This shows the shadow of a teenage girl.
Hakulinen points out that school classes are social networks well suited to research, as people are usually not able to choose their classmates. Credit: Neuroscience News

Teen Social Networks Influence Mental Health Disorder Spread

Summary: Researchers found that mental disorders can spread within school social networks. By analyzing data from over 700,000 Finnish ninth-graders, they discovered that having classmates with mental disorders increased the risk of being diagnosed with similar conditions later in life.

This effect was strongest within the first year and was particularly notable for mood, anxiety, and eating disorders. The findings highlight the importance of preventive measures and early intervention in schools to address mental health issues.

Key Facts:

  • Study analyzed over 700,000 ninth-graders from 860 Finnish schools.
  • Classmates’ mental health status significantly influenced students’ future diagnoses.
  • The effect was most pronounced for mood, anxiety, and eating disorders.

Source: University of Helsinki

Using population-wide registry data, researchers from the University of Helsinki, the Finnish Institute for Health and Welfare, the University of Jyväskylä and the University of Manchester investigated whether mental disorders can be transmitted within social networks formed by school classes.

The study is the largest and most comprehensive so far on the spread of mental disorders in social networks, with more than 700,000 ninth-grade pupils from 860 Finnish schools participating. The adolescents were followed from the end of ninth grade for a median of 11 years.

The researchers demonstrated that the number of classmates diagnosed with a mental disorder was associated with a higher risk of receiving a mental disorder diagnosis later in life.

“The observed link was the strongest during the first year of follow-up in the study. This was not explained by a number of factors related to parents, school and residential area. The link was most pronounced in the case of mood, anxiety and eating disorders,” says Associate Professor Christian Hakulinen of the University of Helsinki.

Research enabled by comprehensive Finnish registers

According to Hakulinen, prior studies have yielded similar results: for example, American researchers have observed indications of depressive symptoms potentially being transmitted from one individual to another in social networks.

In prior research, however, social networks have typically been chosen independently by the research subjects, which may result in bias in the data. Hakulinen points out that school classes are social networks well suited to research, as people are usually not able to choose their classmates.

“Defining the social networks and following adolescents were made possible by extensive Finnish registers. The findings significantly deepen our understanding of how mental health problems develop and affect other people in our social networks,” he says.

Hakulinen nevertheless notes that the connection observed in the study is not necessarily causal. Furthermore, the study did not investigate how mental disorders can potentially be transmitted between individuals.

“It may be possible, for instance, that the threshold for seeking help for mental health issues is lowered when there are one or more people in your social network who have already sought help for their problems. In fact, this kind of normalisation of diagnosis and treatment can be considered  beneficial contagion of mental disorders,” Hakulinen says.

More preventive measures?

Mental disorders are a significant global challenge, adversely affecting individuals, society and the economy. According to Hakulinen, anxiety and mood symptoms in particular have in recent years increased among young people.

Previous studies have shown that, in roughly half of all cases, the onset of mental disorders in adulthood occurs when people are under 18. In fact, Hakulinen emphasises the importance of preventive measures and early intervention.

“When taking preventive measures, it’s worthwhile considering that mental disorders can spread from one adolescent to another,” Hakulinen says.

The study involved a total of 713,809 Finnish citizens born between 1985 and 1997. The adolescents were investigated from the end of comprehensive school until they received their first mental disorder diagnosis, relocated from the country or died. At the latest, the follow-up was discontinued at the end of 2019, resulting in a median follow-up period of 11.4 years.

Funding: The study received funding from the European Research Council (ERC) and the Research Council of Finland.

About this mental health research news

Author: Elina Kirvesniemi
Source: University of Helsinki
Contact: Elina Kirvesniemi – University of Helsinki
Image: The image is credited to Neuroscience News

Original Research: Open access.
Transmission of mental disorders in adolescent peer networks: a Finnish nationwide registry study” by Christian Hakulinen et al. JAMA Psychiatry


Transmission of mental disorders in adolescent peer networks: a Finnish nationwide registry study


 Previous research indicates that mental disorders may be transmitted from one individual to another within social networks. However, there is a lack of population-based epidemiologic evidence that pertains to the full range of mental disorders.


To examine whether having classmates with a mental disorder diagnosis in the ninth grade of comprehensive school is associated with later risk of being diagnosed with a mental disorder.

Design, Setting, and Participants 

 In a population-based registry study, data on all Finnish citizens born between January 1, 1985, and December 31, 1997, whose demographic, health, and school information were linked from nationwide registers were included.

Cohort members were followed up from August 1 in the year they completed ninth grade (approximately aged 16 years) until a diagnosis of mental disorder, emigration, death, or December 31, 2019, whichever occurred first. Data analysis was performed from May 15, 2023, to February 8, 2024.


The exposure was 1 or more individuals diagnosed with a mental disorder in the same school class in the ninth grade.

Main Outcomes and Measures  

Being diagnosed with a mental disorder during follow-up.


Among the 713 809 cohort members (median age at the start of follow-up, 16.1 [IQR, 15.9-16.4] years; 50.4% were males), 47 433 had a mental disorder diagnosis by the ninth grade. Of the remaining 666 376 cohort members, 167 227 persons (25.1%) received a mental disorder diagnosis during follow-up (7.3 million person-years). A dose-response association was found, with no significant increase in later risk of 1 diagnosed classmate (HR, 1.01; 95% CI, 1.00-1.02), but a 5% increase with more than 1 diagnosed classmate (HR, 1.05; 95% CI, 1.04-1.06).

The risk was not proportional over time but was highest during the first year of follow-up, showing a 9% increase for 1 diagnosed classmate (HR, 1.09; 95% CI, 1.04-1.14), and an 18% increase for more than 1 diagnosed classmate (HR, 1.18; 95% CI, 1.13-1.24). Of the examined mental disorders, the risk was greatest for mood, anxiety, and eating disorders. Increased risk was observed after adjusting for an array of parental, school-level, and area-level confounders.

Conclusions and Relevance 

 The findings of this study suggest that mental disorders might be transmitted within adolescent peer networks. More research is required to elucidate the mechanisms underlying the possible transmission of mental disorders.

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