The more television or other electronic media a teenager is exposed to, the more likely he or she is to develop symptoms of depression in young adulthood, suggests a new study.

Researchers from the University of Pittsburgh have found that the likelihood of experiencing symptoms of depression increase by eight per cent for every hour of television a young adult views as a teen.

And for every hour of total media exposure, the likelihood of developing symptoms of depression increases by five per cent.

The study, which was published in the Archives of General Psychiatry, included more than 4,100 subjects who were not depressed when the research began.

The research team asked the teenagers about how many hours per week they watched television or videos, played computer games or listened to the radio. The subjects averaged 5.68 hours of media exposure per day, which included 2.3 hours of television, 0.62 hours of videos, 0.41 hours of computer games and 2.34 hours of radio.

The study was conducted before the widespread use of DVDs and the Internet.

After seven years of follow-up, when the subjects were an average age of 21.8, more than seven per cent of them had developed symptoms consistent with depression.

"In conclusion, the present study breaks new ground in linking media use in adolescence to the development of depressive symptoms in young adulthood, especially relative to television exposure and overall media use," the authors concluded.

The researchers also found that among teens that had similar exposure rates, young women were less likely than young men to develop symptoms.

The findings are significant, the authors said, because depression is often diagnosed in adolescence or young adulthood and is thought to be influenced by a variety of factors, including temperament, genetic susceptibility, relationships, or having had a traumatic experience.

The researchers hypothesized that media exposure may also influence depression risk, since teens are so highly exposed to it.

Media exposure cuts into time that may otherwise be spent on social, intellectual or athletic pursuits, which may have a protective effect against depression, they speculated.

Excessive media exposure may also interrupt sleep, which is a key factor in emotional and cognitive development.

As well, messages teens receive from the media may encourage aggression or other negative behaviours, or inspire fear or anxiety, the authors wrote.

Psychiatrists, pediatricians and other health-care workers should assess media exposure levels in patients who exhibit symptoms of depression.

"If no other immediate intervention is indicated," they said, "encouraging patients to participate in activities that promote a sense of mastery and social connection may promote the development of protective factors against depression."