Friends Are Similar Deep in the Brain

New research suggests that friends process the world in the same way.

Posted May 11, 2018

Jacob Lund/Shutterstock
Source: Jacob Lund/Shutterstock

We tend to be similar to our friends. But you knew that. Aristotle recognized it thousands of years ago. And pretty much everyone is familiar with the cliché about birds, feathers and flocking together. When scientists have tested the assumption over the years, they have found that, sure enough, people are more likely to befriend others of the same gender, age, ethnicity, and so on. 

Still, it’s surprising when new research reveals just how deep the similarities between friends can be. Several years ago, it was reported that we resemble our friends at a genetic level that’s akin to being distant cousins. And now there’s news that the likeness between friends extends to how our brains process the world. 

A first-of-its-kind report in Nature Communications earlier this year revealed that similar neural responses to movies predicted friendships. In other words, based on brain patterns alone, scientists could guess which participants in their study were friends and which were not. The further away from each other in a social network two people were, the less similar their brain responses were. The scientists concluded: “We are exceptionally similar to our friends in how we perceive and respond to the world around us.”

The study was done by Carolyn Parkinson, a neuroscientist at the University of California Los Angeles, and her colleagues at Dartmouth College, Adam Kleinbaum and Thalia Wheatley. First, they characterized the social network of every member of a graduate school program (279 in all) to work out the relationships of each student to every other student. Such an analysis produces a web of connections that’s a scientific version of the Six Degrees of Kevin Bacon game: It tells you how many steps removed each person is from every other person, or who is friends with whom, and whose friends are friends. 

Then they put 42 members of the larger group into a functional magnetic resonance imaging machine. During the fMRI study, the participants watched a collection of video clips that was akin to “channel surfing.” The videos were designed to be captivating and evoke a variety of responses. Because each person saw the same clips in the same order, any differences in their brain’s responses were theorized to be the result of differences in personality or perspective. 

For the imaging part of the research, the scientists began by dividing the brain into 80 different anatomical regions of interest — for example, the amygdala, which is associated with threat response and emotion. For each defined region, “we [track] how the response rises and falls over time to varying degrees as they watch the video in the scanner,” Parkinson explains. “We have these sets of time series for each person and then we’re correlating the time series from corresponding brain regions for each pair of people to look at how similar their responses were within each brain region.”   

Parkinson was surprised by how dramatic the effects were. “We found really strong similarities in areas that are involved in things like how you allocate your attention,” she says. “Your friends might be paying attention to the same parts of the [video] or deploying their attention in similar ways.” 

This was the first time anyone had tried such a technique, and Parkinson calls the study a “proof of concept.” Now that they know their technique can work, the scientists aim to answer much larger questions. Chief among them: What comes first? Are friends with similar brain patterns drawn to each other, or do their neural responses change because they’re friends and spend time together? “We can’t infer much about the direction of the effect from these data,” Parkinson says. 

She and her colleagues are now launching a longitudinal study they hope will shed light on this question. This time around, they are recruiting a group of people entering an academic program and scanning some of their brains before they have the opportunity to meet any of their fellow students. Then they will follow the students to see who pairs up. After a few months, they will repeat the brain scans to see if they can pinpoint how an individual’s responses change after exposure to new friends. They’ll also assess whether those whose responses were more similar to begin with turn out to be drawn to each other. While she won’t be able to say much more until this study is complete in a year or two, Parkinson says, “It seems very likely that [the effect] must go both ways.” 

As to the significance of these findings, Parkinson can only speculate. “There are advantages to surrounding yourself with similar others,” she says. “Potentially, like people might share similar goals and assumptions and experiences, and that could help foster cohesion, empathy, and collection action. It’s interesting to think about. It’s such a pervasive tendency across so many eras and locations and societies to surround yourself with similar others.” 

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