“Show me your friends and I’ll tell you who you are,” the adage goes, but what if I could predict your friendships based on your brain activity? This is exactly what scientists have done in a new study.

Popular wisdom abounds in sayings about how friendships are first formed, such as “birds of a feather flock together” and “friends are on the same wavelength.”

And, as it turns out, there is more than just a grain of truth to these age-old concepts.

A new study led by Carolyn Parkinson — who was formerly based at Dartmouth College in Hanover, MA, but who is now an assistant professor of psychology working at the University of California in Los Angeles — shows that the brains of friends respond in very similar ways to the same stimuli.

Friendship, like romance, is a scientific puzzle: why do we befriend certain people and not others? Is it because we tend to unconsciously choose people who are most similar to us, such as individuals of the same age, sex, or educational background?

Are friendships politically motivated, based on an instinctive understanding of social hierarchy? Or, as we may like to believe, are they explained by more complex, intellectual similarities?

The team’s study, published yesterday in the journal Nature Communications, argues that we tend to associate with people whose brains respond in a similar way to our own to the same preset stimuli.