Television has been around for only 50 years. But the medium has profoundly changed the way people interact with the world. From being a novelty, TV has become an integral part of the lives of billions of people, and many adolescents are among the greatest consumers of what it has to offer.  Now some research indicates that more exposure to TV may predispose those teenagers to depression.

Depression isn't just feeling sad - it's a debilitating condition that prevents people from studying, working and earning to their full potential.  The World Health Organization says depression is the leading cause of morbidity in the world.

"Adolescence is a time of a lot of turmoil. It's also a time during which a lot of people develop behaviors that they will have for the rest of their lives, says University of Pittsburgh Professor Brian Primack, who studies adolescent behavior.

"It's for this reason that a lot of advertising and promotion is particularly aimed at teenagers," he says. "Those marketers know that this is where they're going to get their future market."  

Primack examined data from about 4,000 adolescents. The teens had answered a lot of questions about their habits, including how much TV they watched.  Then, researchers tracked the teens' development for seven years.

"We found that the amount of television that they reported at time one [the beginning of the study] was associated with development of depression over the subsequent seven years," Primack says.   

"But?the amount of video games, video cassette and radio exposure that they reported at time one was not similarly associated with development of depression," he adds.

Primack says he and his colleagues aren't sure why television - but not video games - was a problem for these young people.  

Boys were particularly affected by lots of television watching. The teens all watched an average of two hours of television a day.  But for each additional hour spent in front of the TV, the risk of depression increased by 8 percent.  Primack hypothesizes that hours spent watching TV take away from time spent doing other, more positive, things.

"If you are spending a lot of time with anything, many hours a day, then it displaces your ability to engage in other protective activities," he says.  "You have less time to... have positive social relationships, to engage in sports, music and other activities that might improve self-esteem."

Another hypothesis is that teens might be exposed to too many negative images in the course of watching hours and hours of television each day.

Primack says no one really knows what it is about TV that may contribute to depression.  But he says that it's probably not a bad idea to turn off the tube and do something else.

His research is published in Archives of General Psychology.