According to this article in the New York Times, on average, heterosexual men claim that they have had seven sexual partners, and heterosexual women claim that they have had four.
The maths don’t quite add up.
The possible explanations? According to the article:
One is that men are going outside the population to find partners, to prostitutes, for example, who are not part of the survey, or are having sex when they travel to other countries.
Another, of course, is that men exaggerate the number of partners they have and women underestimate.
Later on the article acknowledges that the prostitute effect, if it exists at all, is likely to be small. And the way I see it, saying that men may overestimate and women underestimate is no explanation at all. The data already seems to be telling us that. It would be much more interesting to know why.