The popularity of “tough-on-crime” policies is nothing new in American politics. Since the 1970s, the general trend in the United States has been toward longer prison sentences for most types of crimes. In 1970 there were just under 200,000 people being held in the U.S. prison system; in 1980 there were just over 315,000; in 1990 there were almost 740,000; and by 2000 there were over 1.3 million. To accommodate such a dramatic increase in convicts, since 1980, about one thousand new prisons and jails have been built in the United States.

One type of “tough-on-crime” policy that has contributed to the increase in prisoners is mandatory minimum sentences. Congress passed more than twenty mandatory sentencing laws between 1985 and 1991—most of them for drug offenses—and most states followed suit. For example, in 2000 in Massachusetts, conviction for possession of two hundred...