"Neural responses to [stimuli such as] videos can give us a window into people's unconstrained, spontaneous thought processes," said Carolyn Parkinson, an assistant professor of psychology at the University of California Los Angeles in a Tuesday press release.
"Our results suggest that friends process the world around them in exceptionally similar ways."
The findings, published Tuesday in the Nature Communications journal, describe how researchers could accurately predict whether people were good friends or not depending on how their brain activity responded to a series of unfamiliar video clips.
The team first asked a first-year graduate school class of 279 students to fill out an online survey about their relationships with each other. Each student was handed a list with the names of every other student in the class and was asked to specify which peers they spent social time with outside of school during the last four months, which marks the start of their graduate program.
The survey results were interesting. The researchers were able to determine which students were friends, which were friends of friends, and so on: the researchers were able to accurately determine six degrees of social ‘distance' between people.
Forty-two students then participated in a functional MRI experiment. The researchers observed the participants' brain activity while they watched 14 unfamiliar video clips consecutively. The videos, which ranged from around 90 seconds to five minutes, reflected different genres and emotions. Some of the clips involved a soccer game, a documentary about baby sloths, political commentary and an astronaut's view of Earth.
When the researchers looked at the brain scans, they noticed that close friends had astonishingly similar neural activity in brain regions regulating emotion, attention and high-level reasoning.
To further test their findings, the researchers controlled for other factors that have been shown to increase friendship probability, including age, gender and ethnicity. Even in this scenario, the team was able to correctly predict friendship by looking at the MRI scans.
"Friends had the most similar neural activity patterns, followed by friends of friends. You can predict who people are friends with just by looking at how their brains respond to the video clips," the research team said in the statement.
"We are a social species and live our lives connected to everyone else. If we want to understand how the human brain works, then we need to understand how brains work, then we need to understand how brains work in combination — how minds shape each other," said Thalia Wheatley, an author of the study and an associate professor of psychological and brain science at Dartmouth.
However, it's a chicken-and-egg situation. Do your friends impact the way you think or do you choose friends with similar brain activity patterns to avoid having to change? The answer is yet to be determined. However, a 2014 study revealed that friends have more gene variations in common than strangers, suggesting that friends might really be the family we choose.