This negative association, called heterophily, is illustrated by the adage "opposites attract," Fowler said. It even affects the choice of a spouse, he noted, indicating that people usually choose mates with a different but complementary immune system to maximize their own health.
"As a child, I would always wonder how both of these [adages] could be true," he said. "This study emphasizes it's going to be true for different attributes."
The study, published online in this week's issue of the Proceedings of the National Academy of Sciences, makes the argument that social networks are not just randomly chosen, but are dictated at least somewhat by genetics, he said.
"We've never known before that we have that correlation between friends," Fowler said. "We live in a sea of the genes of others. We shouldn't just be thinking of the impact our genes have on our outcomes, but what the impact of our friends' genes are on our outcomes."
Ting Wu, a professor of genetics at Harvard University, said she finds the study exciting because genetics has been taught in a "very strict fashion" for many decades.
"We've been trained to think about genes in a particular way," Wu said. "Now we have the time and luxury to look for phenomena we haven't studied."
"It makes sense that what makes us the way we are can influence how we choose our friends," she added. "We know our friends influence us."
For more information about genetics, visit the U.S. National Institutes of Health.
SOURCES: James Fowler, Ph.D., professor, medical genetics and political science, University of California, San Diego; Ting Wu, Ph.D., professor, genetics, Harvard University, Boston; Jan. 17-21, 2011, Proceedings of the Natio
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