'Sex by Numbers', by David Spiegelhalter

A book about people's hidden sex lives, written by a professor of risk at Cambridge university, sounds like it might be fun. And Sex by Numbers, by David Spiegelhalter, does not disappoint. There is, I cannot lie, some eye-popping stuff between these (modestly attired) covers. Much of it, however, is not suit­able for a serious international newspaper.

More usefully, perhaps, Spiegelhalter's work sets out to use research findings to address the "Am I normal?" question and general insecurities that at some point plague everyone. (Don't they?) As he says in his introduction: "Am I having too much? Not enough? Are my experiences different? Or at least, are they really different?"

Spiegelhalter draws widely on the British National Survey of Sexual Attitudes and Lifestyles (Natsal), the widest survey of sexual behaviour since the Kinsey report (which had collected almost 16,000 sex histories by 1953). The Natsal project, and the people who fought to set it up in 1989, come out of this book as heroes: at a time when data on sexual behaviour were desperately needed amid the fear of HIV, prime minister Margaret Thatcher refused funding for the "intrusive" survey. The Wellcome Trust stepped in with £900,000 to rescue Natsal. It was followed by Natsal 2 in 2000 and Natsal 3 in 2010. Tens of thousands of Britons, it turns out, are willing to be interviewed about their sex lives by total strangers.

In his introduction, the author explains the attraction of sex for statisticians, who, "contrary to popular opinion, are also human beings". Sex, he says "occupies a strange boundary between public and private". We learn some surprising new facts from behind the nation's closed doors: not least that British people are, overall, having sex less often than they used to. The three Natsal surveys show a steady decline in the median frequency among those aged 16-44 with an opposite-sex partner in the previous four weeks. It's five times in 1990, four in 2000 and three in 2010. For both sexes.

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The range of topics is suggested by chapter headings that include "By your own hand" and "Sex and not having babies" ("in Britain now, 99.9 per cent of sex does not lead to a pregnancy").

Nor does the author flinch from statistics on rape and coercion. A sobering figure here is that an estimated "19 per cent of women and 3-4 per cent of men are subject to sexual assault in their lives".

And he examines the new pressures that teenagers face in a porn-saturated age. Spiegelhalter concludes, from the research conducted so far, that "young people themselves can see possible problems with an early exposure to sexualised media, but there is also remarkable resilience to influences that might appal older generations. Overall there does not seem to be a huge crisis. But don't expect that story in the media."

Helpfully, all research quoted in the book is rated by the author from 1* ("unreliable") to 4*. The Natsal findings get a 3* "reason­ably accurate" rating, while 4* sex research is rarely achieved because of the tendency to lie and exaggerate - and because sex surveys are often unrepresentative or use self-selected participants.

Modern survey participants are increasingly un-squeamish. Far more striking is the fact that early researchers managed to get people to open up about then-taboo subjects. It is these pioneers who really stick in the mind. Dr Clelia Duel Mosher, for example, was "an extraordinary and underrated contributor to the history of the sex survey". As a student in Wisconsin in 1892, she designed an ahead-of-its-time questionnaire covering married women's attitudes to sex and "how they felt about their intimate lives". She got 45 responses, but never published the data. Later, they were transcribed in a book. As Spiegelhalter notes, "I was the sixth person to take this out of the Cambridge university library in 32 years."

Throughout, Spiegelhalter's writing is erudite but witty, honest and personal. And the wonderful footnotes alone are worth the cover price. Under an analysis of the "pledge" rings worn by young American Christians who have vowed to abstain from sex before marriage, a footnote observes simply: "Miley Cyrus used to wear such a ring, but it has not been in evidence in her recent videos."

Sex by Numbers: What Statistics Can Tell Us About Sexual Behaviour, by David Spiegelhalter, Profile/ Wellcome Collection, RRP£12.99, 360 pages

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