A friend will help you move, goes an old saying, while a good friend will help you move a body. And why not?

Moral qualms aside, that good friend would most likely agree the victim was an intolerable jerk who had it coming and, jeez, you shouldn’t have done this, but where do you keep the shovel?

Researchers have long known that people choose friends who are much like themselves in a wide array of characteristics: age, race, religion, socioeconomic status, educational level, political leaning, even handgrip strength. The impulse toward bonding with others who are like you is found among traditional hunter-gatherer groups and advanced capitalist societies alike.

But new research suggests the roots of friendship extend even deeper than previously suspected.

Scientists have found that the brains of close friends respond in remarkably similar ways as they view a series of short videos: the same ebbs and swells of attention and distraction, the same peaking of interest and boredom.

The neural response patterns evoked by the videos — on subjects as diverse as the dangers of college football, the behavior of water in outer space and Liam Neeson trying his hand at improv comedy — proved so congruent among friends that the researchers could predict the strength of two people’s social bond based on their brain scans alone.

“I was struck by the exceptional magnitude of similarity among friends,” said Carolyn Parkinson, a cognitive scientist at the University of California, Los Angeles. The results “were more persuasive than I would have thought,” said Parkinson, whose research appeared in Nature Communications.

“I think it’s an incredibly ingenious paper,” said Nicholas Christakis, a biosociologist at Yale University. “It suggests that friends resemble each other not just superficially, but in the very structures of their brains.”

The findings offer tantalizing evidence for the vague sense we have that friendship is more than shared interests or checking off the right boxes on a Facebook profile.

“Our results suggest that friends might be similar in how they pay attention to and process the world around them,” Parkinson said. “That shared processing could make people click more easily and have the sort of seamless social interaction that can feel so rewarding.”

Kevin Ochsner, a cognitive neuroscientist at Columbia University who studies social networks, said the new report could well be picking up traces of “an ineffable shared reality” between friends.

Friendship benefits

The new study is part of a surge of scientific interest in the nature, structure and evolution of friendship. Behind the enthusiasm is a virtual Kilimanjaro of demographic evidence that friendlessness can be poisonous, exacting a physical and emotional toll comparable to that of more familiar risk factors like obesity, high blood pressure, unemployment, lack of exercise and smoking.

Scientists want to know what, exactly, makes friendship so healthy and social isolation so harmful, and they’re gathering provocative, if not yet definitive, clues.

Christakis and his co-workers recently demonstrated that people with strong social ties had comparatively low concentrations of fibrinogen, a protein associated with the kind of chronic inflammation thought to be the source of many diseases. Why sociability might help block inflammation remains unclear.

Parkinson and her co-workers previously had shown that people are keenly and automatically aware of how all the players in their social sphere fit together, and the scientists wanted to know why some players in a given network are close friends and others mere nodding acquaintances.

Inspired by the research of Uri Hasson of Princeton, they decided to explore subjects’ neural reactions to everyday, naturalistic stimuli — which these days means watching videos.

The researchers started with a defined social network: an entire class of 279 graduate students at an unnamed university widely known among neuroscientists to have been Dartmouth’s School of Business.

The students, who knew one another and in many cases lived in dorms together, were asked to fill out questionnaires. Which of their fellow students did they socialize with — share meals and go to a movie with, invite into their homes? From that survey, the researchers mapped out a social network of varying degrees of connectivity: friends, friends of friends, third-degree friends, friends of Kevin Bacon.

The students were then asked to participate in a brain scanning study, and 42 agreed. As a functional MRI device tracked blood flow in their brains, the students watched a series of video clips, an experience that Parkinson likened to channel surfing with somebody else in control of the remote.

Analyzing the scans of the students, Parkinson found strong concordance between blood-flow patterns — a measure of neural activity — and the degree of friendship among the various participants, even after controlling for other factors that might explain similarities in neural responses.

The researchers identified particularly revealing regions of pattern concordance among friends, notably in the nucleus accumbens, in the lower forebrain, which is key to reward processing, and in the superior parietal lobule, where the brain decides how to allocate attention to the external environment.

Using the results, the researchers were able to create a computer algorithm to predict, at a rate well above chance, the social distance between two people based on the relative similarity of their neural response patterns.

Parkinson emphasized that the study was a “first pass, a proof of concept,” and that she and her colleagues still don’t know what the neural response patterns mean: what attitudes, opinions, impulses or mental thumb-twiddling the scans may be detecting.

They plan to try the experiment in reverse: to scan incoming students who don’t yet know one another and see whether those with the most congruent neural patterns end up becoming good friends.