Outrage over school shootings has been dominating headlines, not just because the victims are children but also because the attacks occur so randomly and in places—Parkland, Fla., Newtown, Conn.—where it once seemed such a thing could never happen.

It’s much harder to stir a national debate about the persistent problem of gun homicides in the country’s poorest urban neighborhoods, even though more children die in urban gun violence than in school shootings, according to University of Pennsylvania criminologist John Macdonald. Maybe urban gun violence is just too predictable to hold our attention: It is extraordinarily concentrated—“hypersegregated” in Macdonald’s phrase—with a handful of neighborhoods in the 10 largest cities accounting for 30 percent of all gun homicides nationwide.

Now, though, it appears predictability and geographic concentration could actually make urban gun violence easier to prevent. For Columbia University epidemiologist Charles Branas, one answer is a relatively simple and inexpensive infrastructure improvement of derelict or abandoned city lots. These comprise about 7.5 million acres of land and about 15 percent of the area of cities across the U.S.—and significantly higher percentages in midsize cities like Flint, Mich., or Camden, N.J.

Derelict lots often become settings for drug dealing and other criminal behaviors. Thus, they function as a primary threat to nearby residents’ health and safety, according to Branas, lead author of a study published Monday in Proceedings of the National Academy of Sciences. He and his co-authors, including Macdonald, liken efforts to clean up these lots to the 19th-century public investments in sewage treatment and clean water systems that helped curb epidemic diseases and made cities more livable. Instead of cholera, Branas says, the “contagion” this time is urban gun violence. He contends it spreads—and can be interrupted in its course—like any other epidemic.

Abandoned and vacant lot in Philadelphia with accumulated trash, debris, and overgrown vegetation, Summer 2016. Credit: Charles Branas

Branas and his colleagues looked at 541 vacant lots in randomized clusters across Philadelphia, which has one of the country’s highest murder rates. The study, supported by grants from the National Institutes of Health and the U.S. Centers for Disease Control and Prevention, assigned each cluster to one of three experimental options: a control group (meaning no treatment); a basic cleanup; or a “cleaning and greening” treatment, including a lawn, a few trees and a low perimeter fence—“to show that the lot was cared for and to deter illegal dumping.”

The study describes conditions in the worst neighborhoods that make the two treatment options seem like improbable remedies, at first. Some of the vacant lots involved were crisscrossed by footpaths to intravenous drug “shooting galleries.” And some were in areas where dealers paid drug bosses up to $5,000 a week in “rent” for “the right to sell on blocks where inhabited row homes were interspersed with vacant properties,” according to the study. Despite these odds, even the more expensive treatment—cleaning and greening—cost just $9,300 for a typical 1,000-square-foot lot, and about $50 a year for maintenance thereafter. Yet both treatments made a measurable difference.

In response to survey questions—which did not mention vacant lots—residents in low-income neighborhoods that received the cleaning-and-greening treatment reported a 15.8 decrease in their perception of crime incidence and a 61.9 percent increase in their willingness to relax and socialize outdoors. More impressively, police records for the 18-month period following the cleanup showed a 9.1 percent decrease in gun assaults in those neighborhoods, together with significant decreases in burglaries and nuisance complaints. When the researchers reanalyzed their data to weed out areas where maintenance of clean and green lots had lapsed for one reason or another, they found a 29.1 percent decrease in gun violence in neighborhoods where the lots had remained clean. It was such a significant improvement that the funding agencies for the study paid for the 150 lots in the control group to receive the cleaning-and-greening treatment. If extended to vacant lots citywide, the authors wrote, that treatment would translate into 350 fewer shootings a year in Philadelphia alone.

The link between greenery and crime prevention is of course not new. But anticrime initiatives for decades treated greenery mainly as a hazard, and advocated clearing dense vegetation to reduce hiding places as well as trimming trees to create clear lines of sight. Then a landmark 2001 study of Chicago public housing projects turned greenery into a tool for crime prevention, showing trees and green spaces seemed to reduce crime rates by bringing residents out more often, and putting more eyes on the street. But evidence greening actually causesa reduction in crime has proved elusive, making it harder to lobby for public action—until now.

The new study is the first to use a randomized experimental protocol to test the effects of greening on crime. “We knew that violence is generally lower in low-income urban neighborhoods when they’re greener, and we knew that reductions in violence tend to follow greening efforts,” says Ming Kuo, a University of Illinois at Urbana–Champaign specialist on how the physical environment affects human behavior, who was not involved in the new study. “But we didn’t know if the greening was actually responsible for the reduction in violence. Now we know. This is a great advance—one the field has been waiting for,” she notes. “I don’t know if there is much point in asking a scientist what arguments a politician will find compelling. But from a scientific point of view, this should persuade city governments to try. I think it’s fair to say we don’t know for sure that this will have the same effects in every community—but the evidence we have suggests it should.”

Branas says cleaning and greening vacant lots does not “affect legal gun owner rights,” meaning people who are otherwise bitterly divided over gun control could potentially agree on such efforts as a politically acceptable strategy to reduce gun violence. But Mark Kaplan, a public health researcher at the University of California, Los Angeles, Luskin School of Public Affairs, who was not involved in the study, cautions these programs “need to be done alongside other things. You have to address the question of social and economic inequality, and green space alone is not going to fix that.” A major reduction in urban gun assaults is unlikely, he adds, “without regulation of the guns that are contributing to the violence.”

Oddly, though, a strength of the new study could be its minimal approach to larger social and economic inequities: The risk in fixing too many things too fast is that it may attract developers, trigger gentrification and drive out long-time residents. Branas and his co-authors worried about that possibility enough to add a study within a study, aiming to counter the argument that any improvements they measured were the result of gentrification. Cleaning and greening vacant lots might be just enough of a change, Kaplan says, to help build a sense of ownership and neighborhood identity in the affected communities so they can come together and push for larger improvements like better schools and sanitation—and remain in the neighborhood to enjoy them.

For Branas, one of the most poignant moments in the study—and perhaps the beginning of that neighborhood identity—was the experience of having residents venture out from behind locked doors to greet the work crews as they arrived to clean up derelict lots. “And they said, ‘We called you about this 30 years ago! I can’t believe you’ve finally come to do this!’—thinking that we were the City of Philadelphia.”