The Shared City

Homicides Travel Through Social Networks Like HIV

The figure on the left shows the full map of co-offending residents in one Chicago neighborhood, while the dark nodes on the right represent homicide victims. Credit: The American Journal of Public Health

Decades of research on murder have revealed a few truths. Young people are more likely to be homicide victims than old people. African Americans more than white people. Men more than women. The poor more than the rich or middle class. Gang members more than non-gang members. Beyond these simple binaries, however, not much is known. Why, for example, do guns kill some young, black, poor men with gang affiliations, and not others?

Andrew Papachristos and Christopher Wildeman, associate professors of sociology at Yale University, are the co-authors of a new study that just came online in the American Journal of Public Health. The pair set out to examine whether what we know about social ties today can help pinpoint who will, down the road, become a victim of homicide. The hope is that forming a read of the landscape beyond simple demographics might help police and others intervene before the violence takes place.

Papachristos and Wildeman focus on one high-crime, largely African-American community in Chicago, covering a geography of about six square miles that, on race, economic status, and other commonly accepted risk factors, was fairly homogeneous. Using police data, they collected records on every arrest that occurred in the area between 2006 and 2011. From that, they built a network map that connected offenders involved in the same criminal incident. The map captured more than 24,000 people, or nearly one-third of the neighborhood’s 82,000 residents. This astronomically high arrest rate created a data-rich base pattern.

On top of that, the researchers placed 307 gun-related homicides that occurred in the area in the same five-year span.

The deaths were hugely concentrated in parts of the community’s social network. Some 41 percent of all homicides involving guns occurred in just 4 percent of the "network components," or segments tying together co-offenders. That is, being tied to victim of a homicide, even from steps removed — for example, through your running mate’s occasional running mate — makes you far more likely to be the victim of gun violence, too. "Associating with people engaged in risky behaviors — like carrying a firearm and engaging in criminal activities," Papachristos and Wildeman write, "increases the probability of victimization." Simply put, homicides are "socially contagious."

To put the findings in context, the study points to what we know about the spread of HIV. Engaging in needle sharing and unprotected sex aren’t just risky on their own. They also bring you into contact with "situations, behaviors, and people" that raise the likelihood of contracting the virus. Those connections are enormously powerful, Papachristos said in an interview, even if they’re not always known to us. "You might be able to name all your sexual partners," he said, "but while you probably can’t name all their sexual partners, it’s not like they don’t matter to you."

"It’s about finding the behaviors that expose you to pathogens," Papachristos went on. "Guns work the same way [as a virus] because they’re durable goods. You have to be around a gun to get shot or to shoot someone."

There’s much more work to be done on isolating the risk factors that lead to victimization, but Papachristos said that there’s already been some success in using this new insight. For law enforcement, it can represent a shift: "You go from policing everybody to interventions with individuals whom you know to be at risk."

Simply surfacing this knowledge can help, he said, and cities like Stockton, East Palo Alto and New Haven have begun experimenting with it. Papachristos, who works on the ground in addition to doing research, described the interventions that the network mappings make possible: "Sometimes we put these network diagrams up there and say, ‘Look, there have been these four shootings, and you’re in the middle of it." Likely victims are offered everything from bus passes to homework help to drug treatment.

"It gives them sort of an honorable exit," Papachristos said. "It gives them a chance to say, ‘Hell no. Cops showed up at my house with my grandma, and now I can’t go out tonight.’" That can be enough. "People can change their behavior without coming to Jesus," he said — not in the religious sense, but in the transformative one. "You can put down a gun without changing your whole life."

Papachristos is now focused on drilling down further into which networked ties are signal, and which are noise. Co-participating in a robbery, for example, seems like a particularly strong indicator: ("You’re literally approaching someone aggressively with a weapon.") But sharing ties to other criminal activity, like domestic violence and child abuse, don’t seem to point to an increased likelihood of dying at the end of a gun.

But one thing is already clear. "We think about these as tragic, random events," Papachristos said. "But while they’re all tragic, most of them aren’t random."

The Shared City is made possible with the support of The Knight Foundation.

Nancy Scola is a lead writer for the Washington Post’s tech blog, The Switch. Scola’s writing on the intersections of technology and politics has been published by The American Prospect, Capital, Columbia Journalism Review, New York, Reuters, Salon, Science Progress, Seed, and other publications.

Tags: policyshared citychicagopublic safetygun violencelaw enforcement