It is no longer news that the United States incarcerates a higher proportion of its citizens than any other nation: more than 730 per 100,000, compared with less than 150 per 100,000 in Canada, Western Europe and China.

There are now more than 2.2 million inmates in American jails and prisons. Even this figure, however, undercounts the true size of the incarcerated population because it only includes those awaiting trial and those serving criminal sentences after conviction.