Whenever my best friends and I say the same thing in a group chat, we send the wavy dash emoji, 〰️, shorthand for we’re on the same wavelength! The concept gets tossed around in pop culture, though its meaning has always been more symbolic than scientific. Until Wednesday, there wasn’t much proof that friends who think the same thing shared anything but a set of references and some dumb inside jokes.

But in the journal Nature Communications, a team of Dartmouth College scientists provided evidence of what best friends have imagined all along:

“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,” said lead author Carolyn Parkinson in a statement on Wednesday. At the time of the study, Parkinson was at Dartmouth, and she’s currently an assistant professor of psychology and director of the Computational Social Neuroscience Lab at the University of California, Los Angeles.

fMRI of 'me'
fMRI machines are used to measure changes in brain activity in real time.

Taking 280 graduate students of varying degrees of friendship, which participants self-reported, Parkinson and her team wondered whether they could predict which individuals were closer friends based solely on their brain activity while watching the same set of videos. Their hypothesis, a slightly more refined version of pop psychology’s 〰️ theory, was that people who had closer social ties would respond to the videos in more similar ways, which in turn would be reflected in their patterns of brain activity. Plotting the self-reported relationships on a map, the researchers then got to work on finding the links between individuals’ brain activity.

In solitude, 42 of the participants watched the same series of politics, science, comedy, and music videos as the researchers observed their brain activity using an fMRI scanner, a device that tracks changes in blood flow in the brain. The idea is that certain regions of the brain surge with blood — that is, become more active — depending on how the individual responds to the video.

Across the participants, the parts of the brain linked to emotional responses, attention, and high-level reasoning became active, in varying degrees. The analysis revealed that, as the researchers predicted, the people with the most similar brain activity patterns were the closest friends. The strength of the correlation was directly related to the social closeness of the individuals, even when the researchers considered variables like handedness, age, gender, ethnicity, and nationality.

“We are a social species 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,” explained senior author Thalia Wheatley, Ph.D., a study co-author and psychologist at Dartmouth, in a statement.

Mapping the experimental data produced a social network that the researchers could use to predict how close individuals were, solely on the basis of their brain activity. As many of us have already intuited, it seems clear that the experiences we share with our closest friends do cause us to think and respond to things in similar ways, but the exact mechanisms that lead to synchronicity — is it a function of time spent together or laughter shared? — remain to be discovered.