People may unsuspectingly choose friends who have some DNA sequences in common with them, a new analysis finds.
Researchers compared gene variations between nearly 2,000 people who were not biologically related, and found that friends had more gene variations in common than strangers.
The study lends a possible scientific backing for the well-worn clichés, "We're just like family," or "Friends are the family you choose," the researchers said.
"Humans are unique in that we create long-term connections with people of our species," said Nicholas Christakis, a social scientist at Yale University involved in the study. "Why do we do that? Why do we make friends? Not only that, we prefer the company of people we resemble."
The researchers did the study because they wanted "to provide a deep evolutionary account of the origins and significance of friendship," Christakis said.
The new study is based on data from the Framingham Heart Study, which is a large, ongoing study looking at heart disease risk factors in the people living in one town: Framingham, Massachusetts. The researchers looked at data on people's DNA, as well as who was friends with whom.
After analyzing almost 1.5 million markers of gene variations, the researchers found that pairs of friends had the same level of genetic relation as people did with a fourth cousin, or a great-great-great grandfather, which translates to about 1 percent of the human genome.
The most common gene shared by friends was the "olfactory" gene, which is involved in a person's sense of smell.
Although 1 percent may not sound like much, Christakis said in a statement, "to geneticists it is a significant number.
He said, "Most people don't even know who their fourth cousins are, yet we are somehow, among a myriad of possibilities, managing to select as friends the people who resemble our kin."
The findings suggest that choosing friends who share similar genes is a behavior that may have contributed to human evolution, the researchers said.
The study was published Monday (July 14) in the journal Proceedings of the National Academy of Sciences.