Yes. Yes. Yessssss. Finally: a book on statistics that the public will buy and want to read. Sex both sells and is funny. The statistics of sex confirm stereotypes we would like confirming, and dismiss others whose time has come to be dismissed.

Only in France do a majority of the population believe it is acceptable to have an affair. Among the affluent nations, people have sex earliest in Greenland (what else is there to do?) and next earliest in Denmark (Viking blood?). Italian men think they last, on average, 15 minutes between the sheets, but they actually last nine, which is the European mean ¡°intravaginal ejaculatory latency time¡± (or IELT, in sex stats speak).

Black men do not have larger than average penises. Women admit to having more frequent sex if they think they are wired up to a lie detector; men are less inhibited in such admissions. But if you want to know what an average amount of sex is, and the average length of a penis, and a great many interesting things about averages themselves, including how reliable they all are, you¡¯ll have to buy David Spiegelhalter¡¯s book. It is full of gems.

Britain is the ¡°birthplace of onanism¡± where the ¡°antimasturbation rhetoric reached its climax in the years preceding the First World War¡±. This is not a book lacking in double entendres. Anal sex is one of many behaviours that is often ¡°tried for the experience but does not necessarily become a habit. Like swimming at Blackpool.¡±

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Spiegelhalter has worked hard at this book. It would have been easy to have pulled out halfway through and done a quick and dirty job, but he is tenacious and goes at it for longer and harder than the average statistician. He is obscure about his own sexual predilections. The ¡°positionality¡± of the author (as a social scientist might put it) is rarely mentioned, save for his age, which highlights that he is more liberal in his views than his cohort.

He does point out that ¨C apart from humans ¨C bonobos and fruit bats appear to be the only animals to engage in oral sex, suggesting that it is somehow unnatural. However, just because something is rare in the animal world, that does not make it unnatural. Few other mammals, for example, have an opposable thumb, so useful for handling tools.

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This is also a book that does not shy away from the darker side of sex: coercion, rape, unwanted pregnancies and prejudice. It is a caring and careful book. In fact, I¡¯d be tempted to recommend it to teenagers wanting to ask those embarrassing questions. It would also help them with their GCSE maths, although the worked example showing that British men, combined, ejaculate 5 litres of semen a minute (varying by hour of the day) might not go down well with some teachers.

As to the book¡¯s shortcomings, I am not too sure of the author¡¯s conclusions about why the male-female sex ratio at birth has varied as it has. He says he has never before seen the UK ratio plotted over time, which is what he has done here. If that is so, he won¡¯t be aware of the many other theories that exist for such variation ¨C not least fluctuating economic inequality and stress. But if there weren¡¯t at least one questionable conclusion here, you might start to worry that he knows far too much about sex.

Danny Dorling is Halford Mackinder professor of geography, University of Oxford. He is author of , which has just been published in an updated second edition.

Sex by Numbers: What Statistics Can Tell Us About Sexual Behaviour
By David Spiegelhalter
Profile, 368pp, ?12.99 and ?7.99
ISBN 9781781253298 and 9781782830993 (e-book)
Published 2 April 2015

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