Researchers at Harvard and UC San Diego published a paper online in advance in PNAS this week showing that "maps of friendship networks show clustering of genotypes." Using six available genotypes from the National Longitudinal Study of Adolescent Health, the team mapped genetic similarities among more than 3,000 friend pairs and found that, when controlled for population stratification, "one genotype is positively correlated (homophily) and one genotype is negatively correlated (het