The argument for stop-and-frisks is that it cuts crime. But even in places where that has happened — New York being the most well-known example — it has also sown resentment and distrust in black communities, researchers say. That, they say, is why so many cases of public unrest in the past two years, starting with the August 2014 shooting death of Michael Brown in Ferguson, Missouri, have been triggered by encounters that began with a police officer's stopping someone in their car or on foot.
That antagonism led President Obama to create a Task Force on 21st Century Policing that recommended in an expansive report that all police agencies keep track of who it stops and searches, and why. Only then, reformers say, will America be able to better understand where the practice is working and where it's not.
Reformers say that when officers are more thoughtful in their encounters with people on the street — treating them with dignity and respect and empathy — they are more likely to be helpful, have a more positive perception of police, and be willing to give cops the benefit of the doubt when trouble arises.
"Bad encounters come back to haunt you," said Dennis Rosenbaum, a professor of law and criminology at the University of Illinois at Chicago and the outgoing director of the National Police Research Platform. "Good encounters are like putting money in the bank, building up trust."
The timing of Trump's comments underscored that tension: as he spoke, protesters clashed with police in Charlotte over an officer's killing of a man, and two other cities, Tulsa and Columbus, remained on edge over similar incidents.
Jeffrey Fagan, a criminologist at Columbia Law School whose analysis of New York stop-and-frisk data led to a federal judge's ruling that the NYPD's tactics were unconstitutional, said the impact dates much further back — to the late 1960s, when rioting in several American cities began with police stops that went wrong.
"What's changed?" Fagan said.
His research has found that stops are most effective when they target "objective" behavior that an officer deems suspicious, like witnessing a drug deal, or evidence of violence. That cuts out stops of innocent people, and reduces crime, Fagan said.
The folly of Trump's suggestion, he said, is that calling for more aggressive stops around the country forces officers to come up with more subjective reasons to stop someone. "You can't invent suspicion," Fagan said.