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Mental disorders may be socially transmitted among adolescents in a peer group, a Finnish nationwide cohort study suggested.

An analysis of more than 700,000 students showed that those who had multiple classmates in ninth grade with a mental disorder diagnosis had an increased risk of being diagnosed with a mental disorder later in life (HR 1.05, 95% CI 1.04-1.06, P<0.001), according to Jussi Alho, PhD, of the University of Helsinki in Finland, and colleagues.

There appeared to be a dose-response relationship, as students with only one classmate diagnosed with a mental disorder had no increased risk of a mental health condition later in life (HR 1.01, 95% CI 1.00-1.02, P=0.12), they reported in .

"This research suggests that social transmission should be considered a possible factor in the development of mental disorders," Alho told 麻豆传媒 in an email. "It must be noted, though, that this study does not allow drawing conclusions about causality of the observed associations or, in case the association is a causal one, about the mechanisms that would explain the transmission."

The risk appeared highest over the first year of follow-up, with a 9% increase for those with one diagnosed classmate (HR 1.09, 95% CI 1.04-1.14) and an 18% increased risk for those with more than one diagnosed classmate (HR 1.18, 95% CI 1.13-1.24), the researchers found.

Of all the mental disorders examined, risk was highest for mood, anxiety, and eating disorders, with increased risks for students with one diagnosed classmate, and more than one diagnosed classmate:

• Mood disorders (HR 1.04, P<0.001 and HR 1.10, P<0.001)
• Anxiety disorders (HR 1.03, P<0.001 and HR 1.03, P=0.03)
• Eating disorders (HR 1.10, P=0.01 and HR 1.29, P=0.03)

Risks remained elevated after adjusting for a range of parental, school-level, and area-level confounders.

While the findings were similar to those from prior studies, Alho noted that this observational study doesn't prove causality -- and if there is a true relationship, the mechanisms are not well understood.

For instance, it could be that mental disorders become normalized through increased awareness among peer groups. As disorders become more normalized, individuals may become more receptive to receiving a diagnosis and treatment, Alho said.

On the other hand, peer influence could also have a negative effect: "For some diagnosis categories, such as eating disorders, transmission could also occur through processes of peer social influence to which adolescents are particularly susceptible," he said.

Carlene MacMillan, MD, a child and adolescent psychiatrist in Brooklyn, New York, and a member of the American Academy of Child and Adolescent Psychiatry, told 麻豆传媒 that one strength of the research is that it pulled data from schools rather than from "other types of social groupings" that could have been chosen by the kids themselves.

MacMillan, who was not involved in the study, said the findings suggest clinicians should consider asking adolescent patients about their larger social environments.

"We don't necessarily ask about the broader classroom [and] the students in your school," she said. "We tend to focus on if somebody is bullying them, or somebody [with a diagnosis] is their friend. It's an opportunity to inquire more proactively around [the large social setting]."

Alho and colleagues used data from 713,809 Finnish citizens born between the beginning of 1985 and the end of 1997. Students had a median age of 16 years and 50.4% were male. They were followed up from August 1 of the year they completed ninth grade until December 31, 2019.

The researchers used ICD-10 mental disorder diagnosis categories to assess several conditions, including substance misuse disorders, schizophrenia spectrum disorders, mood disorders, anxiety disorders, eating disorders, and behavioral and emotional disorders.

Overall, 47,433 students had a mental disorder diagnosed by ninth grade, and 25.1% of the remaining members received a mental disorder diagnosis during follow-up.

The study was limited by the fact that hazard ratios, while significant, were small. The researchers could not rule out residual confounders.

Nonetheless, Alho and colleagues concluded that when "taking preventive measures, it is worthwhile considering that mental disorders can spread from one adolescent to another. Prevention and intervention measures that consider potential peer influences on early-life mental health could significantly reduce the disease burden of mental disorders in society."

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