Friends are genetically similar, researchers find.

The study, conducted by researchers at the University of California, San Diego and Yale University, found that friends who are not biologically related still share several genes.

"Looking across the whole genome," James Fowler, professor of medical genetics and political science at UC San Diego, said, "we find that, on average, we are genetically similar to our friends. We have more DNA in common with the people we pick as friends than we do with strangers in the same population."

Data for the study came from the Framingham Heart Study, a dataset which has genetic information of participants as well as their friends.

For the study, researchers looked at data on 1,932 unique subjects. They found that friends were more likely to be genetically similar than strangers.

An obvious explanation to the observation could be that people tend to choose to be friends with people who belong to the same ethnicity. However, researchers said that the data included people with primarily European ancestry. So, everybody in the study, whether friends or not, came from the same population.

According to the researchers, friends share about one percent of their genes and are as close in relation as fourth cousins, or people who share great-great-great grandparents.

"One percent may not sound like much to the layperson," Nicholas Christakis, professor of sociology, evolutionary biology and medicine at Yale, said in a news release. "But to geneticists it is a significant number. And how remarkable: 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."

Another surprising finding in the study was that similar genes between friends were evolving faster than other genes. Researchers said that this discovery might explain why human evolution has accelerated in the past 30,000 years or so. Turns out that even our social interactions act as an evolutionary force.

"The paper also lends support to the view of human beings as 'metagenomic,'" Christakis said, "Not only with respect to the microbes within us but also to the people who surround us. It seems that our fitness depends not only on our own genetic constitutions, but also on the genetic constitutions of our friends."

Previous research has shown that people are more likely to have spouses with similar DNA, as well.

The study is published in the journal Proceedings of the National Academy of Sciences and was supported by the National Institute on Aging and the National Institute for General Medical Sciences.