Do we choose our friends because they share our genes?

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"The striking thing here is that friends are actually significantly more similar to one another than we were expecting," says James Fowler, a professor of medical genetics at the University of California, San Diego, who conducted the study with Dr. Nicholas Christakis, a social scientist at Yale University.

In fact, the study in Monday's issue of the Proceedings of the National Academy of Sciences found that friends are as genetically similar as fourth cousins.

"It's as if they shared a great- great- great-grandparent in common," Fowler told Shots.

Some of the genes that friends were most likely to have in common involve smell. "We tend to smell things the same way that our friends do," Fowler says. The study involved nearly 2,000 adults.

This suggests that as humans evolved, the ability to tolerate and be drawn to certain smells may have influenced where people hung out. Today we might call this the Starbucks effect.

"You may really love the smell of coffee. And you're drawn to a place where other people have been drawn to who also love the smell of coffee," Fowler says. "And so that might be the opportunity space for you to make friends. You're all there together because you love coffee and you make friends because you all love coffee."

They also found some interesting differences among friends: They tend to have very different genes for their immune systems. Other researchers have reported similar findings among spouses.

"One of the reasons why we think this is true is because it gives us extra protection. If our spouses have an immune system that fights off a disease that we're susceptible to, they'll never get it, and then we'll never get it," Fowler says. "And so it gives us an extra layer of protection."

"It's obvious that humans tend to associate with other people who are very similar to themselves," says Matthew Jackson, a professor of economics at Stanford University who studies social networks. "This gives us evidence that it's operating not just at a level of very obvious characteristics but also ones that might be more subtle — things that that we hadn't really anticipated."

Taken together, Fowler says the findings could help explain all sorts of things, including how relationships are driven by genetics and how that, in turn, may be influencing human evolution.

"I think the biggest implication is that evolution can't be studied as a Robinson Crusoe phenomenon. We didn't evolve isolated — separate from others. We evolved in communities. We evolved with our friends."

On a more personal level, it could help explain that cozy feeling we get with our friends.

"It's as if we were surrounding ourselves with a new family," Fowler says. "It's the family we chose, rather than the family we're born with."

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