Professor Ross receives NIH funding to study the effect of friendship networks on the health behavior of adolescents. Professor Ross with Professors Fletcher at Yale University and Cohen-Cole at the University of Maryland were awarded a major R21 grant from the National Institute of Child Health and Human Development. Under this award, they will develop and implement new approaches to identify the causal effect of the friendships that a student forms in school on key health related behaviors, such as smoking, drinking, weight gain and sexual relations. In the proposed research, the authors will attempt to isolate the causal effect of friends from confounding factors, such as students sorting into specific friendships based on their unobservables or choosing friends who exhibit similar behaviors by exploiting across grade differences in the environment experienced by students whose families selected into the same school, but who happened to have children of slightly different ages. One aim of their study will be to compare students who made very similar friendships as other students in the same school, but due to their grade were exposed to friends who exhibited different levels of smoking or drinking. In another aim, their study will the examine differences in friendship network structure between adjacent grades and the impact of those differences on health behaviors.