HMN 2025: How Strangers whose brains respond alike to movie clips often become friends later

People often bond with strangers over the books they read or the movies they watch and build friendships that last. Scientists may now have some insight into why this happens. A study published in Nature Human Behaviour found that participants who responded similarly to the same movie clips even before meeting were more likely to become friends later.

As part of the experiment, MRI brain scans were taken of 41 graduate students who had never met each other before, while they were shown clips of movies based on science, food, sports, environment, and social events.

A total of 214 brain regions were analyzed—200 cortical regions associated with functions, such as movement, perception, and sensory processing, and 14 subcortical regions that control movement, autonomic functions, and emotions.

The scans showed that those who became closer over time and were direct friends (just one degree apart in their social network) already showed strikingly similar brain activity in the left orbitofrontal cortex, a region associated with social decision-making. These findings support the idea of neural homophily, which suggests that people tend to connect with others whose brains respond to things in a similar way.

From hunter-gatherer communities to modern societies to online communities, humans have consistently shown a tendency to flock together with people based on shared similarities. The likeness can be based on characteristics such as age, gender, or ethnicity, as well as on shared behaviors and preferences.

Studies have tried to link self-reported personality traits to social closeness, but the evidence is weak or inconsistent, suggesting standard surveys aren’t enough to decode the subtler similarities that define friendships. More recent studies opted to explore whether brain anatomy can provide the clues required to understand these social behaviors.

Neuroimaging studies have found that close friends share similarities in brain structure, neural responses to stimuli such as movies, and even patterns of neural activity at rest. However, most of these studies were cross-sectional, meaning they couldn’t definitely say whether neural similarity preceded the friendship or only emerged among friends due to social influence or shared experiences after the connection was made.

For this study, the researchers designed an experiment that captured pre-existing neural similarities before the participants met each other and then tracked their social connections to check if similar brain responses could predict who would become closer friends.

The MRI scans of the 41 participants were taken shortly after their arrival on campus to minimize the chances of interaction. Two months later the participants and their 246 classmates were asked to take an online survey asking them to identify classmates they spent time with for social activities. The graduate class was again asked to fill out the same form six months later.

The results indicate that people who became closer friends over the period of six months had shown greater similarity in their responses to clips as strangers than those who drifted apart. The researchers note that this pattern could not be explained by demographics or shared interests alone. Thus establishing the fact that similarity in key brain regions was especially predictive of future friendship and social closeness.

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