Think about how you met your closest friend. Maybe you lived down the hall, sat next to each other in class, or kept running into each other at the same coffee shop.
Most of us assume that proximity or shared interests is what sparks a friendship.
But a study published in 2025 suggests the real engine is running somewhere most of us would never think to look.
The ordinary puzzle that researchers decided to measure differently
Throughout the course of their lives, people typically encounter numerous other individuals with different interests, values and backgrounds, but not all of them will become good friends, life partners, or meaningful people in their lives.
Psychologists have spent decades trying to explain why.
Many factors are linked to friendship, including physical proximity and interpersonal similarities. But those factors can only go so far.
They tell you who ends up in the same room. They don’t tell you who clicks.
A team of researchers from UCLA and Dartmouth College decided to look for something deeper, and they found it before participants had exchanged a single word.
What happens inside the scanner before anyone has even said hello
The experiment had a deceptively simple design. Researchers recruited a cohort of incoming MBA students and placed each of them, one at a time, inside a brain scanner.
In the study, researchers scanned the brains of strangers before they even met, while they watched the same set of video clips.
Nobody interacted. Nobody spoke. The participants just watched.
Then researchers sent them out into the world and waited.
The clips covered a range of content, documentary, comedy and debate in style, food, sports and more in topic, designed to trigger different emotional responses in each viewer.
The question was whether those responses matched between people who had never met.
Eight months of real social life used as the measuring stick
Similarities in neural responses to movie clips, acquired before participants met one another, predicted proximity in a friendship network eight months later.
Researchers also examined changes in distances between participants in their shared social network, which resulted from the formation, persistence and dissolution of friendships, between two months and eight months after they met each other.
This was not a self-reported survey of who people thought they liked.
It was a mapped social network of real, mutually confirmed friendships, and the brain scans collected before anyone had met predicted who ended up inside it.
Compared to people who drifted further apart, people who grew closer over this six-month period had been more neurally similar as strangers.
The hidden mechanism: matching brains, not matching bios
The payoff of the study is specific and striking. It was not shared demographics, shared values, or shared neighborhoods that predicted lasting friendship.
It was neural similarity, the degree to which two people’s brains processed the exact same moment in the same way.
Participants with similar neural responses to movie clips were more likely to be friends rather than several degrees of separation apart, according to the study published in Nature Human Behaviour by researchers from the University of California, Los Angeles, and Dartmouth College.
The localization of these results suggests that pre-existing similarities in how people interpret, attend to and emotionally respond to their surroundings are precursors of future social closeness.
“The most striking finding of our study is that brain-to-brain similarity can predict future friendship,” senior author Carolyn Parkinson told Phys.org.
Even after controlling for variables such as age, gender, and background, neural similarity predicted both friendship formation and closeness.
What this means for the friends you haven’t made yet
None of this means you should discount the value of shared routines or common ground. Classic proximity research finds that a large share of friendships form among people living just doors away from one another, and regular contact remains a powerful engine of closeness.
That dynamic showed up clearly when teenagers spending less time in person with friends emerged as a growing social concern.
But the new research adds a layer beneath all of that.
Compared to relationships that form circumstantially and fade quickly, stronger connections may emerge from a deeper compatibility in how people interpret and emotionally respond to their surroundings.
In other words, some friendships that feel instant and effortless, the kind that survive distance and years, may be rooted in something wired in long before the first conversation.
At a time when nearly half of Americans say leisure and real social connection have dried up, the science suggests the brain may already know who belongs in your life. The trick is simply ending up in the same room long enough to find out.
