Watch an hour of prime time TV, and you will probably witness three to five violent acts. Children's programming has even more violence, says Jeffrey Johnson, at Columbia University in New York. "Sports, news, commercials - it's everywhere," he says.

Johnson followed up over 700 families in New York state between 1975 and 2000. He found the link between aggression and TV watching was strongest for males during adolescence and for females, during early adulthood.

The associations held true even after accounting for known risk factors for aggressive behaviour. These factors included childhood neglect, growing up in a dangerous neighbourhood, low family income, low parental education and psychiatric problems. However, the type of the TV programmes watched was not recorded.
 

Moral education

The study confirms for adults what is accepted by many psychologists about children: viewing a lot of violence increases the likelihood that the person will behave that way. 
Craig Anderson at Iowa State University in Ames says that people do not seem to be getting that message: "People don't seem to understand that because they don't notice the way they've changed or the way they treat people, it doesn't mean there is no effect."

But Chris Boyatzis, a psychologist at Bucknell University, Philadelphia, says the link between TV viewing and violence may not be direct: "What may be going on is that families high in TV viewing are also lower in moral and character education."

It is important that parents "filter" what their children watch, he says: "Some studies have shown that about 75 per cent of kids' TV viewing is done without the company of parents, which is tragic."
 

Robbery and threats

Each family in Johnson's study had a child between the age of one and 10 when the study began. In 2000, when the volunteers' average age was 30, they filled out a questionnaire about their aggression, and the researchers double-checked it with FBI and state records.

Johnson found that 45 per cent of the men who had watched three hours or more at age 14 went on to commit an aggressive act against another person, compared to just nine per cent of the men who had spent less than an hour in front of the tube. Over 20 per cent of the three-hour-a day group went on to commit robbery, threaten to injure someone or use a weapon to commit a crime. 

For women aged 30, the strongest TV predictor of violence was watching three hours of more at age 22. Of these women, 17 per cent had committed an aggressive act, compared to none in the group watching less than an hour a day. 

Television viewing seemed to have no bearing on subsequent property crimes, such as arson, vandalism and theft.

Journal reference: Science (vol 295, p 2468)