Aug. 7 (UPI) -- Knowing that your high school friends' genes may affect your risk of substance abuse and psychiatric conditions later in life could make it possible for intervention before these problems occur, a new study shows.
Led by a researcher at Rutgers Robert Wood Johnson Medical School in New Brunswick, N.J., the study was published Wednesday in the American Journal of Psychiatry.
Friends' gene-influenced traits can have ripple effects on their peers, increasing the long-term odds of developing drug and alcohol use disorders, as well as major depression and anxiety, researchers said.
"We find that the genetic predispositions of peers, especially school-based peers in later adolescence, are associated with risk for developing psychiatric and substance use disorders in young adulthood," said the study's lead author, Jessica Salvatore.
Salvatore is an associate professor and director of the Genes, Environments and Neurodevelopment in Addictions Program at Robert Wood Johnson Medical School.
"Individuals who are themselves genetically predisposed to these disorders are especially sensitive to peer social genetic effects," she said.
Sociogenomics -- the influence of a person's genetic makeup on the observable traits of another -- is an emerging field.
Other research has indicated that peers' genetic makeup may have an effect on their friends' health outcomes. To test this theory, Salvatore and her collaborators used Swedish national data to evaluate "peer social genetic effects" for several psychiatric disorders.
Tapping into a database of more than 1.5 million anonymous people born in Sweden between 1980 and 1998 to Swedish-born parents, researchers first mapped out individuals by location and school during their teenage years.
Then, they referred to medical, pharmacy and legal registries recording substance use and mental health disorders for these people in adulthood.
Researchers employed computer modeling to assess whether peers' genetic predispositions foretold individuals' likelihood of experiencing substance abuse, major depression and anxiety disorder in adulthood.
They classified peers' genetic predispositions according to personalized measures of genetic risk based on family history for the same conditions.
Even when researchers accounted for variables, such as people's genetic predispositions and family socioeconomic factors, they found a distinct association between their peers' genetic susceptibility and the individuals' probability of developing a substance use or psychiatric disorder. The effects were more pronounced among peers at school than those within geographic proximity.
Within school groups, researchers observed the strongest effects among upper secondary school classmates, especially those in the same vocational or college-preparatory track between ages 16 and 19. These effects for school-based peers were more significant for drug and alcohol use disorders than major depression and anxiety disorder.
More studies would help illuminate why these correlations exist, Salvatore said, adding that the findings are still compelling.
"Oftentimes, when we think of genetic risk for a disorder, we think about how our own genes impact risk," she said.
"What the results from our study demonstrate is that the genetic predispositions of social partners also matter. This needs to be considered as part of preventive intervention efforts."
The study indicates that parents have reason to be concerned if their teenagers hang around the "wrong crowd" who come from families with drug and alcohol use or depression and anxiety, said Dr. Paul Appelbaum, a psychiatrist at Columbia University Irving Medical Center in New York City. He was not involved in the study.
"Influences on behavior are typically separated into genetic and environmental," said Appelbaum, whose research focuses on the ethical, legal and social implications of advances in genetics.
"This study is a creative effort to illustrate that the effects of environment -- in this case, the impact of one's peers -- can themselves be linked to genetics. It suggests a much broader scope for genetic influences than is usually envisioned."
He noted that the conclusions remained the same even when researchers considered larger groups of peers -- people living in the neighborhood and other students at school, especially those in the same track.
"That suggests an impact of the broader environment in which one lives, without even looking at a person's immediate circle of friends," Appelbaum said.
With this study, researchers moved us "closer to a robust understanding that the genetics of one's social support system" can affect behavior, said Robbee Wedow, an assistant professor of sociology and data science at Purdue University in West Lafayette, Ind.
Their research "underlines the importance of the social networks one finds oneself a part of, bridging the social and genetic components of behavior in a unique way," added Wedow, who also is an adjunct assistant professor of medical and molecular genetics at Indiana University School of Medicine in Indianapolis.
However, the observation of similarities in friends' behavior is common and not surprising, said David Ussery, a professor in the Department of Biomedical Informatics at the University of Arkansas for Medical Sciences in Little Rock.
"There's no reason to claim that genes are responsible for this," Ussery said, adding that "genes are like a musical score -- our health and our activities are the music, and this is strongly dependent on the environment in which we live," Ussery said.
"The 'music of life' is played and controlled not by our genes, but instead in the larger context of our cells and how they interact with each other, and influenced by our life history and events. It is not pre-determined."
It was nonetheless surprising that a friend with a genetic predisposition toward drinking could influence a peer to pick up the habit even if the first person had not yet started drinking much at the time when they interacted, said Alexander Urban, an associate professor of psychiatry and behavioral sciences and genetics at Stanford Medicine in Palo Alto, Calif.
"This was a bit of an unexpected result that will require further study," Urban said.