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The Imprisonment Puzzle: Understanding How Prison Growth Affects Crime

16 Pages Posted: 19 Aug 2011 Last revised: 6 Jun 2012

Joanna Shepherd

Emory University School of Law

Date Written: 2006


Many early empirical studies found that prison growth is associated with substantial reductions in crime. However, recent studies find no statistically significant relationship between growth in prison populations and crime. Even more striking, one recent study finds that, in certain circumstances, imprisoning more people can even increase crime.

I attempt here to briefly solve the puzzle, suggesting how the studies conflicting findings might be reconciled. The differences in the studies’ results may be due to the different characteristics of the periods that the studies address. The earlier studies examined periods when prison populations tended to grow by adding additional violent and property offenders. The studies’ empirical results supported what theory would seem unambiguously to suggest: that putting more offenders in prison should decrease crime by both incapacitating incarcerated offenders and deterring potential offenders. In contrast, during the periods that the later studies addressed, both the War on Drugs and new sentencing reforms caused prison populations to grow in two additional and different ways: by adding drug offenders and by increasing sentence lengths for low-level offenders. I discuss several reasons why imprisoning more drug offenders may not reduce crime and may even increase it. I also evaluate how longer sentences for low-level offenders may have similar, counterproductive effects. Finally, I explore how recent increases both in the numbers of drug offenders and in sentence lengths for low-level offenders might reconcile the studies’ results.

Keywords: economics of crime, sentencing reform, war on drugs

JEL Classification: K00, K14

Suggested Citation

Shepherd, Joanna, The Imprisonment Puzzle: Understanding How Prison Growth Affects Crime (2006). Criminology and Public Policy, Vol. 5, No. 2, pp. 285-298, 2006; Emory Public Law Research Paper No. 11-167; Emory Law and Economics Research Paper No. 11-115. Available at SSRN:

Joanna Shepherd (Contact Author)

Emory University School of Law ( email )

1301 Clifton Road
Atlanta, GA 30322
United States
404-727-8957 (Phone)

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