GUY RAZ, host:
Most friendships are formed because of common interests - books or politics or shopping, or whatever. Race and religion and gender also play a part. But a new study published in the Proceedings of the National Academy of Sciences says biology may also be a big factor.
James Fowler is a professor of medical genetics and political science at the University of California, San Diego. And he says our genes could be making some of those decisions for us.
Dr. JAMES FOWLER (Professor of Medical Genetics and Political Science, University of California, San Diego): Nicholas Christakis and I have been studying social networks for some time. And we've made this argument that social networks are in our nature. And we've been turning our attention to this idea that social networks are not just social; they're also biological.
And so in this particular study, we actually hypothesize that this process, where we would tend to choose friends who are like us, not only exists at the social level but the biological, even the genetic level - that we would find that our friends not only share our behaviors and our interests; they might even share genes in common.
RAZ: We often hear, of course, the clich�d phrase: Birds of a feather flock together. But at the same time, they say opposites attract. I don't know which one to pick. What do you think?
Dr. FOWLER: We've looked at several genes, and we found one gene that exhibited this birds-of-a-feather property - that if you have it, your friend is also more likely to have it. We also found one where if you have a version of the gene, then your friend is likely to have the opposite version.
RAZ: Hmm.
Dr. FOWLER: I think what we're going to find is that there are different systems at play. And so with the dopamine gene that we found - that we share in common with our friends - there's a very intuitive explanation there, because it's related to this behavior that's very social behavior.
But for the opposites-attract genes, we might find things like genes that are related to the immune system. It's been known for some time that people tend to choose spouses who have different immune systems, because you don't want to be exposed to a disease that you're susceptible to. You spend a lot of time with your spouse, and so if they get it, then you're definitely going to get it. You want them to be able to fight off all those diseases that you can't fight off.
We expect that we might find this in friends as well because we spend a lot of time with our friends.
RAZ: This gene that you found is also the same gene that is associated with alcoholism. So how does that translate into picking friends?
Dr. FOWLER: Well, it's interesting. The genes that we would tend to find that are similar to us could have gotten similar in two different ways. One is that you actively choose friends who are like you in a way that's related to the gene. So if the gene is associated with drinking behavior, you might be slightly more likely to drink, and you might choose friends who like to drink because -
RAZ: That seems - right. That seems intuitive.
Dr. FOWLER: That seems - yeah, that's right. Another possibility is that maybe you're not choosing friends actively at all who are like you, but you're drawn to similar environments. And so if you have a version of this gene, you might be more likely to end up in a bar, where other people who have a version of this gene are also more likely to end up - and you might just happen to make friends with people who are near you.
RAZ: I guess this sort of gives new meaning to this notion of a drinking buddy.
Dr. FOWLER: Yes. And, you know, there are some larger points from this study here. I think people, they're very focused on, you know, which gene is it and what does this gene do? But there's a larger point here, which is that we might find ourselves not only biologically susceptible to a particular behavior, but if we are similar to our friends biologically, then we're also surrounded by other people who are susceptible.
And what you get in these networks is, you get these patchworks of localized susceptibility that are created by our genes than genes of those around us.
RAZ: But couldn't this be a problem if people who are alcoholics, for example, are attracted to other alcoholics? I mean, doesn't this actually make the situation worse?
Dr. FOWLER: Absolutely, and this is a point that we make. There can be feedback effects that we never thought of before when we were just looking at your genes and your behavior. Our genes not only influence us, but they may influence the genes of our friends - which in turn, has an additional effect on us.
So for example, the DRD2 gene variant that we study in this particular research has been associated with alcoholism. And if you have this gene variant, your friends are likely to have it, too. So you're not only more susceptible to alcoholism, but you're likely to be surrounded by friends who are susceptible as well.
RAZ: Do you use Facebook, Dr. Fowler?
Dr. FOWLER: I do. Absolutely.
RAZ: And so do your genes affect who you friend on Facebook, or is that...
(Soundbite of laughter)
Dr. FOWLER: I wouldn't say that you are genetically similar to all of the hundreds of friends that you might find yourself surrounded by. But certainly, among that group of people, the real friends, the people that you see every day in real life, I would expect there to be some similarity at the genetic level.
RAZ: That's James Fowler. He's a professor of medical genetics and political science at the University of California, San Diego and the author of "Connected: How Your Friends' Friends' Friends Affect Everything You Feel, Think and Do."
James Fowler, thank you so much.
Dr. FOWLER: Thank you. Transcript provided by NPR, Copyright NPR.