If you've ever been told that you and your best friend are "honestly just so alike" or "could almost be sisters," it may not have been an exaggeration. A new study has found that friends are more likely to share similar genetics than strangers.
The study, published in the journal Proceedings of the National Academy of Sciences, consisted of researchers from Stanford, Duke, and the University of Wisconsin—Madison who compared the genetics of around 5,500 American teenagers.
Pairs of friends were found to be more genetically similar than random pairs of strangers, and had around two-thirds of the genetically similarity of married couples, in same-gender and mix-gender friend groups.
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The study's authors say the genetic similarities could partly be the result of "social homophily." This process is essentially one of like attracting like; it suggests that people are naturally drawn to others with similar characteristics which derive from their genetics, such as height, BMI, and intelligence.
However, the authors also say that external social structures could play a larger part in explaining why friends tend to be genetically similar. According to this theory, a pair of friends could have similar genetics because they're both members of a community, or students at a school where people tend to come from broadly similar backgrounds. So, you might be drawn to the people your life clusters you with, and because those clusters stay the same for many you find genetic similarities.
Speaking to Time, the study's lead author Benjamin Domingue said, "Are individuals actively selecting to be around people who are like them, or is it due to impersonal forces, such as social structures, that we all are affected by?"
"Our evidence, with respect to friends, suggest that it’s largely the effect of social structures."
So next time a friend rubs you up the wrong way, try to bear in mind that it could be because you two are just too damn similar.