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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, NH, 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.

“Neural responses to dynamic, naturalistic stimuli, like videos, can give us a window into people’s unconstrained, spontaneous thought processes as they unfold. Our results suggest that friends process the world around them in exceptionally similar ways.”

The study follows in the footsteps of previous research by Dartmouth College, showing that, when we cross paths with a person we already know, our brain spontaneously signals their hierarchical position in our private social network.

Parkinson and colleagues first recruited 279 graduate students who were quizzed about their friendships, indicating the people in their cohort that they were close to. Then, the researchers estimated the “social distances between individuals” within the cohort’s network using “mutually reported ties.”

In order to explore how brain responses to the same set of stimuli influenced the likelihood of forming a friendship, the scientists conducted functional MRI (fMRI) scans on a subset of 42 participants while they were shown a series of 14 videos.

The videos were shown in exactly the same order to all the participants who underwent brain scans.

Next, the researchers looked at pairs of students, comparing their fMRI-measured responses with the stimuli that they were exposed to. This was done to see whether or not participants who identified as friends also had similar neural responses.

The analysis confirmed that friends had the most similar neural responses overall, with their brain activity indicating compatible emotional reactions, attention-related feedback, and complex reasoning.

These findings remained the same after the researchers adjusted for relevant variables, including participants’ handedness (whether they were left- or right-handed), age, and biological sex.

Moreover, the study revealed that neural responses could also be used to determine the social distance between two people.

“We are a social species,” notes senior study author Thalia Wheatley, “and live our lives connected to everybody else. If we want to understand how the human brain works, then we need to understand how brains work in combination — how minds shape each other.”

The scientists would now like to find out where the causality lies when it comes to friendships. Do we seek people who share our worldview or, to the contrary, do our perceptions shift when we become involved with a particular social group?

A third possibility that they are taking into consideration is that the truth may lie midway: we may both seek out people who are similar to us and shift our perception to better fit our new social ties.