You've heard the claim that what you watch on tv can shape your attitudes and behavior. Watching violent shows can increase aggression, angry conspiracy-theorists on talk shows can fuel paranoia, and the parade of thin beauties on sit-coms and soap operas can convince young girls they need to look a certain way. A recent study even found that watching medical dramas can turn people into hypochrondiacs. 
But a new study from Harvard Medical School suggests it's not so much what you watch that puts you at risk -- it's what your friends watch that shapes how you think, feel, and act.  The researchers carefully tracked the tv habits of 523 teen girls, as well as the social connections among the girls. This allowed them to compute both a measure of "direct exposure" to television (what the girls themselves watched) and "second-hand exposure" (what their friends watched).
The results were startling: there was a strong impact of second-hand tv exposure on body image concerns, pressure to be thin, and symptoms of disordered eating. For example, if a girl's friends watched a lot of television, that girl had a 60% increased risk of having an eating disorder. When the researchers controlled for second-hand exposure, what girls watched themselves had no independent impact on these outcomes.
This study suggests a far different role for media on unhealthy attitudes and behaviors than is typically assumed. While we may be influenced somewhat by what we see on tv, we are far more susceptible to "catching" the ideas, values, and behaviors of people in our direct social network. And, as social network researchers Nicholas Christakis and James Fowler have shown, the behaviors of people we like and feel similar to are the most "contagious." 
The direct influence of tv viewing may be minor, but that influence is multiplied and magnified when many members of a social network are being influenced in the same way. Social interactions reinforce attitudes and behaviors inspired by tv, and those social interactions far eclipse the small power of television on any single person.
The researchers conclude that, "Indirect exposure to media content may be even more influential than direct exposure." They also point to the troubling parenting implications. "If second-hand exposure to media content is, indeed, harmful to children, as this study supports, then the recommendation to parents to limit screen time may be inadequate to protect children from the risk imposed by their social milieu."
1. Ye, Yinjiao (2010). Beyond materialism: the role of health-related beliefs in the relationship between television viewing and life satisfaction among college students. Mass Communication and Society, 13(4), 458-478.
2. Becker, A.E. et al. (2011). Social network media exposure and adolescent eating pathology in Fiji. The British Journal of Psychiatry, 198 (1): 43-50.
3. Christakis, N.A., & Fowler, J.H. (2007). The spread of obesity in a large social network over 32 years. New England Journal of Medicine, 357, 370-379. AND Rosenquist, J.N. et al. (2010). The spread of alcohol consumption behavior in a large social network. Annals of Internal Medicine, 152, 426-33. AND Christakis, N.A., & Fowler J.H. (2008). The collective dynamics of smoking in a large social network. New England Journal of Medicine, 358, 2249-58.