It's a seductive notion, that we could know who will and who won't commit a crime in the future. That has been a dream in criminal justice going back at least as far as the 19th century, when the Italian criminologist Cesare Lombroso claimed he could pick out delinquents from an early age based on physical defects and the shapes of their skulls. And while it may call to mind the science-fiction world of "Minority Report," making judgments about people’s potential to be dangerous is in fact an essential—and routine—part of how the American justice system works. It is what parole boards do, and what sentencing hearings are for.