But none of these explanations have paid much attention to the communities where violence plummeted the most. New research suggests that people there were working hard, with little credit, to address the problem themselves.

Local nonprofit groups that responded to the violence by cleaning streets, building playgrounds, mentoring children and employing young men had a real effect on the crime rate. That’s what Patrick Sharkey, a sociologist at New York University, argues in a new study and a forthcoming book. Mr. Sharkey doesn’t contend that community groups alone drove the national decline in crime, but rather that their impact is a major missing piece.

“This was a part that has been completely overlooked and ignored in national debates over the crime drop,” he said. “But I think it’s fundamental to what happened.”