'Guns add a unique dimension to violence, because it's difficult to defend oneself against a gun. Young people, perhaps rightly, perceive that guns are ubiquitous in our society and there's this constant risk of being shot. That has enormous psychological costs.'

"This study makes us more confident that there is a substantial cause-and-effect relationship between being exposed to violence and perpetrating violence," said study author Jeffrey Bingenheimer, a doctoral candidate at the University of Michigan in Ann Arbor. "Violence can be socially transmitted from person to person in a community through exposure."

The study, published in the May 27 issue of Science, is believed to be the first one to show a cause-and-effect relationship between exposure to violence and committing violence.

Comprehensive Assessment

To reach that conclusion, Bingenheimer and his colleagues analyzed data from the large, longitudinal Chicago Neighborhoods study, which includes about 6,000 children from 78 different Chicago neighborhoods.

The researchers began with a group of about 1,500 children who were either 12 or 15 at the start of the study. The researchers took a comprehensive assessment of family history, home and community environment, health, social support, peer influences, school proficiency, previous exposure to violence and more. Both the adolescents and their primary caregiver -- usually their mother -- provided information for the initial assessment.

A second assessment, which took place about two years later, included 1,239 youngsters from the original group. Study volunteers were asked about their exposure to gun violence: 942 said they hadn't been exposed to gun violence; 283 said they had.

There were many differences between the exposed and unexposed groups, according to the study. Teens exposed to gun violence were more likely to be male, from single-parent households, nonwhite and to be on public assistance.

According to the researchers, those teens were also more likely to use drugs or alcohol, to be truant from school, to commit property crimes, to have family members with criminal problems and to have witnessed domestic violence in their homes.

The third and final assessment took place almost three years after the second, and the researchers were able to get 984 of the original study volunteers to participate.

They found that most -- 856 -- hadn't become perpetrators of violent crime. But, 122 (12 percent) had become perpetrators. That meant they had carried a hidden weapon, attacked someone with a weapon, shot at someone, or been in a gang fight.

Special Statistical Technique

Before controlling for background characteristics, the researchers found a fourfold increase in the chance that someone exposed to violence would become violent themselves.

But, the researchers wanted to control for background factors, such as home and community environment, to see if there truly was a cause-and-effect relationship between exposure to violence and committing violent acts.

Using a special statistical technique called propensity stratification, the researchers used the information on more than 150 characteristics gathered in the first assessment to determine the probability of gun violence exposure, which allowed them to compare those with a high probability of exposure to those who were actually exposed.

After adjusting for these factors, the researchers found that those actually exposed to gun violence were two times more likely to become perpetrators of violence.

Need for Mental Health Services

While this study wasn't designed to look at the reasons why someone exposed to violence might later become violent in turn, Bingenheimer said that those who have been exposed to violence may be more likely to assume that someone is going to be hostile, which "may lead to more preemptive aggressive behaviors."

"When individuals have such traumatic experiences -- witnessing or being exposed to gun violence -- it makes you hypervigilant," added Daniel W. Webster, co-director of the Center for Gun Policy and Research at the Johns Hopkins Bloomberg School of Public Health in Baltimore.

"There are often social stimuli that are ambiguous -- like at a crowded party, someone bumps into you. How do you interpret those stimuli? Most people think, 'It's crowded, it was an accident.' But, youth [who have been exposed to violence] may immediately assume a hostile intent and think someone is trying to challenge them, which can lead to violence, including gun violence," Webster said.

"Guns add a unique dimension to violence, because it's difficult to defend oneself against a gun," Webster added. "Young people, perhaps rightly, perceive that guns are ubiquitous in our society and there's this constant risk of being shot. That has enormous psychological costs."

The study, he said, points to the need for mental health services for those who witness gun violence, as well as additional social support services to help counteract chaotic family lives and problems in school.