Friends are as genetically similar as fourth cousins, a study by Yale and the University of California has found.

A new study from Yale and the University of California suggests that your friends are as genetically close to us as members of our own family.

Scientists working on the study who compared the DNA of unrelated friends were intrigued to find they had the same genetic similarity as fourth cousins, or people who share great-great-great grandparents.

This was constant even after controlling for ethnic, cultural and geographical bias.

Nicholas Christakis, professor of sociology, evolutionary biology, and medicine at Yale said “We are somehow, among a myriad of possibilities, managing to select, as friends, the people who resemble our relatives.”
The researchers identified that friends are most similar in genes affecting the sense of smell and least compatible in genes controlling the immune system.

Recent studies have found that most people appear to choose partners based on differences in immunity.
It has been suggested that forming social groups with others who are able to withstand different pathogens reduces the spread of disease and increases the chances of survival.

Co-author James Fowler, professor of medical genetics at the University of California said “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.”
“However the mechanism for selecting friends or a mate based on genetic traits remains a mystery”.

Previous research has suggested that human pheromones in sweat carry important clues to genetic compatibility and are important to attraction. There are fears that modern medicines could dampen down this natural selection process.

Professor Fowler believes there may be a simpler explanation as to how people with similar genes meet and form friendships. People who like the scent of coffee, for example, may hang out at cafés more and so meet and befriend each other.

The researchers found that the genes that were more similar between friends seem to be evolving faster than other genes.

The findings were published in the journal Proceedings of the National Academy of Sciences.