BERLIN — Colds, measles, the flu: How infectious diseases are transmitted is fairly clear. But could it be that not only physical but also mental conditions — like depression or anxiety disorders — are contagious? Several recent studies have pointed in that direction. But these findings are controversial. Time for a closer look.
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The debate centers on two major studies, one conducted in Finland and the other in Denmark. Both involved large data sets: The Finnish team examined data from more than 700,000 students, while the Danish researchers analyzed records from 250,000 employees at 17,000 companies. Both studies reached similar conclusions:
According to the Finnish study, students are more likely to be diagnosed with a psychiatric disorder if they are in a class with children who are, or have been, mentally ill.
According to the Danish study, the number of diagnoses within companies tends to rise when employees arrive from another company where mental illness was more prevalent.
So how exactly did the two teams reach these conclusions?
Contributing factors
In Finland, a group of psychologists led by Jussi Alho at the University of Helsinki analyzed data on students starting in ninth grade. A quarter of the participants were diagnosed with a mental illness in the years that followed. Those who had more than one classmate suffering from a mental disorder at the start of the school year were at higher risk. Eating disorders and anxiety disorders were particularly likely to be “caught.”
A research team led by Carina Lomberg at the Technical University of Denmark even described the trend as an “epidemic.” They identified managers who either had mental health issues themselves or came from struggling companies as the main “carriers” of mental disorders within a workplace. After all, they tend to interact with a large number of people.
There’s evidence of a real contagion effect.
Not everyone is convinced by the idea of mental illness being contagious. In the end, the researchers found only a statistical correlation, says Andreas Bjerre-Nielsen, professor of social data science at the University of Copenhagen. “Just because the number of diagnoses increases after contact with people suffering from mental illness doesn’t mean that contact caused it.”
He adds that other possible contributing factors were not adequately taken into account. What if the students in a class had something else in common besides mentally ill classmates? For instance, they might have all grown up in a disadvantaged neighborhood.
“What’s more, the studies didn’t record whether the students had the same teachers or whether there were any unusual events in the classroom,” Bjerre-Nielsen says. And what if some students already had mental health issues that had simply not been diagnosed yet? Perhaps they were more likely to seek help if they had classmates or coworkers going through similar experiences.
A new study
A new study now aims to answer some of these questions. A team of researchers in Norway is currently analyzing data from 230,000 young people between the ages of 16 and 19. The findings have yet to be formally published, but they have already been presented at academic conferences. Bjerre-Nielsen has reviewed the data and says he finds the study “significantly more convincing.” He adds that the researchers did a better job of controlling for socioeconomic influences.
Julian Johnsen of the University of Bergen, one of the study’s authors, offers this preview of the results: “We find evidence of a real contagion effect.” This rules out the idea that students were simply more likely to seek diagnosis because of exposure to sick classmates. “If increased awareness led to the diagnosis of pre-existing conditions, we would expect grades to improve since the students could then receive help,” Johnsen explains. But the new study finds the opposite: When more students in a class suffer from mental illness, overall academic performance tends to decline.
The more rigorously a study is designed, the smaller the effect sizes tend to be.
So there does seem to be an effect, but the question is how strong it is. On average, just over 4% of students in Johnsen’s study had a psychiatric diagnosis. When that was the case, the chance that a classmate would later visit their family doctor for mental health concerns was 23.7%. If 10% of the students in a class were already diagnosed, that probability rose to 24.4%. In other words, an increase of just 0.7 percentage points: a very slight change.
This reflects a common trend: The more rigorously a study is designed, the smaller the effect sizes tend to be. Studies like these are only possible in Scandinavian countries. In Germany, for example, linking health registry data with school or workplace information would be out of the question for privacy reasons. The bottom line is: Yes, mental illnesses can be transmitted — but nowhere near as easily as a cold.
