The proportion of the population imprisoned in a society is influenced by four things: crime rates, conviction rates, the tendency to give prison sentences rather than fines or community service, and the lengths of prison sentences. Some societies use imprisonment far less, and when they do imprison people, they put more emphasis on training and rehabilitation so that re-offending rates are also lower.

In the UK, despite falling crime rates, imprisonment rates have been climbing steadily for decades leading to a crisis of prison overcrowding. In the USA only about 12% of the growth in prison populations is due to an increase in crime. In both countries the rise in prison populations over recent decades seems to be primarily the result of more punitive sentencing.

The USA imprisons people at 14 times the rate of Japan, and even within the USA there are big differences, Louisiana imprisons people at a rate six times higher than Minnesota. In a study published in the journal Social Science and Medicine, we have found strong links between imprisonment and income inequality - both internationally and among the 50 US states. We also found that the more equal US states, with lower homicide rates, are less likely to retain the death penalty.