Lead author Justin Trogdon, a health economist at RTI International, found the 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," Trogdon said in a statement. "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."
The study also showed that teens with obese parents were more likely to be overweight themselves.
The resources used data from the National Longitudinal Study of Adolescent Health that surveyed young people in grades 7 through 12.
The study, published in the September issue of Journal of Health Economics, found 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.