Your Best Friend From High School? Here’s Why Their Genes Mattered

Mom always said, “Choose your friends wisely.” Now, a study led by a Rutgers Health professor shows she was right: Their traits can pass on to you, especially the ones in their genes.

A groundbreaking study has found that the genetic makeup of adolescent peers may have long-term consequences for an individual’s risk of drug and alcohol addiction, depression and anxiety.

“The genetic predisposition of peers to psychiatric disorders and substance abuse is associated with an individual’s risk of developing the same disorders in young adulthood,” said Jessica E. Salvatore, an associate professor of psychiatry at Rutgers Robert Wood Johnson Medical School and lead author of the book study published in the American Journal of Psychiatry.

“Our data show how far-reaching socio-genetic effects can be,” she said.

Sociogenomics – the influence of one person’s genotype on the observable traits of another – is an emerging field of genomics. Research suggests that the genetic makeup of peers influence the health outcomes of their friendsTo test this, Salvatore and colleagues used Swedish national data to assess the social genetic effects of peers on various psychiatric disorders.

Using an anonymized database of over 1.5 million individuals born in Sweden between 1980 and 1998 to Swedish parents, the first step was to map individuals by location and school during their teenage years. The researchers then used medical, pharmaceutical, and legal records to document substance use and mental disorders for the same individuals in adulthood.

Models were run to assess whether peer genetics predicted that target individuals would be likely to experience substance abuse, major depression, and anxiety disorders in adulthood. Peer genetics were indexed using family genetic risk scores (personalized measures of genetic risk based on family history) for the same disorders.

Even when factors such as the genetic makeup of the target individuals themselves and family socioeconomic factors were taken into account, the researchers found a clear link between the genetic makeup of peers and the likelihood of the target individuals developing a substance abuse or psychiatric disorder. The effects were stronger among school peers than among geographically defined peers.

Within school groups, the strongest effects were seen among classmates in upper secondary education, particularly those aged 16 to 19 who were in the same vocational or preparatory education. The sociogenetic effects among school-aged peers were greater for drug and alcohol disorders than for major depression and anxiety disorders.

According to Salvatore, more research is needed to understand why these connections exist.

“The most obvious explanation for why the genetic makeup of peers might be associated with our own well-being is the idea that the genetic makeup of our peers influences their phenotype, or the likelihood that peers will also be affected by the disorder,” she said. “But in our analysis, we found that the genetic makeup of peers was associated with the likelihood of disorder in the target individuals, even after we statistically controlled for whether or not peers were affected.”

What is clear, Salvatore says, is what the findings mean for interventions.

“If we want to think about how best to address these socially costly conditions, 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 last at least a decade after school, Salvatore added. “Genic influences from peers have a very broad reach,” she said.

Co-authors include Henrik Ohlsson, Jan Sundquist, and Kristina Sundquist of Lund University in Sweden and Kenneth S. Kendler of the Virginia Institute for Psychiatric and Behavioral Genetics at Virginia Commonwealth University.