Tuesday 28 April 2015

The statistics of sex

Sir David Spiegelhalter (left) and Norman Fenton
Last night I attended the launch of David Spiegelhalter's book "Sex by Numbers"** at the Wellcome Collection in London, which is currently also hosting an exhibition on Sexology.

What makes David's book a very good read is that it not only presents intriguing insights into a broad range of sexual activities, but it does so in a way that explains in lay terms the good, bad and ugly of the underlying statistical methods as well as some of the maths. This includes things like erroneous reporting of sexual habits that falls into the category of prosecutors' fallacy***. There are hundreds of different numbers about sex presented and most get a star rating (ranging from 1* to 4*) based on their reliability; so, for example, the surprisingly high number 48% (births that were formally 'illegitimate' in 2012 in England and Wales) is in the most reliable category (4*), while the number 84% (women emotionally unsatisfied with their relationship) is in the least reliable category (1*). The numbers for favourite sexual positions as presented in the following table are rated as 2*:

Man on top
Woman on top

To give a feel for the range of numbers the book provides those 80% of 25-34 year-olds who have engaged in oral sex in the last year will be interested to know that 3% is the proportion of recommended daily zinc intake contained in an average ejaculation.

**David was one of my co-presenters on the BBC documentary Climate Change by Numbers. The other co-presenter was Hannah Fry, whose book published in February is called "The Mathematics of Love". I deny the rumours circulating that my next book is to be called "The Risks of Marriage"....

***see here for background on the prosecutors' fallacy