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From NPR News this is ALL THINGS CONSIDERED. I'm Robert Siegel.

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And I'm Audie Cornish. People often talk about friends who feel like family. Well, there's new research out today that suggests there's more to that than just a feeling. As NPR's Rob Stein reports, people appear to be more genetically similar to their friends than to strangers.

ROB STEIN, BYLINE: One of the things that makes humans unusual is how much time we spend with other members of our species who are not members of our families not genetically related to us. James Fowler of the University of California, San Diego wanted to know if genes are still playing a role in those relationships.

JAMES FOWLER: Are friends genetically more similar to one another than strangers? And if so, how much more similar are they? And what genes might be involved in this process? What genes do we tend to be the most similar on with our friends?

STEIN: So Fowler and a colleague at Yale analyzed the genes of nearly 2,000 people.

FOWLER: The striking thing here is that friends are actually significantly more similar to one another than we were expecting. In fact, the level of similarity is like what we would see in fourth cousins. In other words, friends are so similar to one another, it's as if they shared a great, great, great grandparent in common.

STEIN: And when the researchers looked at some of the genes friends tend to have in common, they found some really interesting stuff.

FOWLER: We found that those genes that affect how we smell were particularly similar. We tend to smell things the same way that our friends do.

STEIN: 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.

FOWLER: So in the modern day, for example, you might 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. And so that will be the opportunity for you to make friends and you're all there together because you love coffee and you make friends with people who love coffee. That's one possible reason why we're finding so much genetic similarity.

STEIN: They also found some interesting differences among friends - they tend to have very different genes for their immune systems.

FOWLER: One of the reasons why we think we find that friends tend to have different immune systems relates to a similar finding among spouses. Spouses tend to have different immune systems as well.

STEIN: And why would that be?

FOWLER: 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. And so it gives us an extra layer of protection.

STEIN: Other researchers also say the new findings are surprising. Matthew Jackson's at Stanford.

MATTHEW JACKSON: It's obvious that humans attempt to associate with other people who are very similar to themselves. 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 we hadn't really anticipated.

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

FOWLER: 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.

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

FOWLER: It's as if we were surrounding ourselves with a new family. It's the family we choose rather than the family that we're born with.

STEIN: So when we say our friends feel like family, there's apparently an explanation for that on the most basic level. Rob Stein, NPR News.

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