Friends' similarities extend to their genes

Wednesday, 16 July, 2014

It appears that we share more with our friends than we thought, with US researchers discovering that friends who are not biologically related still resemble each other genetically. The study, from the University of California, San Diego and Yale University, has been published in the Proceedings of the National Academy of Sciences.

The researchers conducted a genome-wide analysis of nearly 1.5 million markers of gene variation. They drew on data from the Framingham Heart Study, which contains information on both the friendships and genetics of its participants. Using 1932 subjects, the team compared pairs of friends with pairs of strangers from the same population, none of which were related to each other.

“Across the whole genome, friends’ genotypes at the single nucleotide polymorphism level tend to be positively correlated (homophilic),” the authors said. “In fact, the increase in similarity relative to strangers is at the level of fourth cousins”, with friends found to share about 1% of their genes.

“One per cent may not sound like much to the layperson,” said co-author Nicholas Christakis, “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.”

The researchers found that friends are quite similar in gene variants having to do with sense of smell, but are less likely to share gene variants relating to immunity against specific diseases; in the case of the latter, this reduces interpersonal spread of pathogens. The team suggested that “these systems may play a role in the formation or maintenance of friendship ties” and that friends “may be a kind of ‘functional kin’”.

“Homophilic genotypes exhibit significantly higher measures of positive selection, suggesting that, on average, they may yield a synergistic fitness advantage that has been helping to drive recent human evolution,” the researchers said. For example, said co-author James Fowler, “The first mutant to speak needed someone else to speak to. The ability is useless if there’s no one who shares it.”

It is thus suggested that the social environment itself is an evolutionary force, perhaps helping to explain why human evolution appears to have sped up over the last 30,000 years. Fowler noted, “Human beings are one of the few species who form long-term, non-reproductive relationships with other members of our species. This role of affiliation … ties into the success of our species.”

The researchers were able to develop a “friendship score”, which they can use to predict who will be friends, based on their genes, the same way scientists may predict a person’s chances of obesity or schizophrenia. But as for the mechanisms which drive us towards the people we choose as our friends, further research is still required.

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