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Homicides increased dramatically in Chicago in 2016. In 2015, 480 Chicago residents were killed. The next year, 754 were killed–274 more homicide victims, tragically producing an extraordinary 58% increase in a single year. This article attempts to unravel what happened.

This article provides empirical evidence that the reduction in stop and frisks by the Chicago Police Department beginning around December 2015 was responsible for the homicide spike that started immediately thereafter. The sharp decline in the number of stop and frisks is a strong candidate for the causal factor, particularly since the timing of the homicide spike so perfectly coincides with the spike. Regression analysis of the homicide spike and related shooting crimes identifies the stop and frisk variable as the likely cause. The results are highly statistically significant and robust over a large number of alternative specifications. And a qualitative review for possible “omitted variables” in the regression equations fails to identify any other plausible candidates that fit the data as well as the decline in stop and frisks.

Our regression equations permit quantification of the costs of the decline in stop and frisks. Because of fewer stop and frisks in 2016, it appears that (conservatively calculating) approximately 239 additional victims were killed and 1129 additional shootings occurred in that year alone. And these tremendous costs are not evenly distributed, but rather are concentrated among Chicago’s African-American and Hispanic communities.

The most likely explanation for the fall in stop and frisks that appears to have triggered the homicide spike is a consent decree entered into by the American Civil Liberties Union (ACLU) with the Chicago Police Department (CPD). Accordingly, modifications to that consent decree may be appropriate.

More broadly, these findings shed important light on the on-going national debate about stop and frisk policies. The fact that America’s “Second City” suffered so badly from a decline in stop and frisks suggests that the arguably contrary experience in New York City may be an anomaly. The costs of crime — and particularly gun crimes — are too significant to avoid considering every possible measure for reducing the toll. The evidence gathered here suggests that stop and frisk policies may be truly lifesaving measures that have to be considered as part of any effective law enforcement response to gun violence.