The United States is by a long chalk the most punitive democratic society. Two and a half million of its citizens are in jail – one in every 100 adults. The United Kingdom, no slouch in the punishment stakes and one of the most enthusiastic lockers-up in western Europe, has 84,000 prisoners. If the UK matched the US, its prison population would be 500,000. A hugely disproportionate number of American convicts are black, and incarceration is undermining fragile communities.