Can mental disorders spread among peers? Researchers now say that mental health issues can be transmitted among adolescent social groups, particularly disorders connected to mood, anxiety, and eating.

Earlier studies have shown the possibility of depressive symptoms being transmitted from one individual to another in social networks. In the latest large-scale study involving more than 700,000 ninth-grade pupils from 860 Finnish schools, researchers evaluated the spread of mental disorders in social networks formed by school classes.

The results published in Jama Psychiatry revealed the greater the number of classmates diagnosed with a mental disorder, the higher the risk of an individual 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 areas. The link was most pronounced in the case of mood, anxiety, and eating disorders," said Professor Christian Hakulinen of the University of Helsinki in a news release.

The researchers cautioned the connection observed in the study was not necessarily causal.

However, the study has not investigated the mechanism by which mental disorders are transmitted between individuals. Researchers think the rise in diagnoses among peers might be due to the normalization of seeking diagnosis and treatment. As more people start getting help, it becomes more common and acceptable, leading to more diagnoses.

"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 normalization of diagnosis and treatment can be considered beneficial contagion of mental disorders," Hakulinen said.

Researchers believe that the study results will help in early intervention and prevention of mental disorders in the adolescent stage, the key developmental period when many mental disorders likely occur.

"Understanding the role of peer effects in early-life mental health problems would also offer tools for more successful prevention and intervention measures, thus reducing the economic and societal burden of mental disorders," the researchers wrote.