Genes of school friends may influence your psychiatric fate

They say you can’t choose your family, but you can pick your friends. Research from Rutgers University suggests that secondary school aged children best choose wisely.

Jessica Salvatore works in the burgeoning field of socio-genomics. Flipping the nature/nurture debate on its head, it argues that we can think of the environment we grow up in as having a genotype of its own. This, she argues, is a much ignored determinant in our risk of developing psychiatric conditions in particular.

The new study, published in the American Journal of Psychiatry, found that the genetic makeup of adolescent peers may have long term consequences for individual risk of anxiety, depression, and substance use disorders…

Jessica - We know that from a number of studies peers play a particularly important influence on the risk of developing psychiatric and substance use disorders. What we were doing in this study was bringing a genetic perspective to this question: how do our peers, their genetic predispositions, influence risk above and beyond the peers' behaviour itself? What our collaborative research team did was we used Swedish national data to define schools that people attended as well as the neighbourhoods that they lived in growing up. We were able to characterise then the genetic risk carried by one's peers across these both geographical and school-based levels in adolescents

James - With these conditions it's a complicated picture to put it mildly with regard to what specific genes are associated with them. So how did you index someone's genetic risk for you to then study how this related to health outcomes for their peers?

Jessica - Using what we call pedigree based information. Individuals risk for disorder based on the number of people in their extended families who were affected by these disorders, so an individual who has a number of first degree relatives who are affected by, say, drug use disorder, alcohol use disorder, that person would have what we call a higher family genetic risk score, and then mapping individuals to peer groups to look at peer social genetic effects.

James - We know substance use disorder, for example, has genetic and environmental risk factors, so I trust you controlled for family socioeconomic factors when drawing your conclusions, given that you had access to their address and so on to go off?

Jessica - Absolutely. What we found was, in fact, the genetic predispositions of peers in adolescence was associated with a target individual's likelihood of developing alcohol use disorder, drug use disorder, major depression, or anxiety disorder later on in adulthood. One thing that I also want to mention is that these peer social genetic effects were statistically robust even after we controlled for whether or not the peers were themselves affected by these disorders. So of course, one natural explanation for how peers' genetic predispositions might influence our own outcomes is that the peers at higher genetic risk might also be more likely to develop these disorders themselves, which is in fact correct. But even after we statistically control for whether or not the peers were affected, we still see an association between peers, genetic predispositions, and our target individual's likelihood of developing disorder in adulthood.

James - And were your findings consistent across the conditions you studied: depression, anxiety, substance use disorder?

Jessica - That's actually one of the interesting nuances in these data, which is that we did find peer social genetic effects across all of these disorders, but the effects were stronger for what we call our externalising disorders: that's alcohol and drug use disorder, compared to what we call our internalising disorders: major depression and anxiety disorder. This finding is actually quite consistent with the broader peer influence literature where other research groups have found that there are stronger peer socialisation effects for these externalising disorders compared to internalising disorders.

James - Interesting. So the question becomes, how do we intervene using this information to bring down levels of these unfortunate afflictions in the population?

Jessica - What this study is really underscoring is that our environments have a genotype and that the genotype of these environments matter for our behavioural outcomes. A lot of times when we think about, ‘how do genes influence our behaviour,’ and what can we do about that, it's really thought of at an individual level: how do my genetic predispositions influence my risk? Ergo, intervention should be at the individual level. But what we demonstrate, as we've done in this study, we're going to get the strongest effects for prevention and intervention efforts when we consider social groups and implement the prevention and intervention efforts at the level of socialising units, that that tamps down on some of that disruptive behaviour that we know is an early precursor to disorders like alcohol use disorder and drug use disorder.