Unbroken economic growth provided disadvantaged young people with attractive alternatives to crime. In addition, scholars credit public policies, such as those that led to higher rates of incarceration. The uncertainty that surrounds the New York story is typical of criminology. Based on available evidence, Mr. Eck and Mr. Maguire, associate professors of criminology at the University of Cincinnati and George Mason University, respectively, could draw firm conclusions about none of them. We may never know for sure how policing affects crime.
True experiments require control groups and random samples for reforms introduced one at a time.
That, in turn, rests on finding officials and citizens willing to tolerate differential treatment for the sake of discovery. In the early 1970's, as a young criminologist, Mr. Kelling persuaded the police department of Kansas City to let him test the effect of drive-through police patrols on neighborhood crime. The department increased patrols in five randomly chosen neighborhoods, reduced them in five others, and made no changes in a third group, as a control. Even criminologists' best efforts, however, cannot replicate the laboratory. Scholars have challenged the Kansas City study, on the grounds that the results from the control areas might have been tainted by patrol cars passing through en route to other neighborhoods.