Peers' diagnoses were linked to adolescents' own later mental disorders

Design

Nationwide register-based cohort

Population

604,819 Finnish residents born 1985–2000, followed from age 17

Primary outcome

Time to first diagnosis of a mental disorder

Effect

Peer genetic risk, externalising: HR 1.34 (95% CI 1.29–1.38); peer diagnoses, internalising: HR 1.17 (1.15–1.18)

A Finnish national register study followed 604,819 people born between 1985 and 2000 from age 17, a median of almost 12 years. For each, it measured the mental disorder diagnoses of their school and neighbourhood peers, and those peers' family-based genetic risk, calculated from diagnoses among relatives.

Peers' genetic risk was associated with the same disorder in the adolescent, most strongly for externalising disorders in upper secondary school (hazard ratio 1.34). Peers' actual diagnoses showed the strongest association for internalising disorders such as anxiety and depression in the same setting (hazard ratio 1.17). Internalising diagnoses among peers were linked to both kinds of outcome in the adolescent.

This is association in a register, adjusted for the adolescent's own family risk and parental education and income, but not for everything that brings young people together in one school. It does not show contagion, and it may partly reflect help-seeking and diagnosis becoming more likely in some peer groups. It was published in September 2026.

• When a teenager presents, ask about stress in their friendship group and school, not just at home.
• A diagnosis in a close friend can be an opening to ask a young person how they are coping.
• The effects were modest per person; they matter at school and population level more than for one patient.
• Upper secondary school was the setting with the strongest links in this study.

Why it matters

It puts the school setting, not only the family, into the picture of adolescent risk.

Don't overread it

A register association cannot show that mental disorders spread between peers.

The statistics, in plain English

A hazard ratio of 1.17 means about a 17% higher rate of diagnosis over time; the narrow interval (1.15–1.18) reflects the enormous sample rather than a large effect. With 600,000 people, even small associations are highly statistically significant, so clinical importance has to be judged by size, and these are modest.

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