When we make friends, it can help to have something in common. Researchers and anyone who has ever struck up conversation with a stranger knows this. But a new study published in the Proceedings of the National Academy of Sciences finds that commonality among friends goes far deeper: Friends resemble each other genetically. In fact, the genetic similarity was enough to create a rubric from which researchers can predict, based on genes, who in a group will be friends at about the same level of confidence that scientists currently have for predicting a person's chances of obesity or schizophrenia.
Friends, it turns out, have about the same amount of genes in common as fourth cousins, or people who have the same great-great-great grandparents, which amounts to about 1 percent of genes, according to the study.
"One percent may not sound like much to the layperson, but to geneticists it is a significant number," Nicholas Christakis, professor of sociology, evolutionary biology, and medicine at Yale, and a lead author on the study, said in a press release. “And how remarkable: Most people don't even know who their fourth cousins are! Yet we are somehow, among a myriad of possibilities, managing to select as friends the people who resemble our kin."
The researchers pulled data from 1,932 individuals—most of whom were of European descent—controlled for already well-established factors like people's tendency to befriend those of similar ethnic backgrounds. The study compared pairs of unrelated friends against pairs of unrelated strangers.
"We have more DNA in common with the people we pick as friends than we do with strangers in the same population," James Fowler, professor of medical genetics and political science at U.C. San Diego and a lead author on the study, said in a press release.
Fowler and Christakis found that friends are most similar in genes that affect sense of smell, which comes with no obvious explanation. They hypothesize that people with similar smell-genetics might encounter each other in similarly smelling environments and then become friends, but suspect that “there is more to the story than that.”
On the other hand, friends are the most genetically dissimilar in genes that affect immunity. That difference has a clear benefit, according to the researchers: Interacting with people who withstand different pathogens than you do reduces them from spreading between you and them.
“It seems that our fitness depends not only on our own genetic constitutions, but also on the genetic constitutions of our friends,” Christakis said.
But with no obvious signal that alerts others to our genetic immunity, how do we select friends with that genetic difference? We have no idea.
Next research question—why do dog owners look like their dogs?