Your best friend from high school? Here's why their genes mattered

The old saying goes “Choose your friends wisely.” Now a study shows the old sayers were onto something: Their traits can rub off on you – especially ones that are in their genes.

The genetic makeup of adolescent peers may have long-term consequences for individual risk of drug and alcohol use disorders, depression and anxiety, the groundbreaking study has found.

What the researchers say: “Peers’ genetic predispositions for psychiatric and substance use disorders are associated with an individual's own risk of developing the same disorders in young adulthood,” the lead author of the study, published in the American Journal of Psychiatry. “What our data exemplifies is the long reach of social genetic effects.”

Socio-genomics—the influence of one person’s genotype on the observable traits of another—is an emerging field of genomics. Research suggests that peers’ genetic makeup may influence health outcomes of their friends. To test this the team used Swedish national data to assess peer social genetic effects for several psychiatric disorders.

With an anonymized database of more than 1.5 million individuals born in Sweden between 1980 and 1998 to Swedish-born parents, the first step was to map individuals by location and by school during their teenage years. The researchers then used medical, pharmacy and legal registries documenting substance use and mental health disorders for the same individuals in adulthood.

Models were run to assess whether peers’ genetic predispositions predicted target individuals’ likelihood of experiencing substance abuse, major depression, and anxiety disorder in adulthood. Peers’ genetic predispositions were indexed with family genetic risk scores—personalized measures of genetic risk based on family history—for the same conditions.

Even when controlling for factors such as the target individuals’ own genetic predispositions and family socioeconomic factors, the researchers found a clear association between peers’ genetic predispositions and target individuals’ likelihood of developing a substance use or psychiatric disorder. The effects were stronger among school-based peers than geographically defined peers.

Within school groups, the strongest effects were among upper secondary school classmates, particularly those in the same vocational or college-preparatory track between ages 16 and 19. Social genetic effects for school-based peers were greater for drug and alcohol use disorders than major depression and anxiety disorder.

The lead researcher said that more research is needed to understand why these connections exist.

“The most obvious explanation for why peers’ genetic predispositions might be associated with our own well-being is the idea our peers’ genetic predispositions influence their phenotype, or the likelihood that peers are also affected by the disorder,” she said. “But in our analysis, we found that peers’ genetic predispositions were associated with target individuals’ likelihood of disorder even after we statistically controlled for whether peers were affected or unaffected.”

What is clear is what the findings mean for interventions.

“If we want to think about how to best address these socially costly disorders, we need to think more about network based and social interventions,” she said.  “It’s not enough to think about individual risk.”

This research also underscores the importance of disrupting processes and risks that extend for at least a decade after attendance in school, the researchers added. “Peer genetic influences have a very long reach,” they said.

So, what? The research is surprising that the genotype of one individual can affect others so directly. On the other hand, a mountain of research has shown that humans do tend to adopt the behaviors, and traits, of peers in order to belong to the peers’ group.

We have also known for a long time that depression and anxiety, pessimism and optimism and even resilience are, in some way, contagious. This research goes a long way to explain the mechanism of this contagion.

I strongly suspect that if the researchers had looked at a modern workplace, they would have seen the same effects and come to the same conclusions. In other words, this study shows something about humans generally not just adolescents.