An enjoyable clear popular science book with two main themes.

The first is what do we actually know about sex? Drawing on surveys and research, Speigelhalter presents statistics on everything sex-related including how often people have sex, how old people are when they start having sex, how much sex results in conception, the rates of STDs, and the type of sex people have.

The second is how do we know what we know? With clear and well written discussions, Speigelhalter discusses how anonymised random samples might give different results to face to face interviews with people you know on campus. There are some lovely stories of the heroes who pioneered collecting statistics on sex (and some depressing ones too).

The book is a bit staid in places, Speigelhalter keeps interpreting any concurrent multiple partners as 'cheating' with only the briefest of nods to ethical non-monogamy, is surprisingly kink-shaming, and feels like he is consciously trying to be non-judgemental about same-sex partners, but it isn't coming naturally. But it is still a good summary of basic human sexual behaviour told in an entertaining way. ( )