This study illustrates how social forces are driven by genes and reinforced by the structure of society, causing people to make friends with those that are genetically similar to them. Using data from the National Longitudinal Study of Adolescent to Adult Health, researchers from Stanford, Duke and the University of Wisconsin carried out a series of genetic comparisons in the genetics of around 5,500 American teenagers. They found that friends were more genetically similar than random pairs of people, and about two-thirds as similar as the average married couple. The genetic similarities could be partly the result of “social homophily,” the authors say. This process is essentially one of like attracting like; suggesting that people are naturally drawn to others with similar characteristics which derive from their genetics, such as height, intelligence and weight.