Surprising Links Between Friendships and Genes

We tend to choose friends who share our interests and outlooks, but our selections may have less conscious and more ancient roots. Recent research suggests friends share genetic similarities and that resulting social networks play an important role in human evolution.

In their paper "Friendship and Natural Selection," published in the Proceedings of the National Academy of Science, Nicholas Christakis, a physician and social scientist at Yale, and James Fowler, a professor of medical genetics and political science at the University of California at San Diego, write that the number of genetic markers shared by two friends is about what they would have if they were fourth cousins.

That amounts to about one percent of a human's genetic markers. That doesn't sound like much, but Fowler explains in a Washington Post story that has huge implications for human evolution. Researchers found the genes that friends have in common seem to be evolving faster than other genes, so our social environments and social networks could be a key evolutionary force.  

There's no gene for friendship, and no way to predict friendship among people because of a particular genetic trait. But the genetic data of two people provides clues to whether they will become friends. The researchers developed a genetic "friendship score" that suggests the likelihood of friendship. Individuals don't consciously recognize these similarities, but they are statistically measurable in huge data sets.

Friends are likely to share genes associated with the sense of smell. Being drawn to the same scent could attract us to certain environments, the authors suggest: people who like the smell of coffee might be drawn to coffee shops where they meet others who like the smell. The authors think our sense of smell may be one of the mechanisms humans use to identify genetically similar friends, though they emphasize more research is needed to discover how that happens.

Christakis and Fowler examined genetic information and details of social relationships documented among nearly 2,000 people who participated in the Framingham Heart Study that began in 1948. They and colleagues analyzed nearly 1.5 million markers of gene variations, and compared the data for pairs of unrelated friends and pairs of unrelated strangers. Because nearly all the people in the study had similar European origins, the findings weren't explainable by the tendency to gravitate to others of similar background.

Interestingly, friends are less likely to share genes associated with immunity to specific diseases, the authors note, and that that could be an evolutionary advantage. We're somewhat less susceptible to the things that sicken our friends. 

In their book Connected, Christakis and Fowler write that social networks are in our genes. After studying friendship networks among 1,110 twins drawn from national health data of 90,115 adolescents, they discovered that social network structure was influenced by genes: kids located at the center of their networks had a different genetic makeup than those located at the periphery, and those whose friends were closely connected had different genetic make than those with friends in divergent groups.

In the new paper they discuss the role of genes in a broader social environment where we interact and collaborate with friends and strangers. "Our results support the idea that humans might be seen as metagenomic not just with respect to the microbes within them, but with respect to the humans around them. It may be useful to view a person's genetic landscape as a summation of the genes within the individual and within the people surrounding the individual, just as in certain other organisms."

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