The Search for Causes in an Era of Crime Declines: Some Lessons from the Study of New York City Homicide

Abstract

This article addresses the problem of testing the effects of particular policies on crime rates in an era of general down trends. One illustration of that problem is our recent finding that rates of non-gun homicide had been declining substantially in New York City for 8 years prior to any significant change in policing and could not plausibly be caused by these later events. The article contrasts three different “controls” for time trend effects, naive cross-sectional controls, detailed models of crime causation, and qualitative checks that examine whether the details of crime patterns are changing in ways consistent with theories of policy events as change agents. The qualitative approach is embraced as a necessity. A final section questions whether criminal justice policies should be assumed to affect general crime rates in broad and undifferentiated ways.