Over the past thirty years the US prison population has exploded in a way unparalleled in American history or world experience. Since the mid-1970s it has quintupled in size, from just over 300 thousand inmates to more than 1.5 million. One out of every one hundred adults-and nearly one out of every twenty black males-is behind bars. The United States is home to under 5 percent of the world's population, but it warehouses approximately one out of every three of its prisoners. 1 Such a sprawling penal system is expensive. By 2004, states were spending a total of nearly $40 billion per year to maintain their populations. Correctional spending generally makes up only about 2 percent of overall state budgets, but 10 to 20 percent of state discretionary spending. Even before the current recession, state legislators were beginning to look for ways to trim back prison populations and expenditures. 2 As state budgets have …