What friends' genes say about you

There's a lot of focus these days on tailoring treatments for diseases to individuals based on their genes, but friends' genes might be just as relevant, scientists argue.

It turns out that there are genetic correlations among people in the same social group, a new study in the journal Proceedings of the National Academy of Sciences finds.

"We’re not just a product of our own genes, but those of others around us," said study co-author James Fowler, associate professor at the University of California, San Diego. Fowler and Dr. Nicholas Christakis of Harvard, another study collaborator, previously penned a book called Connected detailing how everything from obesity to happiness to loneliness seems to spread in social networks.

The genetic links found in this new study go beyond the idea that you might be friends with your second cousins other distant relatives. Instead, this research shows that you may actually share predispositions toward certain behaviors with your friends, regardless of whether they share your ancestry.

Long before this relatively recent scientific inquiry into how members of social networks influence each other, organizations such as Alcoholics Anonymous and Weight Watchers were already taking advantage of those principles. In Alcoholics Anonymous, people susceptible to drinking come together with the purpose of quitting, and group members help each other in their determination to abstain from alcohol.

And to some extent, people are already sharing their genetic information, Fowler said. Some services allow users to post results of their genetic tests for friends and family to see.

Fowler and colleagues examined data from the National Longitudinal Study of Adolescent Health.  They found that people who have the genetic marker DRD2, associated with alcoholism, tend to befriend each other, and those who do not have this marker tend to associate with others who lack it.

On other other hand, they found a different relationship with a gene involved in the system that metabolizes foreign substances, such as nicotine.  It seems that people with this particular marker befriend people who don't have it. Fowler suggests that in terms of having social support, there might be some benefit in having close contacts who are better with dealing with toxicity in the environment than you are.

"It’s possible that part of the story is that our fitness as individuals depends not just on our own genes, but on the genes of other people," Fowler said.

It's true that people who drink a lot might naturally gravitate toward each other anyway at a bar, for example. But the result that opposites attract for the other marker may reveal a deeper connection: You also choose friends who would not necessarily find themselves in the same venues that you would, indicating that there could be something going on at the genetic level, too, Fowler said.