In the nineteenth century, British researchers began studying the variation in crime rates between and within cities. Some of these studies offered relatively simple accounts of the variance, in which concentrated poverty led to higher crime. Others went further, asking what explained the disparities in crime rates among poor neighborhoods. Most of this work “offered theories,” the University of Pennsylvania criminologist John MacDonald wrote in a recent paper, “but did not attempt to provide guidance on how to curb crime.” He compared this tradition, unfavorably, with the work of British health scholars, most notably John Snow, whose research on cholera “noted the importance of the spatial environment,” and who “suggested the separation of sewers and drinking water wells to prevent water-borne diseases.”

Of course, social scientists have long influenced crime policies. Consider the “broken windows” theory, which the Harvard political scientist James Q. Wilson and the Rutgers criminologist George Kelling introduced, in a piece in The Atlantic, in 1982. According to Wilson and Kelling, criminals perceive broken windows and other forms of disorder as signs of weak social control; in turn, they assume that crimes committed there are unlikely to be checked. “Though it is not inevitable,” Wilson and Kelling argue, “it is more likely that here, rather than in places where people are confident they can regulate public behavior by informal controls, drugs will change hands, prostitutes will solicit, and cars will be stripped.”

“Broken Windows” is one of the most cited articles in the history of criminology; it’s sometimes called the Bible of policing. Since the nineteen-eighties, cities throughout the world have used Wilson and Kelling’s ideas as motivation for “zero tolerance” policing, wherein officers monitor petty crimes, such as graffiti, loitering, public intoxication, and even panhandling, and courts severely punish those convicted of committing them. “If you take care of the little things, then you can prevent a lot of the big things,” the former Los Angeles and New York City police chief William J. Bratton has said. (Bratton has also applied the theory in overseas consulting work.) In practice, this meant stopping, frisking, and arresting more people, particularly those who live in high-crime areas. It also meant a spike in reports that police were unfairly targeting minorities, particularly black men.

The broken-windows theory always worked better as an idea than as a description of the real world. The problems with the theory, which include the fact that perceptions of disorder generally have more to do with the racial composition of a neighborhood than with the number of broken windows or amount of graffiti in the area, are numerous and well documented. But more interesting than the theory’s flaws is the way that it was framed and interpreted. Consider the authors’ famous evocation of how disorder begins:

A piece of property is abandoned, weeds grow up, a window is smashed. Adults stop scolding rowdy children; the children, emboldened, become more rowdy. Families move out, unattached adults move in. Teenagers gather in front of the corner store. The merchant asks them to move; they refuse. Fights occur. Litter accumulates. People start drinking in front of the grocery; in time, an inebriate slumps to the sidewalk and is allowed to sleep it off. Pedestrians are approached by panhandlers.

Things get worse from there. But what’s curious is how the first two steps of this cycle—“A piece of property is abandoned, weeds grow up”—have disappeared in the public imagination. The third step—“a window is smashed”—inspired the article’s catchy title and took center stage. Debates about the theory ignored the two problems at the root of its story, jumping straight to the criminal behavior. We got “broken windows,” not “abandoned property,” and a very different policy response ensued.

But what if the authors—and the policymakers who heeded them—had taken another tack? What if vacant property had received the attention that, for thirty years, was instead showered on petty criminals?

A few years ago, John MacDonald, the Penn criminologist, and Charles Branas, the chair of epidemiology at Columbia University, began one of the most exciting research experiments in social science. Branas is a leading scholar of gun violence, having become interested in the subject while working as a paramedic. He met MacDonald in the aughts, when they were both working at the University of Pennsylvania, in a seminar on gun violence at the medical school’s trauma center. Both were frustrated by the science that linked crime to neighborhood disorder. “A lot of it, from ‘broken windows’ on, was just descriptive,” Branas told me. “You didn’t know exactly what counted as disorder. And it wasn’t actionable. Outside of policing, which was obviously complicated, there wasn’t much you could do about it.”

The two began meeting on campus. While they were brainstorming, Branas was invited to discuss his research at a conference in Philadelphia. During his presentation, he briefly mentioned his interest in running an experiment on the physical factors related to gun violence. “When I finished, someone from the Pennsylvania Horticultural Society approached me,” Branas recalled. That person was convinced that vacant properties—Philadelphia had tens of thousands of empty lots—were driving up violent crime in poor neighborhoods. The Pennsylvania Horticultural Society, or P.H.S., had incredible data, and offered to help.

Branas and MacDonald were excited about the idea. There was, after all, an established literature on the relationship between abandoned properties and crime. In 1993, the criminologist William Spelman published a paper showing that, in Austin, “crime rates on blocks with open abandoned buildings were twice as high as rates on matched blocks without open buildings.” In 2005, the sociologist Lance Hannon showed that, in New York City’s high-poverty areas, the number of abandoned houses in a given census tract correlated with homicide levels. But Branas and MacDonald wanted to draw from an even deeper study, which required collecting an enormous amount of data and designing an experiment. They invited more scientists to join them: a health economist, a professor from Penn’s Department of Emergency Medicine, and a medical anthropologist.

One of the team’s first research projects involved two natural experiments in Philadelphia. In one, they examined violent crime around 2,356 abandoned buildings that had been in violation of Philadelphia’s anti-blight ordinance. A set of six hundred and seventy-six buildings had been remediated by the owners, which meant they had been “treated” with replacement doors and windows; the rest had not. Every month, for a three-year period between 2010 and 2013, the researchers compared violent-crime levels around the treated buildings with violent-crime levels around a randomly selected, geographically matched group of buildings that remained in disrepair.