New research finds the brains of close friends tend to respond to stimuli in very similar ways.

(Photo: Craig Whitehead/Unsplash)

How do we choose our friends? Beyond family ties, there has always been something of a mystery as to why we form close bonds with certain individuals. Sometimes, it seems, two people just click.

According to a new study, that fantastic feeling reflects the fact you and your bestie are, neurally speaking, mirror images of one another.

"These results suggest that we are exceptionally similar to our friends in how we perceive, and respond to, the world around us," writes a research team led by psychologist Carolyn Parkinson of the University of California–Los Angeles.

So similar, in fact, that it's possible to predict the intensity of a friendship by analyzing how two people's brains respond to a range of stimuli. The "I barely know the guy" defense in criminal conspiracy cases may have just suffered a big blow.

The study, published in the journal Nature Communications, utilized a survey in which "an entire cohort of students in a graduate program"—279 in all—"indicated the individuals in the program with whom they were friends." The researchers charted the friendships reported by both parties, as well as students in their social circle (friends of friends), and more distant relations (friends of friends of friends).

A subset of 42 students then participated in a study in which they watched a series of video clips while their brains were scanned using fMRI technology. The clips, which ranged from 90 seconds to four minutes, were highly varied.

They included footage of an astronaut describing the view of Earth from space: a cable-news debate about Barack Obama; footage of a gay wedding ceremony; highlights from a soccer match; a comedy skit in which grown men portrayed teenage girls; and excerpts from the television program America's Funniest Home Videos.

The results were unambiguous. Neural responses to the clips—that is, which sections of the brain were activated by each—"were significantly more similar among friends than among people farther removed from one another in their real-world social network," the researchers report.

Brain regions implicated in motivation, learning, and integrating information into memory all responded in similar ways among mutually acknowledged friends, as did those responsible for "processing language and the narrative content of stories, and sense-making more generally."

The researchers note that these areas are responsible for "attending to interpreting the sensory environment, as well as emotional responding." In other words, they are integral to both what we perceive, and what feelings those perceptions evoke. Agreement on those fronts can foster a friendship.

Parkinson and her colleagues, Adam Kleinbaum and Thalia Wheatley of Dartmouth College, note that it's possible our response patterns gradually begin to resemble those of the people we are closest to—especially in still-developing young adults. Future research will explore this question.

But it's notable that their findings hold true even after taking into account such potentially distancing factors as ethnicity, nationality, and gender. Similar brain-response patterns, it seems, override all such obstacles, lowering defenses and allowing the formation of a strong connection.

Sure, I like the guy: He thinks just like I do.