The United States has a strange obsession with putting people in prison.

The USA officially has the highest incarceration rate in the world, according to data from the International Center for Prison Studies. For every 100,000 people who lived in the US in 2011, 716 were in jail. Most European countries, like France, Germany and the UK, have between 50 and 150 prisoners per 100,000 residents.

Despite falling crime rates, the US prison population has grown from 307,276 in 1978 to a high of 1.6 million prisoners in state and federal prisons in 2009, according to a report from the Pew Research Center. Since then, in the last three years, the prison population has declined, but only slightly.

Joe, have you read The Spirit Level: Why Equality is Better for Everyone?It was written by Richard Wilkinson and Kate Pickett, two English academics with a background in "health inequalities", and first published in 2009. They offer evidence to show that many aspects of society - from life expectancy to mental illness, from violence to illiteracy - are determined not by how wealthy a society is, but how equal it is. It's a fascinating account, and the USA and the UK have a great deal in common.