“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.

The study, a project that included interviews of children and teenagers from Chicago neighborhoods, used statistical advances and detailed information about the study subjects to go beyond the factors typically considered by social scientists to determine violent behavior.

By comparing teens with similar likelihood of exposure, the researchers were able to isolate the independent contribution made by seeing gun violence. It turned out to swamp 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 interviewed about social, academic, and personal factors. Two years later, the subjects were interviewed to see which of them had witnessed gun violence. Finally, three years later, they were interviewed to determine who had participated in violent acts.

Researchers must decide whether violence is a product of families, or something like an environmental contaminant, lurking in some communities. Based on this study’s results, Earls feels the best model may be a socially contagious disease.