Do you and your best friend finish each other’s sentences? Could you look at each other and know what you’re thinking? There may be a scientific explanation for that. A study from 2018 found that close friends share similar — or identical — brain patterns. So you’re not actually “reading each other’s minds”; your brain activity is imitating each other.
“Neural responses to dynamic, naturalistic stimuli, like videos, can give us a window into people’s unconstrained, spontaneous thought processes as they unfold. Our results suggest that friends process the world around them in exceptionally similar ways,” said lead author Carolyn Parkinson.
Science Says Friends Have Similar Brain Patterns
The Dartmouth College study involved about 280 students filling out a survey about friendships. After that, they studied the brain patterns of 42 participants as they watched videos, which included footage of a wedding ceremony, an astronaut at a space station, America’s Funniest Home Videos, and CNN’s Crossfire. These videos were intended to be engaging while evoking different responses. Therefore, each person’s brain responded to each clip. Since all of the participants watched them in the same order, it was easy to see the patterns or lack thereof. The researchers theorized that the different responses occurred because of different personalities or perspectives. 
“…We [track] how the response rises and falls over time to varying degrees as they watch the video in the scanner,” Parkinson said. “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.”
Strong Similarities Between Friends
Therefore, these fMRI scans discovered that close friends shared similar brain activity. The closer the friendship, the more identical brain activity. In fact, the researchers were able to accurately guess which students were friends. “We found really strong similarities in areas that are involved in things like how you allocate your attention,” said Parkinson. “Your friends might be paying attention to the same parts of the or deploying their attention in similar ways.” 
According to the authors, “People who responded more similarly to the videos shown in the experiment were more likely to be closer to one another in their shared social network, and these effects were significant even when controlling for inter-subject similarities in demographic variables, such as age, gender, nationality, and ethnicity.”
In other words, people gravitate to friends who are similar to them. So the more similar they are externally, the more similar their brain patterns are. And the more different two people are, the less similarity was in their brain activity. The researchers agreed, “We are exceptionally similar to our friends in how we perceive and respond to the world around us.”
“We are a social species and live our lives connected to everybody else. If we want to understand how the human brain works, then we need to understand how brains work in combination— how minds shape each other,” explained Thalia Wheatley, Ph.D., co-author and psychologist at Dartmouth. 
This discovery raised another question. Do these similar brain patterns develop as the friendship evolves? Or do people instinctively seek out people who are already similar to them? One of the researchers, Adan Kleinbaum, thinks both are the case. In fact, he adds that close friends with similar brain patterns “may be rewarding because it reinforces one’s own values, opinions, and interests.”
Continuing the Research
Furthermore, Parkinson continued this line of research with another study published in 2020. She and her colleagues studied a rural village in South Korea and conducted MRI scans to study 591 people’s social ties. The researchers found that individuals with more similar brain patterns were closer socially. As in, this reinforced the original study from 2018. 
“This is a groundbreaking study that focuses on the important idea that similarities across individuals might explain why some people have closer connections to one another than to others,” said Leonhard Schilbach, a psychiatrist at the LVR Clinic Düsseldorf in Germany.
However, since this research involved mostly older adults, Parkinson and co-author Yoosik Youm, a sociologist at Yonsei University in Seoul, plan to conduct another study on teenagers. This would involve a freshman class in high school. The researchers would monitor them before they bond with their peers and examine their brain patterns and as they evolve their social networks. This may answer the question of whether the similar brain activity is based on pre-existing similarities or how developed the friendship is. 
- “Similar neural responses predict friendship.” Nature Communications. Carolyn Parkinson, Adam M. Kleinbaum, Thalia Wheatley January 30, 2018
- “Friends Are Similar Deep in the Brain.” Psychology Today. Abigail Fagan. May 11, 2018
- “BEST FRIENDS REALLY DO SHARE BRAIN PATTERNS, NEUROSCIENTISTS REVEAL.” Inverse. Yasmin Tayag. February 2, 2018
- “Similarity in functional brain connectivity at rest predicts interpersonal closeness in the social network of an entire village.” PNAS. Ryan Hyon, Yoosik Youm, Junsol Kim, Jeanyung Chey, Seyul Kwak, and Carolyn Parkinson. December 29, 2020
- “Friends appear to share patterns of brain activity.” PNAS Blog. Amber Dance. January 15, 2021