In a study designed to isolate the root causes of violent behavior, Harvard Medical School researchers found that young teens who witnessed gun violence were more than twice as likely as non-witnesses to commit violent crime themselves in the following years. The study will appear in the May 27 issue of Science.

"Based on this study's results, showing the importance of personal contact with violence, the best model for violence may be that of a socially infectious disease," says Felton Earls, MD, HMS professor of social medicine and principal investigator of the study and of the the Project on Human Development in Chicago Neighborhoods. "Preventing one violent crime may prevent a downstream cascade of 'infections'. And the lessons learned in Chicago should be broadly applicable. Generalizing this to any large city should be valid," Earls said.

The study, a five-year project that included interviews of over 1,500 children and teenagers from 78 Chicago neighborhoods, used statistical advances and extremely detailed information about the study subjects to go beyond the correlations and associations typically used by social scientists to determine violent behavior. "We have a broad range of factors, and a long course of study, so we can tease out the causal mechanisms," said first author Jeffrey Bingenheimer, currently a doctoral candidate at the University of Michigan who will be joining the Harvard School of Public Health in September as Robert Wood Johnson Health and Society Scholar.

Previous work has shown that a large network of factors pushes or pulls young people away from or into violent crime. Researchers suspected that exposure to violence in the community played a role, but many argued that a common factor, perhaps in family structure or personality, might be the common cause of both exposure to violence and later acts of violence. Demonstrating cause and effect with a controlled experiment, deliberately exposing some children to mayhem, would be ethically impossible. But by grouping together and comparing teens with similar likelihood of exposure, some of whom were and some of whom were not actually witnesses to violence, the researchers were able to isolate the independent contribution made by seeing gun violence. And it turned out to be large, swamping other single factors like poverty, drug use, or being raised by a single parent.

The researchers studied the subject teens at three points in their adolescence. Initially they and their caregivers were intensively interviewed and data was collected about their families, personalities, neighborhoods, school performance, and many other factors; this allowed the researchers to group the teens by their propensity to witness gun violence. Two years later, the subjects were interviewed to see which of them had actually seen someone being shot, or shot at. Finally, almost three years further on, they were interviewed again to determine who had participated in gang violence or other violent actions.

After finding that witnessing violence more than doubled the risk that teens would violently offend, the team looked at their statistics to check whether an unknown factor could be hiding from them. They found that something significant would have to be at work to change the findings substantially, and it would have to be uncorrelated with the factors they did examine. "And honestly, it's very difficult to think what we might have left out," Earls said, pointing to the 153 variables that were embraced in the study.

There is no shortage of medical ways to view urban violence, but the challenge for social medicine researchers is to choose the best one - is violence a product of families, akin to a hereditary disorder? Or is violence like an environmental contaminant, lurking in some communities and leaving others unscathed? Based this study's results, showing the importance of personal contact with violence, Earls feels the best model may be an socially contagious disease.

From Harvard Medical School