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Sex by Numbers:

What Statistics Can Tell Us About Sexual Behaviour

David Spiegelhalter

(London Times)

SEX statistics undoubtedly offer a voyeuristic thrill. When you read that 80% of British 25- to 34-year-olds had oral sex last year, or that the average heterosexual British couple had sex three times in the past four weeks (down from five times 20 years ago) or that 50% of British men and 40% of British women aged 25 to 34 have tried heterosexual anal sex at least once, it is hard not to imagine all these people actually doing these things.

You might find that alluring or alarming. What makes this book so rewarding is that it shows how sexual behaviour has changed over time. David Spiegelhalter is Cambridge University’s Winton professor for the public understanding of risk (a statistician) and he also manages to make the figures side of sex stats seem as interesting as the sexual, which is quite a feat.

He draws chiefly on the UK's National Survey of Sexual Attitudes and Lifestyles (Natsal), which has taken the pulse of British sexuality every 10 years since 1990. Natsal is big (the 2010 survey interviewed 15,162 people) and methodologically about as good as it gets. Still, its researchers are asking about sex, which makes it hard to secure a representative sample or ensure respondents tell it straight. Women tend to underestimate the number of sexual partners they have had, for example - unless the interviewer says they are being monitored by a lie detector.

Spiegelhalter refers to an array of other studies, going right back to the first sex survey of 1902, which inquired into masturbation among YMCA members. Cautiously, he gives them star ratings according to how reliable he thinks the numbers are. Alfred Kinsey's famous reports of 1948 and 1953, for instance, get just two stars - meaning they are 'rough ballpark figures'. (He does repeat one dodgy Kinsey statistic as if it were a fact: Kinsey reckoned that the average erect penis was six inches long; modern surveys tend to put it at something a little over five.) Shere Hite's feminist surveys of the 1970s and 1980s get just one star, for 'unreliable', which will shock some.

Below that come zero-starred 'made up' statistics, such as the popular notion that men think about sex every seven seconds. Astonishingly, American researchers looked into this in 1992, concluding that men actually think about sex 19 times a day, and women 10 times.

Beyond myth-busting and statistical re-education, Spiegelhalter is most interesting on social change. The British are getting more tolerant and sexually adventurous, as you would expect. The average age of first sex for women now stands at 16, three years younger than it was for women born around 1930. 'Only a few years' reduction,' Spiegelhalter observes, 'but a very important few years.'The rate of teenage pregnancy, however, is not so different in 2012 to 1945. In both years, around 2% of British women aged 15–19 gave birth. As an aside, Spiegelhalter notes that seaside towns deserve their 'notorious reputation': 15- to 17-year-olds made up 5% of pregnant women in Blackpool and Hastings, compared to less than 1.5% in Windsor and the well-named Maidenhead. What has changed, especially over the past 30 years, 'is that now almost none of these births is to a married mother'. Spiegelhalter is not moralising, however - there is none of that here.

His book focuses generally on heterosexual couples, but the most extravagant British changes relate to homosexuality. More than half of British people now agree that male same-sex partnerships are 'not wrong at all', up from 25% in 1990. The figure is only just over 50%, though, which will surprise some. Still, it compares strikingly with the 95% of Ghanaians and Egyptians who believe homosexuality is 'morally unacceptable'.

One of the deepest behavioural swings, however, relates to British women. Almost one in five female 20-year-olds now reports having had a same-sex experience, which compares with one in 40 women aged 70. Even taking into account the discretion of older respondents, that signals a hugely significant shift. It is a far steeper rate of change than for male same-sex experiences, too; those have remained relatively constant across recent generations.

Otherwise, the difference between male and female experience shown here is far smaller than you might imagine. British women interviewed for Natsal complained more than men about finding it hard to reach orgasm, and not enjoying sex, but the sexes were only a few percentage points apart. A more significant difference concerns desire: 34% of British women said they had felt a lack of interest in sex for at least three months over the last year - roughly twice the percentage of men.

British women masturbate much less than men, too. Two thirds of men had pleased themselves in the last four weeks, Natsal found, but only one third of women. As ever, Spiegelhalter's special skill is getting behind the headline numbers. Here he reveals that British men tend to have sex with themselves when there is no one else to do it with. For women, by contrast, masturbation is more of a supplement than a compensation.

Spiegelhalter’s analysis falls short on anal sex, however. His headline figure, again from Natsal, suggests something significant is happening. In 2010, 18% of British men reported having had anal sex with a female partner in the last year. In 2000, the figure was 12%. In 1990, 6%. What changed so dramatically? Everyone can imagine a probable answer - pornography - but Spiegelhalter refuses to approve it, asserting that the causes of trends in sexual behaviour cannot be explained by statistical analysis.

He does unpick another anal-sex statistic cleverly, though, noting that while almost half of young British people have tried it, roughly half of them have not done it again within the last year. Spiegelhalter interprets this as a behaviour 'tried for the experience' not the reward. He also observes that anal sex is less commonly practised among more affluent people than less affluent, while for oral sex it is the opposite way around.

Frustratingly, he still refuses to speculate on the causes. The same goes for that decline in how often British heterosexuals are having sex - down from five to three times in the past four weeks. Are more of us living on our own? Are Britons 'just too busy with their fancy new tablet computers', as one member of the Natsal team wondered? Spiegelhalter refuses to say. This is no doubt statistically proper, but it does rather illustrate that numbers, while fascinating, can only take you so far.

This is a clever, hugely readable book, sometimes titillating and occasionally astounding. Yet most interesting sex still happens in our heads, it seems, where even the best statisticians cannot go.

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