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Since 1988, every US state has established a database of criminal offenders' DNA profiles. These databases have received widespread attention in the media and popular culture, but this paper provides the first rigorous analysis of their impact on crime. DNA databases are distinctive for two reasons: (1) they work mainly by increasing the probability that a criminal is punished rather than the severity of the punishment, and (2) they exhibit enormous returns to scale. I exploit the details and timing of state DNA database expansions in two ways, first to address the effects of DNA profiling on individuals' subsequent criminal behavior and then to address the impacts on crime rates. Using a regression discontinuity analysis, I show that DNA profiling deters criminal activity for both violent and property offenders. Then, using an instrumental variable approach, I show that larger DNA databases reduce crime rates, especially in categories where forensic evidence is likely to be collected at the scene---e.g., murder, rape, assault, robbery, and vehicle theft. Back-of-the-envelope estimates of the marginal cost of preventing each crime suggest that DNA databases are much more cost-effective than traditional law enforcement tools.
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