Teens with classmates who have a mental illness have a significantly greater risk for a psychiatric diagnosis later in life, even after controlling for parents’ mental health history and other factors, a new study suggested.
The research provides new evidence that adolescents within a specific peer network may possibly “transmit” mental disorders such as depression and anxiety to each other, the investigators noted.
The study is said to the be the largest to date on the topic, including data on more than 700,000 ninth graders in Finland who were followed for up to 18 years.
At least one expert noted that the numbers are higher than he would have expected, but the investigators were quick to caution the study doesn’t prove having a classmate with a mental illness leads to later psychiatric diagnosis among peers.
“The associations observed in the study are not necessarily causal,” lead investigator Jussi Alho, PhD, a postdoctoral researcher at the University of Helsinki, Finland, told this news organization. “The study did not investigate the mechanisms that explain the observed associations.”
The results were published online on May 22 in JAMA Psychiatry.
Few Data
Previous studies have reported a clustering of mood symptoms, eating disorders, and other psychiatric illnesses among adolescent and adult social networks. But most involve self-selected peer groups.
“Investigating the transmission of mental disorders is especially important in childhood and adolescence,” the authors noted. “Yet, despite a few survey studies reporting that adolescents may experience increased mental health symptoms when exposed to friends or peers with mental health problems, large-scale studies on the potential peer influences of mental disorders in youth are lacking,” the authors wrote.
Researchers used a database of 713,809 students in the ninth grade, about half boys and half girls. All were born between January 1, 1985, and December 31, 1997. About 47,000 were excluded as they had a mental disorder diagnosis before the study began.
Some 666,000 students in 860 schools were followed from ninth grade until the first diagnosed mental disorder, death, emigration, or the end of the study in 2019. Median follow-up was 11.4 years.
Diagnoses were gathered from Finnish registries for inpatient, outpatient, and primary care and included ICD-9 and ICD-10 diagnoses for substance misuse disorders, schizophrenia spectrum disorders, mood disorders, anxiety disorders, eating disorders, emotional and social-functioning disorders, and hyperkinetic and conduct disorders.
The authors adjusted for sex, birth year, school and ninth-grade class size, area-level urbanicity, area-level morbidity, area-level education, area-level employment rate, parental educational level, and parental mental health, with a random intercept per school.
Dose-Response Relationship
Overall, a quarter (167,227) of the students were diagnosed with a mental disorder.
The risk of being diagnosed with any mental disorder was 3% higher during the entire follow-up period (hazard ratio [HR], 1.03; 95% CI, 1.02-1.04). Risk was highest in the first year of follow-up (HR, 1.13; 95% CI, 1.08-1.18) and then rose again in years 4 and 5, when the risk was 5% higher with one diagnosed classmate and 10% higher with more than one diagnosed classmate.
The risk was significantly increased for mood, anxiety, and eating disorders in each follow-up time window. Investigators also noted a dose-response relationship: The more classmates with a psychiatric illness, the greater the risk for later mental illness.
“These findings suggest that mental disorders may be transmitted within adolescent peer networks,” the authors wrote.
The researchers chose to describe the spread of mental disorders among peer classmates as “transmission” in part because it has been previously used in the literature, Dr. Alho said.
Alho said the researchers also believe that transmission is an accurate term to describe the potential mechanisms by which mental disorders may spread.
The authors hypothesized that more students might be diagnosed when disorders are normalized, through increased awareness and receptivity to diagnosis and treatment.
Conversely, the rate of disorders might also have increased — especially in the first year of follow-up — if there were no students in the peer network who had been diagnosed, the authors added. Without an example, it might discourage a student to seek help.
The authors also noted that it’s “conceivable that long-term exposure to a depressive individual could lead to gradual development of depressive symptoms through the well-established neural mechanisms of emotional contagion.”