In a new study, scientists found that groups of friends show patterns of genetic similarity.
The study was conducted by lead author James Fowler, a social scientist at the University of California, San Diego, and his colleagues.
Their findings are based on patterns of variation in two out of six genes sampled among friends and strangers.
But the claim is a hard sell for some geneticists, who say that the researchers have not analysed enough genes to rule out alternative explanations.
The team looked at the available data on six genes from roughly 5,000 individuals enrolled in unrelated studies, and recorded the variation at one specific point, or single nucleotide polymorphism (SNP), in each gene, and compared this between friends and non-friends.
After controlling for genetic likeness due to sex, age, race or common ancestry, friends still tended to have the same SNP at one position in a gene encoding the dopamine D2 receptor, DRD2.
Friends also showed more variation at one position in a cytochrome gene, CYP2A6, than non-friends.
An 'opposites attract' phenomenon may account for the variation in CYP2A6 among friends, the authors said.
The study also suggested that genetic patterns don't always show up for friends who connect through similar activities, such as running marathons or playing musical instruments.
The ultimate function of DRD2 or CYP2A6 is not clear. But the authors pointed out that previous studies have associated both genes, albeit controversially, with traits that influence social behaviour: DRD2 with alcoholism and CYP2A6 with 'openness' 2,3.
"When people choose friends with similar genotypes, an individual's fitness — or survival until reproduction — not only reflects their own genes but also the genes of the friends they've chosen," Nature quoted Nicholas Christakis, a social scientist at Harvard University in Cambridge, Massachusetts, and an author on the study, as saying.
The study appears in The Proceedings of the National Academy of Sciences.