Excerpt taken from "Children and television violence" webpage.
Children and television violence
Violence on television affects children negatively, according to psychological research.
Children often behave differently after they've been watching violent programs on television. In one study done at Pennsylvania State University, about 100 preschool children were observed both before and after watching television; some watched cartoons that had many aggressive and violent acts; others watched shows that didn't have any kind of violence. The researchers noticed real differences between the kids who watched the violent shows and those who watched nonviolent ones.
Children who watched the violent shows were more likely to strike out at playmates, argue, disobey authority and were less willing to wait for things than those children who watched nonviolent programs.