During video-clip watching, friends show the highest neural similarity (leftmost brains). As social distance increases, neural similarity decreases. Warmer colors indicate high neural similarity; cooler colors indicate low neural similarity.

Carolyn Parkinson, Adam Kleinbaum, and Thalia Wheatley

Humans have long suspected—and researchers have confirmed—that people often become friends with those similar to themselves. Demographics alone, however, do not predict everything, as people do not always befriend those from the same age group or ethnicity. Recently, non-invasive neuroimaging has provided researchers with an alternative method to investigate why people connect with some individuals better than others.

Dartmouth College social neuroscientist Thalia Wheatley and her colleagues wondered about the biological roots of friendship and whether friends might perceive and process the world in similar ways. Her team tested these questions in a two-part experiment. To begin, they surveyed a cohort of 279 first-year university graduate students enrolled in the same leadership course. In an online survey, the students, who had arrived on campus three or four months earlier, disclosed their social relationships to fellow students in the program. Researchers paired this information with the students’ demographic data and analyzed students’ social ties to construct a web of relationships that showed the degrees of separation between students. For example, they only considered two individuals “friends” if both reported the friendship.

In the next phase, researchers used functional magnetic resonance imaging (fMRI) to measure the brain activity of forty-two of these students. Such technology allows researchers to “look under the hood and see how the mind works moment by moment,” says Wheatley. All of the students in this phase watched the same series of video clips while undergoing fMRI scanning. Presented sequentially in a TV “channel surfing” style, the videos covered diverse subjects, from sports and music to nature and politics, and included various types of humor and emotional appeals. The fMRI scans indirectly measured what parts of the students’ brains were active during different points in the videos.

When the research team mapped how similarities in brain activity manifested within the broader social network, friends showed the greatest similarities in neural activity during video clip watching. Furthermore, regions of similarity appeared all over the brain, including areas involved in attention, reasoning, and the processing of emotion, sounds, and visual information. As social distance between participants increased, neural similarity dropped off.

The scientists also constructed an algorithm to predict social relationships based on similarities in brain activity, while also taking into account demographic similarities or differences. Their model could predict social relationships (e.g., whether participants were friends, friends of friends, etc.) based on those neural similarities with an average accuracy of forty-one percent, a significant improvement over chance, which predicts with only twenty-five percent accuracy.

“What we don’t know,” says Wheatley, “is why friends cluster in this way.” She hopes future studies could reveal whether friends who similarly experience the world tend to seek each other out or friends become more alike over time—or perhaps both. (Nature Communications)

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