MONDAY, Feb. 2 (HealthDay News) -- Teens who spend long hours watching television are at higher risk for depression as adults, a new study finds.

Participants faced significantly greater odds of being depressed seven years later, and the risk rose with each hour of daily television viewed, according to a report involving more than 4,000 teenagers.

The same association was found for exposure to other electronic media, the researchers noted.

"We cannot be sure it is cause-and-effect," stressed study author Dr. Brian A. Primack, an assistant professor of medicine and pediatrics at the University of Pittsburgh School of Medicine. "The reason that the study suggests it might be cause-and-effect is that the television viewing came first. It did not include people who had symptoms of depression when the study began."

His team published its findings in the February issue of the Archives of General Psychiatry.

The more than 4,100 adolescents in the study were first asked in 1995 about the number of hours they had spent the previous week watching television or videocassettes, playing computer games, or listening to the radio. They reported an average daily exposure of about 5.7 hours, including 2.3 hours of television viewing.

Seven years later, at an average age of almost 22, 308 (7.4 percent) of the young people had developed symptoms consistent with depression. The incidence of those symptoms was directly related with the number of hours of exposure to television and other electronic media reported at the start of the study, the researchers noted.

However, "while we were able to control for a lot of variables such as socioeconomic status and education, in the final analysis we cannot be sure it was cause-and-effect," Primack said.

It's possible to speculate about why watching television might lead to depression, he said. "One theory is that you see a lot of depressing events on television and are likely to internalize them," Primack said. "Television emphasizes bad news, and repeated exposure to it might be internalized."

TV commercials might also have an influence, he said. "You see about 20,000 television advertisements a year, and a large proportion of them dwell on the fact that life is not perfect," Primack noted.

TV viewing might also replace social, intellectual or athletic activities that protect against depression, and late-night watching might disrupt normal sleep, which is essential for emotional and intellectual development, he said.

It would be desirable to know whether specific kinds of television exposure might be linked to later development of depression, Primack said. "Probably there are certain kinds of exposure that are depression-genic," he said. "If we learned more about whether certain exposures increase the risk, we might be able to do things about them."

The study was called "a very good first step toward demonstrating this relationship between television and depression," said David S. Bickham, a research scientist at the Center on Media and Child Health of Children's Hospital, Boston.

"It demonstrates a high level of sophistication," Bickham said. "While it doesn't prove causality, it is evidence in that direction. More studies are needed to explore the issue further and see what is going on."

Another report in the same issue of the journal -- on a different matter of concern for parents -- found that marijuana use by European and North American teenagers appears to be decreasing.

Data on more than 93,000 15-year-olds analyzed by Swiss researchers found a decrease in most countries between 2002 and 2006.

The team found that pot use is higher among teenagers who frequently go out with friends, and the number of evenings that teens typically spent with friends declined during the four years of the study.

More information

Tips for parents on TV and teens are offered by the Rhode Island Department of Health.

SOURCES: Brian A. Primack, M.D., assistant professor, medicine and pediatrics, University of Pittsburgh School of Medicine; David S. Bickham, Ph.D, research scientist, Center on Media and Child Health, Children's Hospital, Boston; February 2009 Archives of General Psychiatry