A man holding a gun.
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Murder is contagious and may spread like the flu, new research suggests.
The researchers relied on the same techniques public-health officials use to track the spread of diseases, but applied them to the spread of homicide in Newark, N.J., over a 26-year span from 1982 to 2008.
And just as in other epidemics, certain neighborhoods were more susceptible than others. Diverse, immigrant-rich communities looked to be protected against homicide's spread in the research, while the poorest neighborhoods were more vulnerable. These findings suggest communities could inoculate themselves against murder waves by addressing the underlying risk factors, said study co-author April Zeoli, a criminal justice researcher at Michigan State University.
"People speak about violence, crime and homicide as being contagious. The idea is that violence begets violence," Zeoli told LiveScience. [The History of Human Aggression]
As for what makes violence beget violence, Zeoli speculated that being in an area with a murder epidemic makes people fearful and more likely to resort to violence as a defensive measure.
"They may get their own firearms and be willing to use lethal force just so they can stay safe."
What causes crime is one of the most studied, yet most elusive questions. Past studies have tied murder rates to dozens of factors, from gun ownership and incarceration rates to a belief in hell. But until recently, scientists didn't apply the principles of infectious disease to understand exactly how crime spreads.
In their study, Zeoli and her colleagues found Newark experienced epidemic levels of violence during the 26 years: Homicide rates ranged from between 3.5 and five times higher than the national average on average.
Zeoli's team found that murders mimicked patterns of infectious disease. At the beginning of their study, homicides spiked in central Newark, but over the study period, central Newark killings waned while the murder epidemic moved into the southern and western wards of the city. Like other infectious diseases, murder rates spiked in certain areas and spread to nearby regions.
Interestingly, the city's homicide epidemic didn't spread to the northern and eastern sections of the city — regions that are more diverse than the city as a whole, Zeoli told LiveScience.
"It's potentially that diversity that helps keep homicide out of those areas," she said.
The northeastern part of the city's high fraction of Latino immigrants may also reduce violence there, perhaps because they tend to be more cohesive, she said.
In theory, that may mean people feel less need to resort to crime, Zeoli said.
Poor areas were found to be most susceptible to a murder wave.
Because murder moves like an infectious disease through a community, similar tools could potentially help stop its spread, Zeoli said.
For instance, if poverty increases susceptibility to murder because residents have no job or educational prospects, then providing those opportunities could help ward off a murder wave, she said. Over the long run, that could make murder less infectious in poor communities, she added.
"For homicide, if you start trying to revitalize a community, are you going to stop a homicide that would have been committed tomorrow? Probably not," she said. "But you are maybe going to prevent homicides that would have been committed a year from now."
The study is detailed in a forthcoming issue of the journal Justice Quarterly.