It's often said that friends are a family you choose, but a new study published in the journal Proceedings of the National Academy of Sciences proves there's more than just choice among friendship. Friends are as genetically close to us our own family members.
Researchers compared the DNA of friends who had no family ties with each other, or were unrelated, and found that they had the same genetic similarity as fourth cousins, they are the people who share the same great-great-great grandparents. Even after controlling ethnic, cultural and geographical biases, the finding remain the same.
Nearly 1.5 million markers of gene variations were observed, and the study showed among friends a similarity in the genes affecting the sense of smell. They also observed that they were least compatible in genes controlling the immune system. 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 kin."
Recent studies have found that people appear to choose partners based on differences in immunity. It has been suggested that this type of group forming with people who are able to withstand different pathogens increases the chance of survival and reduces the spread of diseases.
"Looking across the whole genome we find that, on average, we are genetically similar to our friends," said co-author James Fowler, professor of medical genetics at the University of California. Explaining that people have more DNA in common with the people they pick as friends than they do with strangers in the same population. Researchers believe this pattern indicates towards "evolutionary advantages"
However, it is still unclear as to how the mechanism of selecting similar gene friends or a mate occurs. Since prehistoric periods sweat was an important factor in attracting a partner, researchers also believed that the pheromones in the sweat carry clues to genetic compatibility and are important to attract a partner. They also fear that modern medicine may dampen down this natural selection process.
Referring to an experiment called the 'Smelly Tshirt experiment' performed by Swiss zoologist Claus Wedekin in 1994, Professor Dan Davis of the University of Manchester, who recently published The Compatibility Gene, said that though the results previously were clear, attempts to repeat the experiment now had shown mixed results.
"It is fascinating that there seems to be some underlying biology going on, but this area is very controversial and the evidence of this in humans is still extremely weak. It is certainly feasible that genetics is driving some kind of selection behaviour. We see it in animals but in humans there are so many other factors to consider that the findings have been inconclusive," he said.
However, Fowler believes that there may be an underlying simpler explanation to explain the process of how people with similar genes become friends. He said that people who like the scent of coffee, for example, may hang out at cafés more and so meet and befriend each other.
The study was conducted on 1,932 subjects and pairs of unrelated friends and unrelated strangers were compared. They found that the similar genes between friends were evolving at a faster pace than the other genes. Fowler and Christakis said that this could be a possible reason for the rapid evolution over the last 30,000 years, suggesting that the social environment is a highly determining factor, evolving at an equal pace.
"It seems that our fitness depends not only on our own genetic constitutions, but also on the genetic constitutions of our friends," added Christakis. The team also developed a 'friendship score' that will determine the compatibility of two individuals and predict if they would be good friends based on genetic similarity.