Your risk for health issues including drug addiction and psychiatric disorders could in part depend on the genetic makeup of our childhood and adolescent social circles, according to a new study.
Researchers from Rutgers University in New Jersey looked at more than 650,000 Swedish health registry records for data on individuals aged 17 to 30 and their extended families, mapping risks for substance abuse and mental health problems. From this they determined their family genetic risk scores – the likelihood that their addiction or mood disorders had inherited features.
Referencing the information against location and school information, they searched for signs of a relationship between the genetic risk scores and tendency for school peers and other members of the local community to also experience similar outcomes.
Here's what they found: if you're hanging around with people who have a higher genetic risk of a variety of certain health problems, you're more likely to develop them yourself, even if you don't have the same risk mapped out in your own genes.
"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," says Rutgers University psychiatrist Jessica Salvatore.
"What our data exemplifies is the long reach of social genetic effects."
Known as socio-genomics, this emerging field of research looks at how one person's genotype can influence the observable traits of another. While there's evidence of people 'rubbing off' on others in this way, it's not yet clear why it happens.
The research looked at issues including drug use disorder, alcohol use disorder, major depression, and anxiety disorder. The likelihood of being affected by a peer group varied depending on the health problem, but it was higher for the substance use disorders – up to a 59 percent increased risk among those who attended high school in the same peer groups.
According to the data, a smaller but still significant influence could be seen among those living in the same area, and is most noticeable between the ages of 16 to 19 – though the health issues can develop later on in life, and study data was collected up to the age of 30.
What is particularly interesting is that the association held even if the genetic risk hadn't manifested itself: so, for example, being at school with someone more genetically likely to develop an alcohol issue would affect your own risk, even if that person wasn't actually drinking to excess.
"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," says Salvatore.
Basic group dynamics would suggest you're more likely to join in with what your friends are doing and think in similar ways, but this genetic link indicates there might also be something else going on at a deeper biological level.
The researchers are keen to investigate further to improve diagnosis and treatment methods.
"Peer genetic influences have a very long reach," says Salvatore. "It's not enough to think about individual risk."
The research has been published in the American Journal of Psychiatry.