You may have more in common with your friends than a shared sense of humor or a penchant for the same bar. A new study finds that friends often resemble each other at the level of their genomes – though certain genes may influence people to bond with others who aren't like them.

Research has shown that birds of a feather do, in fact, flock together most of the time: People tend to become friends with people similar to themselves. Studies have also shown that that people's friendship styles – the number of friends they have and how central they are in their social network – are influenced by genetics.

These findings raised the question of whether genes play a role in whom we choose to become friends with, said James Fowler, a professor of political science and medical genetics at the University of California, San Diego. Fowler and his co-author Nicholas Christakis explored these theories in their book "Connected: The Surprising Power of Our Social Networks and How They Shape Our Lives" (Little Brown, 2009).

"We hypothesize that we're not just similar to our friends socially or behaviorally," Fowler. "We may also be similar to them biologically or genetically."

Friends and Genes

To find out if that theory holds water, Fowler and his colleagues culled data from two large, long-running studies, the National Longitudinal Study of Adolescent Health and the Framingham Heart Study. Both studies collect genetic information as well as data on who knows whom through friendship, marriage and other relationships.

The researchers tested the friendship pairs from the studies for six genes known to influence human behavior. Most of the genes tested influence the neurotransmitters serotonin or dopamine. (Serotonin is associated with mood and happiness, while dopamine plays many roles in motivation, reward-seeking behavior, learning and attention.)

People who live near each other tend to have similar genomes simply because they're more likely to have shared an ancestry. To control for this "population stratification," Fowler and his colleagues controlled for ethnicity and included siblings of the subjects in the analysis. Including data from siblings allowed the researchers to see how much of the correlation between genes and friendship was due to population stratification and how much was a real effect.

Of the six genes tested, two clustered among friends in unexpected ways. The first, the DRD2 gene, codes for a dopamine receptor in the brain, and some variants have been associated with alcoholism in previous studies. People with similar variants of DRD2 tend to stick together, the researchers report this week in the journal Proceedings of the National Academy of Sciences.

A second gene, the CYP2A6 gene, had the opposite effect. People with similar variants of the CYP2A6 gene tended to befriend people with different CYP2A6 gene variants. Little is known about this gene, the researchers wrote. It's responsible for a protein that metabolizes several compounds, including nicotine. One earlier study associated the gene with personalities that are open to new ideas.

A sea of genes

While the CYP2A6 gene effect is somewhat mysterious, it's not hard to imagine that a person with alcoholic tendencies might be drawn to people and places that non-drinkers would avoid, Fowler said. Researchers can't yet say if people's genes influence their friendships mostly by pushing people into environments with like-minded (and like-genomed) individuals, or if people are choosing pals based on genetically influenced personality traits. The finding that opposite CYP2A6 variants attract suggests, however, that some individual choice is involved, Fowler said.

"With the negatively correlated gene, we can rule out the possibility that you and I have been drawn to the same environment because we have the same genotype, because we don't have the same genotype," Fowler said.

The findings are a "first step," Fowler said. He and his colleagues hope to repeat the study with whole-genome scans to test all 25,000 genes humans are estimated to have instead of just the six the researchers initially tested. 

If the results hold (or if more influential genes are discovered), they could add another wrinkle to the mystery of gene-environment interaction. If genes influence a person's social environment, that social environment could, in turn, influence the person's behavior: Imagine an alcoholic who befriends other alcoholics. Hanging out in an alcoholic crowd could encourage the person to drink more.

But people may also choose friends under the subtle influence of evolution, Fowler said. Previous research has found that people tend to choose spouses with different immune-system genetics than their own, perhaps to increase their protection against contagious diseases (if you can't fight off a bug, you want to be sure your partner won't get ill and transmit that same bug to you). Perhaps the CYP2A6 gene, while not linked to the immune system, plays a similar protective role.

"We live in a sea of the genes of others," Fowler said. "We are not just influenced by our own genes… We think that we are going to find more and more biological processes underlying the social networks that we live in."

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