In 2004, Steven Alm, a state trial judge in Hawaii, was frustrated with the cases on his docket. Nearly half of the people appearing before him were convicted offenders with drug problems who had been sentenced to probation rather than prison and then repeatedly violated the terms of that probation by missing appointments or testing positive for drugs. Whether out of neglect or leniency, probation officers would tend to overlook a probationer’s first 5 or 10 violations, giving the offender the impression that he could ignore the rules. But eventually, the officers would get fed up and recommend that Alm revoke probation and send the offender to jail to serve out his sentence. That struck Alm as too harsh, but the alternative—winking at probation violations—struck him as too soft. “I thought, This is crazy, this is a crazy way to change people’s behavior,” he told me recently.