
The three factors identified in the study explain about two-thirds of the differences in the number of sexual partners reported by men and women, according to the researchers.
"Accurate reporting of sexual partners is crucial for many reasons, including assessing individual risk of sexually transmitted infections (STIs) and estimating the rate of STI/HIV transmission," said study author Kirstin Mitchell, from the University of Glasgow.
"To our knowledge, our study is the first attempt to look at all the key types of explanation for the gender discrepancy within the same large and representative sample," Mitchell said in a journal news release.