What Caused the 2016 Chicago Homicide Spike?

An Empirical Examination of the “ACLU Effect” and the Role of Stop and Frisks in Preventing Gun Violence

Homicides increased dramatically in Chicago in 2016. In 2015, 480 Chicago residents were killed. The next year, 754 were killed—274 additional 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 street stops is a strong candidate for the causal factor, particularly since the timing of the homicide spike so directly coincides with the decline. Regression analysis of the homicide spike and related shooting crimes identifies the street stop 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 candidate that fits the data as well as the decline in stops.

Our regression equations permit quantification of the costs of the decline in street stops. Because of fewer stops in 2016, it appears that (conservatively calculating) approximately 245 additional victims were killed and 1,108 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 stops that appears to have triggered the homicide spike is a consent decree on the subject 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 ongoing national debate about stop and frisk policies. The fact that America’s “Second City” suffered so badly from a decline in street stops 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 police street stop activities may be truly lifesaving measures that must be considered as part of any effective law enforcement response to gun violence.

The full text of this Article is available to download as a PDF.