Researchers from Stanford, Duke and the University of Wisconsin—Madison studied 5,000 pairs of adolescent friends using data from Add Health, a long-term study of people who were in grades seven through 12 during the 1994-1995 school year.
Study author Benjamin Domingue, an assistant professor at the Stanford Graduate School of Education, says this similarity is strong enough to detect, but not nearly on the same level as siblings, for example.
Classmates were about half as genetically similar as friends and significantly more similar than unaffiliated individuals — which suggests that a shared environment and background may account for a good chunk of the genetic likeness observed among friends, Domingue explains.
“Geneticists need to pay attention to the social context when they’re estimating genetic influences on [traits] like education attainment,” she says.
“It’s important to pay attention to these shared genetic effects that we speculate are really due to social structure.”