Almost every bad thing a teenager does gets attributed to the 'crowd' they hang out with - few parents confess to having the troublemaker kid. Now an RTI International researcher says that goes for weight gain in kids too.

It isn't your fault for letting them eat junk food; it's their friends, for also being overweight.

The study, published in the September issue of Journal of Health Economics says that friends' weight is correlated with an adolescent’s own weight even after considering demographics, smoking status, birth weight, and household characteristics such as parental obesity.

Females were more impressionable than men, the study says - peer effect on weight was strongest among females and among adolescents who were at risk of becoming overweight.

"Our results may help explain the dramatic rise in obesity among adolescents in the past few decades," said Justin Trogdon, Ph.D. a health economist at RTI and the paper's lead author. "Peers can influence all of the significant weight-related choices for teens, including eating patterns, diets and physical activity. Peers also affect teens’ perceptions of an acceptable weight."

Whew. That absolves the junk food culture being instilled in every cartoon they see from birth.

But are friends making teens fat or do teens seek out teens similar to themselves? It's unclear, since the study also showed that teens with obese parents were more likely to be overweight themselves.

That means heavy girls should hang out with models, right? But models are the most oppressed demographic in America, with constant criticism for their lower weight.

"Research has also indicated that peers may help adolescents to lose weight," Trogdon said. "Better understanding peer influence on weight will help improve policies and prevention efforts aimed at reducing adolescent weight."

The study used data from the National Longitudinal Study of Adolescent Health, a study that RTI helped conduct, that surveyed youths in grades 7 through 12. The researchers looked at students from 16 schools included in the data and defined peer groups based on friendships and grade level.