Today I came across an article in the Telegraph that states that an average man has 9 sexual partners in his lifetime, whilst a woman has only 4.

Let's assume for a moment that in fact all these men and women are heterosexuals. In that case, each partnership contains one man and one woman. So, in total, the number of partnerships that women enter into is exactly the same as the number of partnerships that men enter into. Given a few other pretty well-known facts - that men are roughly as numerous as women, and live roughly as long, we deduce that on average men enter into roughly as many sexual partnerships as women.

There is, of course, a potential flaw in this - we know not all partnerships are in fact heterosexual. We know that heterosexual partnerships must have as many female participants as male, but we know no such thing about homosexual ones. Perhaps homosexual men have many, many more partnerships than heterosexual men, or women of any inclination? Whilst there is some evidence in favour of this concept, I don't really think it's going to be a big enough effect to skew the entire ratio this much. In order to have a ration of 9 to 4, most male partnerships would have to be homosexual ones.

A better theory is that the data is not very good. Perhaps men tend to exaggerate the number of sexual partners they have, even in anonymous surveys. Perhaps women tend to emphasise their degree of virginity. Perhaps the distributions are different - perhaps for example, most women have fewer partners than most men, but some women have lots? And perhaps this sort of information is lost in the survey?

Probably the only safe conclusion we can draw from these figures is that journalists and statistics don't mix.