They say that birds of a feather flock together, and human beings tend to choose friends that look like them. Now surprising new results of a genetic comparison reveals that friends share some amazing genetic similarities, raising the question of whether we choose our friendships or are they, at least in part, determined by genetics.

Researchers at Yale and the University of California, San Diego (UCSD), report this week in the Proceedings of the National Academy of Sciences that “friends may be a kind of ‘functional kin.’” In other words, we tend to befriend people who not only look similar to us but also have certain genetic similarities to us as well. These genetic similarities are complex too.

“Your friends don’t just resemble you superficially, they resemble you genetically,” said Nicholas A. Christakis, a co-author on the report. Christakis is a physician and social scientist at Yale University.

The scientists used comparisons of single-nucleotide polymorphisms (SNPs) among friends and strangers to determine to what extent and in which genetic regions or genes the similarities and differences occur. The similarities in genetic abberations, the SNPs, were on the level of fourth cousins, the researchers note.

“We can do better than chance at predicting if two people are going to be friends if all we have is their genetic data,” said James Fowler, senior author and professor of medical genetics and political science at UCSD.

What was particularly interesting among the findings of the study is that the genetic similarities were mainly in two types of genes: olfactory, or sense of smell, and immunity. Even so, the overall genetic similarity amongst friends is only about one percent. There are no single genes that link friends, either. Still, the implications are tremendous.

“Social networks are an important engine for human evolution,” Fowler said. “Our friends are sort of like family members. They’re functional kin.”