For years, researchers have tried to pinpoint the factors behind why the U.S. homicide rate had fallen so swiftly for so long that it was cut in half between the early 1990s and 2015. They recently found a missing piece of the puzzle: Everyday citizens who decided to improve their communities, without much fanfare, are among the nation's most effective crime fighters.

As higher-profile solutions — more cops, stop-and-frisk, an increase in the prison population — were being hotly debated, neighborhood nonprofits were quietly going about their work. While many of those programs were not specifically designed to combat crime, they were doing so anyway. That's the intriguing new finding from Patrick Sharkey and a team of researchers at New York University. For every 10 community programs created in a city of 100,000 residents, researchers found a 6 percent drop in violent crime, 4 percent drop in property crime — and a 9 percent drop in the murder rate.

One of the primary criticisms of community-level groups and everyday residents who have spent the past few years protesting police brutality was that they weren't doing enough in their own communities to combat crime. Supposedly, they spent all their time being angered by police killing people but not caring much about residents killed by other residents in the community. That criticism was never based on reality, given the numerous prayer groups, candlelight vigils and groups of men and women who canvass their own neighborhoods to prevent or report crime — even at personal risk.

None of those everyday residents and volunteers and founders of unsung small nonprofits had to don a Batman suit to fight crime; all they had to do was establish mentorship and after-school reading programs and erect playgrounds and provide young men returning from prison second chances and kids a place to appreciate the arts or participate in sports. Even unorganized, spontaneous efforts to improve communities suffering from high crime rates proved valuable.

While more research must be done to more finely pinpoint which of the programs are most effective and efficient, we now know that efforts at the local level are making a difference, often in ways that have gone unappreciated. That progress is being accomplished without alienating longtime residents of those communities or creating more distrust between police and those they serve the way aggressive policing, such as stop-and-frisk, does. (Since a judge outlawed stop-and-frisk in New York, the crime rate there has continued to improve.)

A doubling-down on such efforts — with more investment from philanthropists and guidance from experts in the business and educational fields — could even help police departments by eliminating the unrealistic expectation that a badge and gun are the best tools to combat society's most pressing issues.