Friends David Gyngell, left, and James Packer. Photo: Newslink

Washington: Friends often look alike. The tendency of people to forge friendships with people of a similar appearance has been noted since the time of Plato. But now there is research suggesting that, to a striking degree, we tend to pick friends who are genetically similar to us in ways that go beyond superficial features.

For example, you and your friends are likely to share certain genes associated with the sense of smell.

Our friends are as similar to us genetically as you'd expect fourth cousins to be, according to the study published on Monday in the Proceedings of the National Academy of Sciences. This means that the number of genetic markers shared by two friends is akin to what would be expected if they had the same great-great-great-grandparents.

"Your friends don't just resemble you superficially, they resemble you genetically," said Nicholas Christakis, a physician and social scientist at Yale University and a co-author of the study.

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The resemblance is slight, just about 1 per cent of the genetic markers, but that has huge implications for evolutionary theory, said James Fowler, a professor of medical genetics and political science at the University of California at San Diego.

"We can do better than chance at predicting if two people are going to be friends if all we have is their genetic data," Dr Fowler said.

This is a data-driven study that covers hundreds of friendship pairs and stranger pairs, plus hundreds of thousands of genetic markers. There's no single "friendship" gene driving people together. There's no way to say that a person befriended someone else because of any one genetic trait.

The research suggests that genetic factors are like a subtle breeze in the background, strong enough to be measured statistically in a big data set even if people in their day-to-day lives aren't consciously aware of it.

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Research: Human friendships based on genetic similarities

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