Smithsonian.com

A new study debunks the idea that friendships are influenced by shared genes. Image via Flickr/DuncanCV

If asked how you’ve chosen the people you count as close friends, you might give one of several logical answers: that they’re people with whom you share interests or personality traits, or that you enjoy their company, or even that you became friends out of pure happenstance.

In recent years, though, as DNA sequencing has gotten increasingly quicker, cheaper and easier, some researchers have looked at individuals’ genes and come to a surprising finding—that people who are friends are disproportionately likely to share certain similarities in their genetic makeup.

Some scientists have even hypothesized that this is the result of an evolutionary advantageous strategy, similar to the theory of inclusive fitness for kin: As a prehistoric human, if you tended to stick together and support others with whom you share genes, helping them survive led to the survival of your own genes, even if you personally didn’t make it to pass your genes on to your offspring. Under that theory, we’re able to recognize our non-family genetic brethren and, consciously or not, become friends with them based on that similarity.

A group of social scientists led by Jason Boardman of the University of Colorado, however, was skeptical. They doubted whether genetic similarity was really driving the way we pick our friends—and had a suspicion that, instead, other social factors drove us to become friends with people we happen to share genes with. In order to test their hypothesis, they dove deep into data from the National Longitudinal Study of Adolescent Health, which gathers a wide range of data on thousands of middle- and high-school students across the country, on everything from risk-taking behavior to  particular genetic alleles to relationships with others.

Their findings, presented in an article published yesterday in the Proceedings of the National Academy of Sciences, strongly rebut the idea that genes determine friends and instead present an alternate idea: that social mechanisms simply put us into situations where we’re exposed to people we share genes with, and that we become friends with them based on this context. Ultimately, they write, “our work highlights the fundamental role played by broad social structures in the extent to which genetic factors explain complex behaviors, such as friendships.” In other words, genes alone aren’t sufficient to explain a complicated decision-making process like choosing friends.

The researchers came to this conclusion by using survey data to compare schools that varied in how many friends shared genetic similarities. Confirming previous work, they did find that, as a whole, a pair of students that listed each other as close friends tended to share certain alleles for particular genes.

However, they also found that students in schools with the greatest levels of social stratification and racial segregation were most likely to form genetically-similar friendships. As it turns out, students from the same ethnic background are much more likely to share these particular genetic alleles to begin with. This also holds true for social class, because ethnic background strongly correlates with economic standing in the schools included in the data set.

For the researchers, this paints a very different picture of how genes affect friendships than previously understood. Instead of students discerning the genes of others and forming friendships based on the DNA they shared, it’s much more likely that—in most American schools at least—they’re simply given the most exposure to other students like them starting at an early age. Instead of a sunny lesson about evolutionary altruism, they say, we’ve merely found an indirect reminder of the continuing degree of de facto segregation in schools.