America has one-quarter of the world’s prisoners. More than 7 million people are under correctional supervision in this country. These staggering statistics—no other country comes close in percentage terms, let alone raw numbers—have serious consequences. For one thing, there is the fiscal cost: The corrections system lags only Medicaid in government spending growth on the state level. Yet most prisons are overcrowded, underserviced, and exponentially more dangerous than any decent society should tolerate.