Adolescent Friendships, BMI, and Physical Activity: Untangling Selection and Influence Through Longitudinal Social Network Analysis


  • This research was made possible by William T. Grant Foundation Awards 10690 to David R. Schaefer and Sandra D. Simpkins and 7936 to Simpkins, and by Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) Grant R21-HD060927 to David R. Schaefer and Steven A. Haas. This research uses data from Add Health, a project directed by Kathleen Mullan Harris and designed by J. Richard Udry, Peter S. Bearman, and Harris at the University of North Carolina at Chapel Hill, and funded by Grant P01-HD31921 from the NICHD, with cooperative funding from 23 other federal agencies and foundations. Special acknowledgment is due Ronald R. Rindfuss and Barbara Entwisle for assistance in the original design. Information on how to obtain the Add Health data files is available on the Add Health website (http:// No direct support was received from Grant P01-HD31921 for this analysis.

Requests for reprints should be sent to Sandra Simpkins, School of Social and Family Dynamics, Arizona State University, PO Box 873701, Tempe, AZ 85287-3701. E-mail:


Bioecological theory suggests that adolescents’ health is a result of selection and socialization processes occurring between adolescents and their microsettings. This study examines the association between adolescents’ friends and health using a social network model and data from the National Longitudinal Study of Adolescent Health (N = 1,896, mean age = 15.97 years). Results indicated evidence of friend influence on BMI and physical activity. Friendships were more likely among adolescents who engaged in greater physical activity and who were similar to one another in BMI and physical activity. These effects emerged after controlling for alternative friend selection factors, such as endogenous social network processes and propinquity through courses and activities. Some selection effects were moderated by gender, popularity, and reciprocity.