IT is well known that young males commit a disproportionately large share of many serious crimes.  Some persons think the solution to this problem is for juvenile courts to “get tough”; others think that if the penalties given to adult offenders by the courts were made sufficiently swift, certain, or severe, juvenile offenders would be dissuaded from becoming adult criminals. Still others doubt that anything the courts can do will affect juvenile crime. Until recently, however, it was almost impossible to say anything about the relationship between what courts of any kind do and a given individual’s criminal career, in large part because the juvenile and adult courts are separate institutions with separate (or non-existent) records. In the real world, people grow older one year at a time, but as far as the criminal justice system is concerned, each person has two entirely separate livesone as a juvenile (from birth to age 17 or 18), another as an adult (over age 17 or 18). And until recently, almost all analyses of the likely effect of sentencing policies on crime were confined to adult criminals, as if juvenile offenders did not exist.