Across the genome, friends tend to have similar genotypes at the SNP level, a research duo from Yale University and the University of California, San Diego, report in the Proceedings of the National Academy of Sciences.
The pair conducted a genome-wide analysis of data from the Framingham Heart Study to determine whether there was a correlation in genotypes between friends. Such homophily and the formation of friendships, the duo says, could be a sort of extended kin detection system, and that "genetically similar (but unrelated) friends are a kind of 'functional kin.'"
After scanning more than 466,600 SNPs in nearly 2,000 people who were part of one or more friendship pairs, Yale's Nicholas Christakis and James Fowler at UCSD found that, as compared to strangers, friends are, on average, as genetically similar to one another as fourth cousins.
In particular, Christakis and Fowler note that friends tended to share some olfactory gene variants.
"Olfactory genes have a straightforward explanation: People who like the same smells tend to be drawn to similar environments, where they meet others with the same tendencies," Fowler tells Reuters.
However, the study included people of similar ancestries, and the University of Chicago's John Novembre cautions that people tend to have friends with similar ancestral backgrounds. He tells the Huffington Post that even after controlling for that — which is difficult — the effect is "very subtle."
Still, the study may help untangle why people have friends. "The making of friends is exceedingly rare in the animal kingdom," Christakis tells Reuters. "Certain other primates, elephants, and whales are the only other mammals who do this, and this alone aroused our curiosity."