Friends often look alike. The tendency of people to forge friendships with people of a similar appearance has been noted since 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, said the study published 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.

The resemblance is slight, just about 1 percent 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.”

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. The research suggests that genetic factors are like a subtle breeze, 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.

A wrinkle: You tend to pick friends whose immune systems are dissimilar. This might seem to contradict the initial hypothesis, but it reinforces the broader thesis that there could be a subtle biological influence on friendships. The preference for people with different immunity may have advantages. If you are immune to pathogen X, and your friend is immune to Y, neither of you can catch either the diseases from the other.

How, exactly, do we sniff out these biologically congenial people? That’s not clear. “Social networks are an important engine for human evolution,” Fowler said. “Our friends are sort of like family members. They’re functional kin.”

Washington Post