In October 1993, twelve-year-old Polly Klaas was kidnapped from her home in Petaluma, California. Two months later, Richard Allen Davis, a felon on parole who had served prison time for burglary and two kidnappings, confessed to Polly’s kidnapping and subsequent murder. Outraged that Davis had been freed from prison despite his record, Polly’s family worked to establish a “three strikes, you’re out” law to keep criminals in prison and to deter those already convicted of one crime from committing another. The law requires that criminals convicted of their third violent felony receive a sentence of twenty-five years to life in prison. It also mandates that judges double the sentences for defendants who have one prior violent felony conviction. Five years after the passage of the three-strikes measure, however, opponents and supporters are still debating the...