Researchers conducted a large-scale study of genetic data and found that people who are friends are more genetically similar to one another than people who are strangers. Calling your best friend your family may not be such a stretch, after all.

The study found that friends are as genetically similar to one another as fourth cousins or people with the same great-great-great-grandparents.

The team of researchers from University of California, San Diego, and Yale University controlled for ethnic similarities between friends by retrieving data from the Framingham Heart Study. The study was conducted in Europe, and the subjects did not contain vast ethnic dissimilarities.

They study looked at 1,932 subjects. Unrelated pairs of friends and unrelated strangers were compared, and their genetic data was analyzed.

"Looking across the whole genome we find that, on average, we are genetically similar to our friends. We have more DNA in common with the people we pick as friends than we do with strangers in the same population," says James Fowler, professor of medical genetics and political science at UCSD.

The study examined 11.5 million genetic markers for comparison. On average, friends shared 1 percent of their genome. Don't be fooled by the small number -- it seems inconsequential, but in genetics it's actually quite significant, according to Yale professor and co-author of the study, Nicholas Christakis.

The study was published in the Proceedings of the National Academy of Sciences. Along with the published results, Fowler and Christakis also developed a "friendship score." The score predicts future friendships as accurately as scientists can genetically predict the probability of a person's obesity or chances of being diagnosed with schizophrenia.

How does this work? How do we manage to "choose" friends based on genetic similarities? The study argues for an evolutionary explanation. Friends with similarities may have an evolutionary advantage over strangers because any positive attributes both friends have are more advantageous when shared. With a push from evolution, unrelated kin become friends.

Additionally, Fowler explains, some traits need someone else with those same traits to be functionally relevant. He uses speech as an example.

"The first mutant to speak needed someone else to speak to. The ability is useless if there's no one who shares it. These types of traits in people are a kind of social network effect," Fowler says.

Interestingly, Fowler, Christakis and colleagues also found that the genes that friends shared seem to evolve faster than unshared genes. They believe this explains the rapid acceleration of evolution over the last 30,000 years.

Finally, the researchers make an argument for the possibility that evolutionary fitness -- the ability to survive and reproduce -- strongly depends on friendships. In other words, keep your friends close. 

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