It may be time to give our friends a spot on the family tree.

New research by scientists at Yale University and the University of California-San Diego reveals that people tend to pick friends who resemble them genetically. In fact, according to the study, close friends are the genetic equivalent of fourth cousins, on average.

The study appears in the Proceedings of the National Academy of Sciences. Nicholas Christakis, professor of sociology, evolutionary biology, and medicine at Yale, co-authored the study with James Fowler, professor of medical genetics and political science at UC San Diego.

“This gives us a deeper accounting of the origins of friendship,” Christakis said. “Not only do we form ties with people superficially like ourselves, we form ties with people who are like us on a deep genetic level. They’re like our kin, though they’re not.”

Christakis and Fowler looked at some 1.5 million gene variants in order to do their analysis. They drew from a dataset known as the Framingham Heart Study, which offered details on both the friendships and genetics of its participants.

Using 1,932 unique subjects, researchers genetically compared pairs of friends with pairs of strangers from the same population. None of the pairs involved people who were related to each other.

What they found was that friends share about 1% of the same gene variants – a highly significant number in the eyes of geneticists. They also uncovered telling details within that analysis.

For example, friends are quite similar in gene variants having to do with the sense of smell. They are less likely to share gene variants relating to immunity against specific diseases.

The researchers said this might indicate that while it’s advantageous for friends to prefer the same smells, it might also be beneficial for them to have genetic shields for different disease threats.

In essence, the study said, friends are “functional kin.” The researchers even developed a “friendship score” that suggests how likely two people are to become friends, based on their genetics.

Christakis said one of the more interesting aspects of the study has to do with the rate of evolution for genes shared by friends. Those gene variants are seeing the most evolutionary activity overall, meaning that friendship may play a role in the speed of human evolution.

“The fitness of many genes may depend on whether similar genes are in evidence in people we befriend,” Christakis said. “My genetic fitness depends on my own genes and my friends’ genes.”

Researchers emphasized that the study is not a statement about ethnicity or race. The study’s dataset, they said, was dominated by people of the same European extraction. In other words, even within an ethnically similar population, people tended to choose friends with a closer genetic profile. “These results are evident above and beyond any tendency people might have to associate with other people of the same ethnic or racial group,” Fowler said.

The study also points to the need for further scientific examination of the role of friendship.

“Human beings are one of the few species who form long-term, non-reproductive relationships with other members of our species,” Fowler noted. “This role of affiliation is important. It ties into the success of our species.”

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