"It's much easier to stay below the radar, respond to calls, engage in what we call reactive policing," said Rosenbaum, who has worked with Chicago police officials on research projects. "For an individual officer on the street, they need to know that somebody has their back and if they feel that they're going to get thrown under the bus when they do something, why would you proactively get out of your car or stop a group of kids who ... seem to be up to no good?"