Friends who are particularly close and feel like family often share genes with one another, new research reveals.

Yale and University of California (UC) researchers examined 1.5 million genetic markers of 1,932 people, using data from the Framingham Heart Study. This earlier study was rare in collecting both genetic information, as well as data on friendships. This allowed investigators to compare the genetic codes of friends with strangers.

"Looking across the whole genome, 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," James Fowler, professor of medical genetics and political science at UC San Diego, said.

Ancestry and race were taken into account in the study, in order to eliminate the effect of people befriending those of the same race. Most of the subjects in the Framingham study were of European ancestry. Although this could cause challenges with some studies, the lack of genetic diversity benefited this research.

Friends share around one percent of their genes, which is significant in terms of genetic code. That is roughly the percentage shared by people who had a pair of the same great-great-great grandparents, making them fourth cousins.

"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," Nicholas Christakis, professor of sociology, evolutionary biology, and medicine at Yale, told the press.

Genetic similarities could have assisted friends in the days when humans were hunters and gatherers. Those sharing similar levels of hunger would be on nearly-identical hunting schedules, aiding food gathering. People with unusual traits could seek out others with the same characteristics with whom they could socialize.

A friendship score was developed by the team, allowing the researchers to be able to predict how well people will get along, based on their genetic markers. This was found to be as accurate as similar genetic tests for schizophrenia and obesity.

Sense of smell was most similar among close friends, while immune system responses were least-alike. This could have provided a better chance for one person to be able to care for another during times of illness.

In what may be one of the remarkable conclusions of this research, Christakis and Fowler noted the genes shared between friends are evolving faster than other genes. This could explain the acceleration in evolution in humans over the last 30,000 years, the study suggested.

The actual mechanism by which we select our friends based on genes remains a question.

Study of the role of gene in determining friendships was published in the Proceedings of the National Academy of Sciences.

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