Adolescent Peers' Diagnoses and Genetic Predispositions and Subsequent Risk of Mental Disorders

Importance: Peer networks may influence early psychiatric vulnerability, both through direct exposure to peers' mental disorders and via peers' genetic predispositions. Clarifying these peer influences could inform preventive strategies aiming to improve adolescent mental health. Objective: To determine whether adolescents' exposure to peers' diagnosed mental disorders or genetic predispositions is associated with subsequent risk of mental disorders and whether these associations differ across disorders or peer network contexts. Design, Setting, and Participants: Nationwide register-based cohort study including Finnish residents born between 1985 and 2000. Cohort members were assigned to 4 peer network contexts: lower secondary school, upper secondary school, postal code area, and 1000-m square. Follow-up extended from age 17 years until first mental disorder diagnosis, emigration, death, or December 31, 2023. Study data were analyzed from October 2025 to March 2026. Exposures: (1) Peers' family-based genetic risk scores (FGRS) for mental disorders, estimated from diagnoses in first- through fifth-degree relatives and (2) peers' own diagnoses of mental disorders. Main Outcomes and Measures: Time to an incident diagnosis of the disorder after age 17 years. Associations were estimated using Cox proportional hazards models adjusted for sex, birth year, proband FGRS, parental education and income level, and peer network size. Results: Of 604 819 cohort members (mean [SD] age, 17 [0] years; 303 967 male [50.3%]), 234 117 received a mental disorder diagnosis over a median (IQR) follow-up of 11.7 (7.5-16.5) years. Peer FGRS was associated with subsequent risk of the same disorder across disorders and peer networks, with the greatest associations for externalizing disorders in upper secondary school (hazard ratio [HR], 1.34; 95% CI, 1.29-1.38). In contrast, peer diagnoses showed the strongest associations for internalizing disorders in upper secondary school (HR, 1.17; 95% CI, 1.15-1.18). Cross-disorder associations were also observed, with peer FGRS for externalizing disorders associated with proband internalizing outcomes, and vice versa. However, whereas peer internalizing diagnoses were associated with both proband internalizing and externalizing outcomes, associations between peer externalizing diagnoses and proband internalizing outcomes were substantially weaker than those with proband externalizing outcomes. Conclusions and Relevance: Findings of this cohort study suggest that peer environments, particularly in upper secondary school settings, are associated with mental health trajectories. More research is needed to elucidate the mechanisms underlying these findings.