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It's probably not a coincidence that you consider your friends to be like family.

That's because, according to a collaborative study between the University of California San Diego and Yale University, friends who aren't biologically related tend to resemble each other genetically.

The study, published in the Proceedings of the National Academy of Sciences, consisted of doing a genome-wide analysis of nearly 1.5 million markers of gene variations that relied on data from the Framingham Heart Study, a multi-generational cardiovascular study on the residents of Framingham, Mass.

Data used in the heart study contain extensive genetic detail and information on people who describe themselves as friends, offering researchers a chance to closely examine the relationships.

"Looking across the whole genome we find that, on average, we are genetically similar to our friends," wrote James Fowler, a professor of medical genetics at UCSD and the study's co-author.

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The findings do not suggest people seek out friends with a similar ethnic background, Fowler said. All of the subjects were from the same geographical region.

Friends with Benefits

How similar are your friends to you? On average, they're as "related" as fourth cousins, or people who share great-great-great-grandparents.

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"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," said Nicholas Christakis, a professor of sociology, evolutionary biology, and medicine at Yale and co-author of the study.

The researchers also found that shared attributes among friends, or "functional kinship," can bring a variety of evolutionary advantages. For instance, if your friend feels cold when you do and builds a fire, you both benefit.

In other cases, the trait only works if your friend also shares it.

According to Fowler: "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."

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Perhaps the most interesting finding from the study is that genes that were more similar between friends seem to evolve faster than other genes.

This may help explain why human evolution appears to have sped up in the last 30,000 years, Christakis and Fowler said. They suggest a social environment itself is an evolutionary force.

If friends are practically family and there are additional benefits to our relationships, then it's just another reason to keep them close.

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