L. Rowell Huesmann and Brad Bushman, both of the University of Michigan, reviewed more than half a century of research on the impact of exposure to media violence and found that U.S. children spend an average of three to four hours a day watching television and more than 60 percent of television programs contain some violence and about 40 percent contain heavy violence.

"Children are also spending an increasingly large amount of time playing video games, most of which contain violence -- video game units are now present in 83 percent of homes with children," Huesmann said in a statement.

"Exposure to violent electronic media has a larger effect than all but one other well-known threat to public health. The only effect slightly larger than the effect of media violence on aggression is that of cigarette smoking on lung cancer."

The findings are published in a special issue of the Journal of Adolescent Health.