Are men more promiscuous than women? The results of the latest National Survey of Sexual Attitudes and Lifestyles (Natsal) appear to suggest so. However, in a comment in The Lancet - one of the most prestigious medical journals in the world - Tom Underwood has highlighted that this is mathematically impossible.
Natsal, one of the largest and most comprehensive surveys on sexual behaviour in the world, aims to provide
much-needed up-to-date and accurate information regarding sexual practices in the UK. There have been 3 Natsal surveys: in 1990-1991, 1999-2001, and 2010-2012. The data from these surveys is widely used, and have played an important role in shaping sexual and reproductive health policy in the UK.
One of the results of the survey, which was picked up by BBC News and The Telegraph, is that the average number of (opposite-sex) sexual partners which a person has had throughout their lifetime differs significantly between men and women - it is much higher for men. Specifically, the values are 14.1 for men and 7.1 for women for the age range 16-74 years.
"In Natsal, the average for men is twice that for women. Half the time, for men, it seemingly takes one to tango." Tom Underwood, Institute for Condensed Matter and Complex Systems
In his comment, Tom pointed out that the mean lifetime number of opposite-sex sexual partners is necessarily identical for men and women, given the following assumptions: firstly, that the population under consideration consists of equal numbers of men and women; and secondly, that for each member of the population, all of their sexual partners also reside within the population. With this in mind, he then suggested that the large difference in the averages for men and women obtained from Natsal casts doubt on the extent to which the survey results reflect the true sexual behaviour of the general population.
There are a number of explanations for the discrepancy in the averages. One is that either men are over-reporting or women are under-reporting their number of lifetime sexual partners. Another possibility is the breakdown of one or both of Tom's assumptions - which at first glance appear reasonable with regards to the general population. In this regard, the authors' reply to Tom's comment is interesting. In the reply, the authors of the Natsal study suggest further possible reasons for the discrepancy, including:
1) Under-representation of female sex workers in the survey.
2) Men having sex with foreign women while either the men are abroad, or the foreign women are visiting the UK, more than the opposite situation (ie women having sex with foreign men while either the women are abroad or the foreign men are visiting the UK). The idea is that foreigners are not picked up by the survey, violating the assumption of a closed population.
3) More complicated effects involving generational changes in promiscuity, the fact that men generally take younger sexual partners, and the fact that women generally live longer than men.
The source of the discrepancy is not clear, and requires further investigation. For the population 'of all human beings who have ever lived' Tom's assumptions are valid, and the averages are therefore identical. This reflects the truth that 'it takes two to tango': each opposite-sex partnership involves one man and one woman, and hence promiscuity (which, at the risk of misusing the word, we have here equated to the number of lifetime sexual partners) is necessarily the same for both sexes. The crux of Tom's observation is that this truth is not borne out in the results of Natsal.
Tom is based in the Computational Materials Physics research group, in the School's Institute for Condensed Matter and Complex Systems.