Black and Hispanic police officers stop, arrest and use force on people significantly less often than their white counterparts, according to a first-of-its kind study using data from the Chicago Police Department. 

The findings, released Thursday in Science, provide a quantitative backbone to the arguments of police reform advocates who say that diversity within police departments will reduce discrimination and violence against racial minority groups. 

The results were particularly striking for Black officers, who made 15.16 fewer stops, 1.93 fewer arrests and used force 0.10 fewer times over 100 shifts in comparison to their white counterparts in similar working conditions. 

Hispanic officers made 2.84 fewer stops, 0.44 fewer arrests and used force 0.04 fewer times per 100 shifts in comparison to white officers — a smaller but still significant pattern, according to the researchers.

Both Black and Hispanic officers were less likely than their white counterparts to interact with Black people while on the job, the researchers found. 

“Across the board, all the differences we find are driven by less engagement with Black civilians,” said co-author Roman Rivera of Columbia University at an American Association for the Advancement of Science briefing. 

Rivera wrote the paper, titled “The role of officer race and gender in police-civilian interactions in Chicago,” alongside Bocar A. Ba of the University of California, Irvine; Dean Knox of the University of Pennsylvania; and Jonathan Mummolo of Princeton University. 

The researchers also found significant differences in police behavior by gender: Female police officers of all races made 0.61 fewer arrests and used force 0.09 fewer times per 100 shifts than male officers.

Rivera, Ba, Knox and Mummolo used data from 2.9 million officer shifts and 1.6 million enforcement actions between 2012 to 2015. The study covered nearly 7,000 Chicago Police Department officers and included detailed information about daily patrol assignments. 

Examining granular shift-by-shift data is crucial when contrasting the actions of police officers, the researchers said, though that data can be quite hard to obtain. 

“Until now, it has been extremely difficult to make this sort of apples-to-apples comparison,” said Ba. 

While some academics work with law enforcement in order to gain access to data, the researchers said they did not obtain their data through cooperation with the Chicago police and have not informed the department of their results. 

“We took a different approach, which was to do this analysis completely independently, to seek public data using the open records laws available,” said Mummulo. “The trade-off here is in the time that’s necessary to conduct that sort of study.” 

The researchers hope their paper will lead police departments and local governments to be more transparent. 

“With access to the necessary data we can answer longstanding questions,” said Mummulo. “For too long, policing agencies have closely guarded the data.”

He added that he hopes researchers in other regions will seek to replicate the study to see whether the results are specific to Chicago, where both non-violent protests and riots occurred in the summer of 2020 following the police killing of George Floyd in Minneapolis. 

“There are about 18,000 police agencies [in the U.S.]," said Mummulo. “These dynamics could very well work differently in different places.” 

Mummulo said that going into the project, he saw “common sense” behind advocacy for diversity in police departments. However, he said that he was unsure whether the researchers were going to find a significant difference in the behavior of Black, Hispanic and white officers working for the same police department. 

“I study bureaucratic politics more broadly. There’s a lot of evidence in that literature that an institution would cause homogeneous behavior,” he said. 

The researchers are now in the process of examining the effectiveness of police bodycams, which have been widely adopted by departments across the U.S. following the 2014 police killing of Michael Brown in Ferguson, Missouri. 

Initial research has generally shown that bodycams have little effect on officer behavior, potentially due to the fact that they create hours upon hours of footage that is difficult to analyze. 

“It just creates a massive amount of work for supervisors to go through those videos,” said Knox. 

Mummulo said they plan to come up with a strategy to “computationally analyze” bodycam footage, giving both police supervisors and outside researchers more data on the “subtleties of police interactions.” 

The paper, titled “The role of officer race and gender in police-civilian interactions in Chicago,” is forthcoming in Science. The co-authors are Bocar A. Ba of the University of California, Irvine, Dean Knox of the University of Pennsylvania, Jonathan Mummolo of Princeton University and Roman Rivera of Columbia University.