Brain activity can predict whether strangers will become friends

Strangers are more likely to become friends if their brains respond similarly to movie clips, suggesting neural activity can predict relationships.

Across cultures, humans tend to surround themselves with like-minded people. This phenomenon, known as homophily, explains why past studies have found neural similarities amongst friends. But researchers didn’t know whether this is because friends grow more alike over time or because people gravitate towards those with similar thought processes.

Carolyn Parkinson at the University of California, Los Angeles and her colleagues collected brain scans from 41 students before they began a graduate programme. During the scans, participants watched a series of 14 movie clips spanning a range of styles such as documentary or comedy, and various topics including food, sports and science. The researchers then analysed the neural activity of each participant across 214 brain regions.

The participants – along with the 246 other students in their programme – completed a survey two months afterwards and again after an additional six months asking who they enjoyed spending their free time with. People who were friends at the eight-month mark had more similar responses in a portion of the left orbitofrontal cortex, a brain region involved in processing subjective value, than those who were furthest apart in the social network – that is, friends of friends of friends. This effect remained significant even after accounting for similarities in taste based on how much people rated their own enjoyment or interest in the movie clips.

Two months into the programme, the neural similarities between friends and non-friends were no different, suggesting people may initially form friendships based on proximity before finding closer friends over time. This was further supported when the researchers looked at how friendships changed between the two surveys. Participants who grew closer over this period had significantly greater similarities in the activity of 42 brain regions than those who drifted apart. The link remained significant even after accounting for factors such as age, gender and hometown. “Sociodemographic factors, at least in terms of what we were able to measure here, just seem to explain part of the picture,” says Parkinson.

Many of these regions are involved in brain networks that direct attention and help us make sense of stories, suggesting friendships form, in part, because of similarities in how people understand the world around them, says Parkinson. “People whose thinking processes are more similar find it easier to get on,” says Robin Dunbar at the University of Oxford. “When they say something, they just know what the other is thinking because it is how they think themselves.”

Dunbar, who wasn’t involved with the research, doesn’t find these results surprising. Rather, they confirm what many have long suspected – “that like attracts like, rather than people thrown together by accidents draw closer in their traits”, he says. “In other words, close friends are born, not made.”