We are more likely to share genetic similarities with our friends than with strangers because of our social structure, a new study has found.
Not only do friends tend to be genetically closer than strangers, they are on average two-thirds as similar to each other as the average married couple, according to a study published in PNAS.
Past research has suggested a likeness between the genome of spouses and adult friends but why or how this happens has been unclear.
One explanation underlying these genetic similarities may be social homophily, whereby individuals are drawn to one another based on shared characteristics such as social background or physical traits, for example height or weight, which themselves can be shaped by genetics. Another cause may be social structuring the authors write, the idea that people form friendships due to a shared social environment, which again can be traced back to genetics.
To examine this further, scientists from Stanford, Duke and the University of Wisconsin studied social and genomic data from 5500 American adolescents collected during the National Longitudinal Study of Adolescent to Adult Health.
When analysing the genome of schoolmates, the scientists demonstrated that classmates were half as genetically similar as friends and significantly more similar than unaffiliated individuals. The researchers further revealed that socially affected traits, such as body mass index and educational attainment, were more similar than traits that weren't influenced so much by social enviroment, such as height. So people weren't necessarily subconsiously seeking out friends who are genetically like them, but rather they tend to be surrounded by people of similar genetics because of where they live and the circles they move in, and that's where they make friends.
Speaking to Time, lead author Dr Benjamin Domingue at Stanford University concluded that a shared environment and background is likely to be the main factor underlying the genetic likeness observed.
'Are individuals actively selecting to be around people who are like them, or it is due to impersonal forces, such as social structures, that we all are affected by? Our evidence, with respect to friends, suggests that it's largely the effect of social structures,' he said.
The authors add that social homophily and social structuring are not mutually exclusive and may complement one another. Professor Kathleen Mullen Harris, fellow author and University of North Carolina Chapel Hill, suggests careful consideration of this 'social genome' in future social science research.
'Geneticists need to pay attention to the social context when they're estimating genetic influences on [traits] like education attainment,' she said. 'It's important to pay attention to these shared genetic effects that we speculate are really due to social structure.'