If I asked you to think of a murderer, what's the image that springs to mind?
If you're like most people, you'll probably think of an evil psychopath, or someone bent on revenge. Perhaps you'll see a criminal mastermind, who eliminates rivals on his way to riches. Or a strung-out drug addict, who kills because she needs money to get high.
All of these images have something in common: As a rule, we tend to associate murder with the behavior of individuals who behave in aberrational ways.
"We think of individuals who commit homicide as being unlike the rest of us," said April Zeoli, a public health researcher at Michigan State University's School of Criminal Justice. "They are crazy, or substance users, or had a bad childhood. There is some reason specific to the individual that they are committing homicide."
Zeoli recently decided to test that theory using the lens of public health research: When scientists study the outbreak of an infectious disease like AIDS or the flu, they don't ask what it is about specific individuals that made them sick. They look for broader patterns, knowing that illness in any individual stems from a process of contagion.
Along with colleagues Jesenia M. Pizarro, Sue C. Grady and Christopher Melde, Zeoli asked whether homicide might follow the same principles of contagion.
"We looked at homicide as an infectious disease," Zeoli said in an interview. "To spread, an infectious disease needs three things: a source of the infection; a mode of transmission; and we need a susceptible population."
The researchers studied every homicide that occurred in the city of Newark, N.J., over a period of a quarter century, from January 1982 to September 2007. In all, Newark had seen 2,366 murders in that period, a rate of homicide some three times as high as that of the general U.S. population.
The researchers tracked down the time and location of every single murder. They plugged the data into a software program that has previously been used to track infectious diseases: When you put in the geographical location and the time of infection of each victim of the infectious disease, the program creates a model that shows how the epidemic is spreading and where it might go next.
"We hypothesized that the distribution of this crime was not random, but that it moved in a process similar to an infectious disease, with firearms and gangs operating as the infectious agents," the researchers wrote in a paper they published in the journal Justice Quarterly.
The analysis showed that homicide spread through Newark very much like an infectious disease. The value of tracking murder in this fashion, Zeoli said, was not just to let police know where murder was happening police already track hot spots and direct resources to those areas but to make predictions about where homicide might spread next, based on the path of the epidemic.
Zeoli said that the model could make specific predictions about how and where homicide would spread in the future information that could prove very valuable to police and other city officials.
Studying homicide via a broad public health lens, Zeoli added, also allowed researchers to identify positive outliers: "We actually had some areas within Newark that were resistant to homicide, despite being surrounded by areas with high homicide rates. So we need to investigate why those little islands exist."
To use the language of infectious disease research, Zeoli said, once researchers figure out what makes some neighborhoods "resistant" to homicide, despite having the same risk factors as areas with high homicide rates, policymakers could apply those insights to "inoculate" other areas in order to prevent homicide from spreading.
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