Witnessing even a single episode of gun violence doubles the chance that a young person will later engage in violent behavior, according to a new longitudinal study of Chicago adolescents. The results, published 27 May in Science, may once again stoke the debate over juvenile violence; it has already triggered controversy over the unusual statistical method it employs.
The work is part of the decade-old Project on Human Development in Chicago Neighborhoods, run by Harvard University psychiatrist Felton Earls. For 5 years, researchers followed more than a thousand adolescents, initially aged 12 to 15. They first gathered extensive data on factors related to risk of exposure to gun violence, including family structure, temperament, IQ, and previous experience with violence. Halfway through the study, the subjects were asked if, in the prior 12 months, they had been exposed to firearm violence. In the end, the 984 subjects remaining were asked if they were involved in a fight in which somebody got hurt or were part of a gun-related incident.
Since it is not feasible to test community violence with randomized tests, the scientists instead ranked the subjects according to "propensity" scores: a cumulative tally of 153 risk factors. They were then divided up according to whether or not they had reported exposure to gun violence and whether or not they had subsequently engaged in violent behavior. In order to control for all the risk factors, those with the same propensity scores but different exposures were compared. The results suggested that exposure to gun violence predicted a doubling of the risk for violent behavior--from 9% for unexposed to 18% among the subjects who reported exposure, says lead author Jeffrey Bingenheimer, a Ph.D. candidate at the University of Michigan School of Public Health, Ann Arbor.
According to Jeanne Brooks-Gunn, a developmental psychologist at Columbia University in New York City and one of the scientific directors of the Chicago project, the study shows that a single exposure might have a profound effect, even on a hitherto nonviolent individual. But others disagree. Psychiatrist Richard Tremblay of the University of Montreal says the study does not demonstrate that "those who are nonviolent to begin with will become violent." Economist Steven Durlauf of the University of Wisconsin, Madison, calls the study an "implausible modeling of violence exposure." Nobel Prize-winning economist James Heckman of the University of Chicago says the study fails to demonstrate cause and effect and calls it "potentially very misleading."