WASHINGTON (Reuters) - Teen-agers who watch more than an hour of television a day are much more likely to become violent than the rare adolescent who watches less, researchers reported on Thursday.

One of the most definitive studies yet to link watching television with violent behavior finds both men and women are affected by violent programs on television -- but teen-aged boys are especially at risk.

"We saw the jump was between less than one hour and more than one hour a day. There was a four-fold increase," Jeffrey Johnson of Columbia University in New York, who led the study, said in a telephone interview.

His advice: "Parents should try not to let children watch more than one hour a day on the average."

Johnson, a psychiatric epidemiologist who studies patterns of behavior, said 60 percent of TV programming contained violence.

An average hour of television portrays three to five violent acts, the American Psychological Association says.

Johnson's team tracked 707 children, most of them white and Catholic, who took part in a study in upstate New York.

The children, aged between 1 and 10 when the 17-year study started, were interviewed several times. The researchers also checked state and federal arrest records.

WATCHING FOR THE WRONG REASONS

The link between watching television and behaving violently was clear even after the researchers accounted for other factors such as childhood neglect, low family income, or a psychiatric disorder during adolescence.

Researchers said that in some families these factors did in fact lead to more television watching.

"Childhood neglect, growing up in an unsafe neighborhood, low family income, low parental education, and psychiatric disorders were significantly associated with time spent watching television at mean age 14 and with aggressive behavior reported at mean age 16 or 22," they wrote.

The study, published in Friday's issue of the journal Science, found that 5.7 percent of the adolescents who watched less than one hour of television committed aggressive acts against other people in later years, as compared to 22.5 percent of those who watched between one and three hours a day.

And 28.8 percent of those who watched three or more hours of television daily committed aggressive acts. Broken down by sex, this equaled 45 percent of males and 12.7 percent of females.

Violent acts by males included assault and fighting that led to injuries, while violent behavior by young women included robbery and threats to injure someone.

Johnson said several mechanisms are at work. "One of the most important one is the tendency to imitate behavior that people see on TV," he said.

"We are social beings and we tend to want to try out things that we see other people doing, especially if we see the person rewarded for what they did or portrayed as a hero for it."

Johnson said many studies had shown that people simply become inured to violence when they see a lot of it -- either in real life or on television.

"It has been shown that viewing media violence leads to a desensitization effect," he said. "The more violence that they see, the less negative, the more normal, it seems to them."

Perhaps people who watch lots of television lose their social skills, Johnson said, or never develop them.

"So when they get into a conflict with somebody else, whether it is road rage, whatever the situation might be ... they may not be able to work their way out of it gracefully. They may resort to something like verbal aggression and they may even start throwing verbal punches because they don't know what else to do," he said.