UNTIL now, most public discussion of crime prevention has emphasized the highly visible and dramatic role of the police. Yet, in all probability, the police have less impact on the extent of crime than do the decisions of judges to sentence convicted criminals to prison or to let them remain free on probation. The judges’ decisions are crucial because such a large proportion of those arrested for crimes-from roughly 35 to 55 per cent, depending on the way one measures it-are “recidivists,” i.e., repeaters. And whether a person recidivates seems to be significantly affected by the type of sentence and treatment he gets upon conviction for his initial crime.