Explaining the dramatic rise of incarceration in the United States has been surprisingly difficult. Theories abound, but they are continually defeated by the vastness and complexity of the American criminal justice system. For a time, the prime suspect was the War on Drugs, which President Obama described as “the real reason our prison population is so high.” Numerically, this never made sense, given that drug offenders are a small fraction of state prisoners.1 Mandatory minimums and three-strikes laws were tangible reforms that attracted a great deal of attention. But as causal explanations they, too, wither under scrutiny. “There’s not a lot of evidence that the amount of time spent in prison has changed that much,” as law professor John Pfaff recently observed.2 Even landmark pieces of federal legislation—think of the 1994 Violent Crime Control and Law Enforcement Act, which dogged Hillary Clinton during her 2016 campaign—probably had minor statistical impact.