Sex, as you will have gathered, looms quite large in this book, at least at the start; Levitt and Dubner know the importance of softening up their readers with a bit of smut before hitting them with the heavier stuff. Their research into Chicago prostitution reveal that prostitutes’ wages have plummeted in real terms in the last 60 years. Why? Simple really. Sex is much easier to come by than it was then. Between 1933 and 1942, more than 20 per cent of American men had their first sexual experience with a prostitute. Now it’s around five per cent.