Researchers have long known that people choose friends who are much like themselves in a wide array of characteristics: of a similar age, race, religion, socioeconomic status, educational level, political leaning, pulchritude rating, even handgrip strength. The impulse toward homophily, toward bonding with others who are the least other possible, is found among traditional hunter-gatherer groups and advanced capitalist societies alike.
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 reward processing here, boredom alerts there.
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, compared to patterns seen among people who were not 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.” Dr. Parkinson and her colleagues, Thalia Wheatley and Adam M. Kleinbaum of Dartmouth College, reported their results in Nature Communications.
“I think it’s an incredibly ingenious paper,” said Nicholas Christakis, author of “Connected: The Power of Our Social Networks and How They Shape Our World” and 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. It’s about something we call good chemistry.
“Our results suggest that friends might be similar in how they pay attention to and process the world around them,” Dr. 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 N. Ochsner, a cognitive neuroscientist at Columbia University who studies social networks, said the new report is “cool,” “provocative” and “raises more questions than it answers.” It could well be picking up traces of “an ineffable shared reality” between friends.
Dr. Ochsner offered his own story as evidence of the primacy of chemistry over mere biography. “My wife-to-be and I were both neuroscientists in the field, we were on dating websites, but we were never matched up,” he said.
“Then we happened to meet as colleagues and in two minutes we knew we had the kind of chemistry that breeds a relationship.”
Dr. Parkinson — who is 31, wears large horn-rimmed glasses and has the wholesome look of a young Sally Field — described herself as introverted but said, “I’ve been fortunate with my friends.”
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, smoking cigarettes.
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.
Dr. 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.
Researchers have also been intrigued by evidence of friendship among nonhuman animals, and not just in obvious candidates like primates, dolphins and elephants.
Gerald G. Carter of the Smithsonian Tropical Research Institute in Panama and his colleagues reported last year that female vampire bats cultivate close relationships with unrelated females and will share blood meals with those friends in harsh times — a lifesaving act for animals that can’t survive much more than a day without food.
Through years of tracking the behaviors of a large flock of great tits, Josh A. Firth of Oxford University and his co-workers found that individual birds showed clear preferences for some flock members over others. When a bird’s good friend died or disappeared, the bereft tit began making overtures to other birds to replace the lost comrade.
Yet when it comes to the depth and complexity of bonds, humans have no peers. Dr. 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.