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Compared to other cities, a larger share of homicide suspects in Chicago consists of adolescents, although the majority of all homicide suspects are in their 20s or older. The increase in gun violence occurred disproportionately in several disadvantaged neighborhoods on the city’s South and West sides, which now account for an even larger share of the city’s homicides. Another change is that from 2015 to 2016, the share of homicides that CPD believes stemmed from an altercation, as well as the share of homicide offenders who were recorded by CPD as having a gang affiliation, seemed to decline. What caused Chicago’s sudden surge in gun violence in 2016 remains a puzzle. Weather cannot explain the surge in homicides and shootings, since monthly temperatures in 2016 were close to their historical averages. City spending on social services and public education did not change much in 2016 compared to previous years, and while the state budget impasse disrupted funding for many community organizations, this did not seem to change sharply in December 2015. 4 Most relevant measures of police activity did not change abruptly enough to explain the surge in gun violence. Overall arrests declined in 2016, driven by narcotics arrests, but arrests for violent crimes, including homicides and shootings, barely changed. One policing measure that declined was the chance of arrest for homicides and shootings (the “clearance rate”), which was a result of arrests for these crimes not keeping pace with the increase in gun violence. Another policing measure that declined was the number of investigatory street stops. However, for this to explain why shootings increased in Chicago would also require an explanation for why the previous dramatic decline in street stops in New York City did not lead to more gun violence there. We also cannot know the effect of factors not measurable in the available data, such as any change in street gangs or the use of social media. However, given the timing of the recent increase in gun violence, for any alternative explanation to make sense it would need to involve something that changed abruptly near the end of 2015 and disproportionately affected gun crimes. Not knowing the definitive cause of Chicago’s sudden and substantial increase in gun violence does not mean the city should be paralyzed in crafting a response. The solution to a problem need not be the opposite of its cause. One key implication of these data is the importance of a policy response that is focused on the core problem: violence concentrated largely in a moderate number of our most disadvantaged neighborhoods, carried out by teens and young adults in public places with illegally owned, and perhaps increasingly lethal, firearms.