The effect broke along racial lines: The majority of the decline in calls took place in black neighborhoods. “It shows what a deep rift events like this cause in the social fabric, in predominately black communities,” Mr. Desmond said.

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An abandoned home, a magnet for drugs and prostitution, became a target of crime-fighting efforts in Milwaukee. Credit Darren Hauck for The New York Times

Such events didn’t need to be local to have an impact. The researchers also looked at how the volume of calls to 911 in Milwaukee changed after news accounts of police violence in other, distant cities. In one of the other two cases they studied, they found a significant impact on crime reporting.

The change in calls is unusual because the relationship between crime and calls to the police is typically strong. “If crime is going up in Milwaukee, calls should also be going up,” Mr. Papachristos said.

The advantage of using 911 data is that it’s somewhat of a hybrid between survey and administrative data. With surveys, the best a researcher can do is ask what a person might do in a given situation; it’s not clear whether what people say in a survey will match with what they’ll actually do in real life. The record of 911 calls, by contrast, is data created by residents in their moment of need.

“This is derived from what people are doing — it won’t be as biased as crime reports,” Mr. Papachristos said. “This is the first time that we’ve seen a result in citizen activity.”

It’s not as if people are silent when a crime takes place. Quite the opposite: News spreads fast from house to house.

“Residents are very willing to tell you about what’s happening in their neighborhood,” said Adrian Spencer, who at one point lived in a predominately black neighborhood in central Milwaukee across the street from a tavern that had become a magnet for fights, drag races and shootings. “But it’s much more difficult to get them to talk directly to the police. Or come to a hearing.”

To Ms. Spencer’s surprise, she and her mother seemed to be the only ones calling 911 to report crime connected with the tavern. When she asked other people in the neighborhood, some of whom had lived there longer than she had, the usual response was: What were the police going to do?

She suspects this reluctance to call is rooted in skepticism that law enforcement can make much of a difference or be fair to the residents. “There’s a huge fear of retaliation,” she said. “It’s not happening on the level that they’re thinking that it is. And then you look on TV and you see what’s happening on TV with the police, you definitely don’t want to come in contact with the police if you don’t have to.”

The new study focuses only on data that is 12 years old and primarily focused on Milwaukee. But Mr. Desmond says that the effect may also be true elsewhere: “I think it has implications for what we’re seeing in Cleveland, in Charlotte, in Baltimore, with very publicized cases of police violence. Milwaukee is similar to places like Baltimore and Cleveland in its level of segregation. I think that probably has a lot to do with the story. ”

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