Researchers from the University of Colorado Boulder found that genetic similarities may help to explain why human birds of a feather flock together, but the full story of why people become friends "is contingent upon the social environment in which individuals interact with one another".
People are more likely to befriend genetically similar people when their environment is stratified, when disparate groups are discouraged from interacting, the study found. When environments were more egalitarian, friends were less likely to share certain genes.
Scientists debate the extent to which genetics or environmental factors - 'nature' or 'nurture' - predict certain behaviours, said researcher Jason Boardman.
"For all the social demographic outcomes we care about, whether it's fertility, marriage, migration, health, it's never nature or nurture. It's always nature and nurture. And most of the time it has a lot more to do with nurture," he said.
The research team used data from the National Longitudinal Study of Adolescent Health. The team focused on 1,503 pairs of friends in seventh through 12th grade in 41 schools.
The study found that some pairs of friends shared certain genetic characteristics.
The team tested the evidence, arguing that if genes were the driving friendship factor, genetically based friendship should emerge most often and easily in schools with the least amount of social friction but they found the exact opposite.
In the most socially equal environments, genetic homophily (or love of the same) was 'pretty weak', meaning that friends were less likely to share genetic traits.
"It was in the most unequal social environments that we saw the highest level of genetic homophily," he said in a statement.
In a socially stratified school, "students from different populations within the school may be effectively off limits for friendships," the team wrote.
"To me, to say whether genes predict friendships without understanding the context within which these friendships may or may not occur just doesn't tell the whole story," Boardman said.
The study was published in the Proceedings of the National Academy of Sciences.
