After decades of stability from the 1920s to the early 1970s, the rate of imprisonment in the United States more than quadrupled during the last four decades. The U.S. penal population of 2.2 million adults is by far the largest in the world. Just under one-quarter of the world's prisoners are held in American prisons. The U.S. rate of incarceration, with nearly 1 out of every 100 adults in prison or jail, is 5 to 10 times higher than the rates in...