We take it for granted that certain aspects of our social behavior—whether we chat easily with strangers at a party, for instance, or prefer to be a wallflower—are influenced by genetics. But now researchers at the University of California, San Diego, and Harvard University have shown that genes have a much broader sway, affecting the kinds of social networks people form and the positions they occupy in them.

James Fowler, a political scientist at U.C.S.D., and his colleagues studied the social networks of 1,110 adolescent fraternal and identical twins. They found that three aspects of the twins’ social networks appeared to be shaped by genetics. How many times each teen was named by others as a friend and how likely each youth’s friends were to know one another were both approximately 50 percent related to genetic factors. Whether a teen was located at the center of a network or toward the edge was about 30 percent genetic.

“We have innate characteristics that give us a tendency to gravitate toward one part of a network,” Fowler explains. “We vary in the tendency with which we’ll attract people as friends, and we vary in our tendency to introduce our friends to one another.” The genetic foundation uncovered in the study, he posits, is probably a broad combination of genes that are mostly linked to personality traits such as humor, generosity or extroversion.

Fowler and his co-authors have previously shown that health-related traits and behaviors, including obesity and smoking, seem to spread through social networks—people whose close friends gain weight, for example, are likely to bulk up themselves. Now that the researchers have shown that social networks have a genetic component, they are moving on to the next question: Is it possible that certain genes associated with obesity are not acting directly on the body but are influencing the structure of someone’s social network in a way that makes that person more likely to “catch” obesity? “Social networks might be a conduit through which genes act,” Fowler says. “It’s a pretty big and speculative hypothesis, but this is the first step.”

Note: This article was originally printed with the title, "Popularity Genes."

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