We've all had this moment: You are desperately avoiding making eye contact with a friend, because if you do, you know you'll both burst out laughing. Or that thing where you and your bff say the same thing at the same time, and then marvel at the coincidence.
It often seems like our friends know just what we're thinking, or mirror our thoughts and reactions. In a new study published this week in Nature Communications, researchers demonstrated that that's not far from the truth. A group of social and cognitive neuroscientists found that friends' brains activated in similar areas in response to videos they were shown in the lab. The researchers were even able to predict how close two people were (or weren't), based on how similar their brain responses were.
Their findings line up with a concept called homophily, which is essentially that with similarity brings connection. It applies in nearly every relationship you can think of: marriage, friendship, work, support, and more. In romantic relationships, it’s been shown that people pick partners that look like their brothers, or their parents, or (cutting out the middle man) who are exceptionally similar to themselves.
We know that friends fit this pattern too. “We resemble our friends on many traits: height, athleticism, religion, politics, ethnicity, educational attainment, it goes on and on,” Nicholas Christakis, a sociologist and physician at Yale University, who wasn’t involved in the study, tells me.
The question this new work starts to answer, says first author Carolyn Parkinson, is if similarities among friends are deeper than those demographic categories. “Do we perceive, respond to, and interpret the world in a way that’s more similar to our friends too?” She says.
To find out, Parkinson and her group characterized the social connections of all 279 students in a graduate program at Dartmouth University. They asked each student whom they were friends with, and charted a social network based on the mutually reported friendships.
Then, they observed the neural responses of 42 of the students, using fMRI, as the students watched a variety of clips, like music videos, a scene from the documentary Food Inc., or footage of a sloth. The videos were chosen to include many subjects—from politics to cute videos to comedy—so that from person to person, the responses could be different; the parts of their brains that activated while watching would vary depending on their pre-existing knowledge of the content, how excited or bored they were by it, or what other thoughts and feeling the videos triggered.
Parkinson and her team found that the neural responses friends had were significantly more similar than among people farther removed from one another in their social network. "Friends responded more similarly than people who are friends of friends, who in turn are more similar than friends of friends of friends," says senior author Thalia Wheatley. The effect remained even when they controlled for demographics like age, gender, nationality, and ethnicity.
Parkinson says that they don’t want to make too many inferences yet about the specific brain areas that were tied to social connections, because they have many functions. In general, though, they “may be associated with similarities in how individuals attend to, interpret, and emotionally react to the world around them,” the paper notes.
What this paper doesn’t yet show is if those shared neural responses come about from spending time together, or if we gravitate towards people who already see the world as we do. Parkinson says it's tough to disentangle, and they’re now pursuing research that will get at that question more directly: Asking if people who befriend one another have similar neural responses before they ever meet.
The tendency to have friends that are similar to us leads to a larger biological question: Why would we do that?
In Parkinson's new work, it’s likely that it’s a mixture of social and biological influences: We befriend people that are like-minded and that is reinforced and perpetuated by the experiences that we share. But in Nicholas Christakis’s previous work, he has shown that friends are also more genetically similar to one another. “My genes don't affect your genes,” he says. “If our genes are similar, it must be because we sought each other out.”
That finding led him to a more abstract consideration: Why do we have friends at all? If we’re choosing friends that are genetically, neurally, and physically similar to us, is there some evolutionary force that’s pushing us to do so?
“It’s not hard to provide an account for why we humans form long-term reproductive unions with other members of our species; in other words, why we have sex with them,” Christakis says. “But we humans do something else, which is extremely unusual in the animal kingdom. We form long term non-reproductive unions with other members of our species; namely, we have friends. And there are very few animals who do this.”
In a 2014 paper, he found while your friends are not your relatives, they are likely to be as genetically similar to you as if they were your third or fourth cousins. “That to me is very interesting,” he says. “And it suggests that natural selection has played a role in the processes by which we form our friendships.” The new findings about neural responses, he says, adds in yet another layer of resemblance.
Evolutionarily speaking, if we don’t make babies with our friends, why should it matter? Christakis says that it still favors your genes to befriend those with similar genes as you, because it gives any unique variants you have the opportunity to flourish. Imagine, he says, that you’re the first person on the planet to evolve the ability to talk. You won't get any benefit from those genes, unless you have someone to talk to who has a similar genetic mutation.
Parkinson says that besides these underlying genetic motivations, it can just feel better to be around people like you: “Being around people who are similar to yourself can reinforce your own values, opinions, and interests,” she says. “That can be inherently rewarding.”
Dan Belsky, an assistant professor at Duke University in the department of population health sciences, co-authored a paper earlier this month that found that among adolescents, friends were, on average, more genetically similar than non-friends. He thinks studying homophily in friends is important because it can highlight the effects of the social genome, or the ways your friend’s DNA might be impacting you, even if they can’t pass it on or share it with you. Belsky's research has shown that the genetics of a person’s friends and schoolmates might influence their own educational outcomes, independent of their own DNA.
While it’s a nice thought that you and your bestie have matching brain activity, the idea of homiphily can also tint your friendships with a utilitarian streak. If your friends are your friends because they’re genetically and neurally the same as you, did you choose them at all? Or are we just biologically drawn together?
“Does this suggest you’re not a free agent when you choose your friends?” Christakis says. “It suggests that you’re slightly less a free agent. But by no means significantly less of a free agent. Not at all.”
He says natural selection seems to have equipped us with a “taste of homphily,” but that the genetic effects he and others have found are not overwhelming; they’re just noticeable enough to not be attributable to chance. Parkinson too says that her work doesn't explain every single friendship or every kind of mental processing.
Studies that reveal homophily usually zero in on our similarities, which are important and can be pretty cool to learn about. “But we also have lots of differences,” Belsky says. “The fact that friends, on average, have slightly more similar neural responses doesn’t tell us that friends never have different responses. We found that friends are, on average, a little more genetically similar to one another than they are to random other people. But that doesn’t mean that people with completely different genomes don’t sometimes form powerful friendships.”
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