Depressed and Anxious Together: Decomposing the Within- and Between-Subjects Components of the Clustering of Internalizing Symptoms in Adolescent Friendship Networks

Adolescent depression and anxiety have reached historically high levels, and research consistently shows that these symptoms cluster among adolescent friends. Yet a critical question remains underexplored: does this clustering reflect the stable social environments adolescents are embedded in, or does it reflect dynamic, real-time emotional co-variation between friends? Using five waves of data from a large, diverse cohort of Southern California high school students (N = 3,612), we applied mixed-effects models to decompose the within- and between-person effects of peer network exposure to depression and generalized anxiety on adolescents' own internalizing symptoms, while adjusting for bullying, substance use, social media use, and sexual, gender, and racial/ethnic identities. Results revealed significant within- and between-person effects of network exposure on both depression and anxiety. Between-person effects were more pronounced, indicating that adolescents chronically embedded in more symptomatic peer networks tended to report meaningfully higher symptom levels. Supplemental cross-lagged models confirmed that this co-clustering was largely driven by stable between-person differences rather than peer influence: exposure to peers' symptoms did not predict subsequent increases in adolescents' own symptoms, though adolescents' depression did predict subsequent increases in peers' depression, a pattern more consistent with social selection than contagion. Bullying and social media use independently explained both stable and acute variation in symptoms, and sexual and gender minority adolescents faced disproportionately elevated symptom burdens. These findings underscore the importance of attending to the peer environments in which adolescents are socially organized, not just individual-level symptom profiles, when assessing and addressing adolescent mental health.