John Pfaff, professor of law at Fordham, whose research on mass incarceration I have covered several times before in this blog, has published a new book – “Locked In: The True Causes of Mass Incarceration—and How to Achieve Real Reform” – which uses empirical data and statistical analysis to show that most of what we think we know about the subject is wrong. It is not the war on drugs, mandatory minimums, or the prison-industrial complex, but too much discretion resting with prosecutors. Most reform efforts focus on getting people out of prison by shortening or abolishing minimum sentences, hastening the work of parole boards, awarding “earned time” for good behavior, and the like. Pfaff’s most potent — and perhaps contentious — argument is that reforms should instead focus on bringing fewer felony charges against Americans in the first place. And that means zeroing in on prosecutors.Pfaff’s major data epiphany was that, during the 1990s and 2000s, as violent crime and arrests for violent crime both declined, the number of felony cases filed in state courts somehow went up. A lot. “In the end, the probability that a prosecutor would file felony charges against an arrestee basically doubled, and that change pushed prison populations up even as crime dropped,” he writes. Pfaff suggests several explanations for this. There were tens of thousands more prosecutors hired across the country in the 1990s and aughts even after the rising crime of the 1980s had stalled out, and the position of district attorney simultaneously became a more politically powerful one. Prosecutors’ discretion, always great, was expanded by courts and legislatures. And public defenders, stuck at the same or lower levels of funding, have not kept up with the growing caseload.Reformers have overlooked the role of prosecutors, Pfaff reasons, in part because there is no good data on how they use their discretion, and in part because they are simply less visible; about 95 percent of cases end in plea bargains worked out behind closed doors. “We see the police every day; no one is more high-profile in the criminal justice system,” he said in an interview. “Then we think of the judge imposing the sentence.” But prosecutors, and how they work, remain something of a mystery.Pfaff’s plea, then, is for advocates of reform to look for ways to curb the aggressiveness of prosecutors. He offers a tentative menu of options: establish guidelines for charging and plea bargaining, which New Jersey has already done; make prosecutors pay from their county budgets for the bed space they use in state prisons; and provide more funding for public defenders. And, last but not least, attack public complacency. In 46 states, prosecutors are elected — and 85 percent of them run without opposition.