only during the early years of the present decade (Blumstein and Wallman, 2006a; Zimring, 2006). In the past few years, attention has turned to increases in violence observed in some cities across the United States (e.g., Police Executive Research Forum, 2006).

What explains these recent shifts in crime rates? Are they the result primarily of modifications to the quantity and quality of policing and incarceration? Were shifts in abortion laws, demographics (e.g., age structure and immigration), or the economy (e.g., unemployment and wages) important? Have changes in illicit drug (e.g., crack cocaine) involvement or alcohol consumption played a role? Do these or other factors help explain the substantial degree of variability in crime trends observed across places? And, ultimately, which factor or set of factors has contributed most to shaping recent crime trends? These are the questions of primary interest in this volume and the ones that I have been asked to examine in this chapter. Addressing these questions is difficult but vitally important for shaping perceptions of public safety among citizens, informing public policy debates about how best to respond to crime, and identifying conditions that are most apt to produce or prevent major shifts in crime.

THE STATE OF KNOWLEDGE

Policy makers, the media, and other citizens have rightly pressed for answers to the puzzling changes in U.S. crime rates over the past three decades. However, the existing empirical research on recent crime trends is in the early stages of development and is not at the point of sufficient breadth or depth to provide definitive evidence on which factors mattered a lot, which mattered relatively little, and, importantly, which mattered the most. The research community appears to have been reluctant to admit this apparent fact, concluding instead that the available evidence supports either the conclusion that virtually all of the dozen or so factors implicated in the theoretical literature played some role in shaping recent crime trends or the conclusion that one or more specific factors were very important and others mattered little. In my view, neither of these conclusions is supported strongly by the available evidence, which consists primarily of inconclusive circumstantial patterns and empirical evidence based on limited data and models that, as elaborated below, simply do not permit strong conclusions one way or another. Despite bold claims about which factors mattered and which did not (e.g., Levitt, 2004), as Travis and Waul (2002, p. iii) summed up after a national forum on the subject, although a good deal has been learned from prior research, there are no “definitive answers to the questions raised by the recent crime [trends]—that would require more research, new data, and a sustained effort to reconcile every competing claim” (see also Blumstein and Wallman, 2006b; Rosenfeld, 2004).



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