Bocar Ba spent a good part of his youth “scrambling” from police, he says. The University of California, Irvine, economist grew up in the notorious banlieues of Paris, and as a young Black man, he often encountered law enforcement in ways that most scholars do not. Years later, as a doctoral student of public policy at the University of Chicago, he continued to think about police behavior and its impact on civilians.
In 2015, while attending a seminar, he was struck by the assumptions fellow academics were making, based on a few ride-alongs in a police car. And so, like many other academics in the wake of the shootings of Michael Brown and other Black civilians by police, he decided to probe an important social question: Does a police officer’s race matter when it comes to the use of force, or even the decision to stop someone in the first place? Although many activists, academics, and even police departments have answered yes—and hired more minority officers to improve community relations—a link has never been proved, and the field has been riddled with contradictory studies.
Now, Ba and his colleagues have added some certainty to that discussion. After combing through millions of police records from 2012 to 2015 and analyzing them for the nature of the action, the time of day, the race of the civilian and the officer, and many other factors, they found that Black, Hispanic, and female officers in Chicago made fewer stops and arrests than their white male counterparts, especially for petty crimes.
The study, the most fine-grained of its kind to date, “is really valuable,” says Jeffrey Fagan, a professor of law at Columbia University who was not involved in the work. “A lot of other studies out there have tried to use officer race as a benchmark test for discrimination … [but this one] got much more precise estimates of what the police workplace actually looks like.”
Getting the initial data for those estimates was a stroke of luck. In 2014, an investigative group of journalists and lawyers in Chicago called the Invisible Institute had won a court ruling forcing the police department there to release thousands of complaint records from several previous years. Ba approached the organization and suggested that together they build a fuller view of police-community relations by looking for additional data.
Ba studied police union contracts, which helped him work out which city departments might host the records. “I said, ‘Give me a couple of lawyers and I’ll tell them exactly where to get the data we need,’” he recalls. Soon a pattern developed: Ba would direct the institute toward data, and it would sue for additional records—a process that continued for several years. “Bocar wanted to understand the nuance, so he asked us all sorts of challenging questions,” says Chaclyn Hunt, a civil rights attorney with the institute.
In 2019, Ba started to collaborate with Dean Knox at the University of Pennsylvania’s Wharton School and Jonathan Mummolo at Princeton University—quantitative social scientists who use massive amounts of data to analyze social policies and behaviors. Together with Ba’s collaborator, Roman Rivera, they set out to scrutinize the behavior of Chicago police in the most fine-grained way possible.
To make the most valid comparisons, the researchers first had to “prune” their collection. For example, two officers could have the same badge number if one retired and passed it on, or the same names if they were father and son, so the team also had to use birthdates. To properly compare arrest and stop records, they had to geolocate each incident. They also threw out the records of higher ranking officers and explosives technicians, who tended not to interact with civilians. The result, Mummolo says, “was a real Frankenstein data set.” But it allowed the team to refine its list to nearly 7000 officers, who, over 3 million shifts, had engaged in 1.6 million stops, arrests, and uses of force over a period of 3 years.
When they analyzed their data, they found Black officers in Chicago made far fewer stops and arrests, by 29% and 21%, respectively, than their average white counterparts. They used force 32% less frequently. Overall, Black officers stopped 17% fewer white civilians than their white counterparts, and 39% fewer Black civilians. Most of the differences involved discretionary stops for “suspicious” activity or minor violations; the researchers saw little difference in stops and arrests for violent crimes, they report today in Science. They saw a similar pattern with Hispanic officers. Across all races, female officers made fewer stops for minor violations than male officers.
The results make a strong case that diversifying a police force can reduce conflicts between officers and the community without making any trade-offs in public safety, other researchers say. “These are not small numbers,” says Phillip Atiba Goff, professor of African American studies and psychology at Yale University and co-founder of the Center for Policing Equity, whose commentary on the study appears today in Science. “They imply that you can save lives by increasing the number of Black and women officers.”
“The whole question whether diversity makes a difference or not in policing is a huge policy issue,” says Christopher Winship, professor of sociology at Harvard University. “A paper like this can have a huge impact on the local level going forward.”