Besties Share Everything – Even Their Brains

Have you ever met someone you just clicked with? Right away. You had that connection. Chemistry. Same interests, same beliefs, same reactions. That feeling you share one brain.

We know from social psychology that we – not only humans by the way – somehow tend to pick people who are very similar to us to be our friends. The scientific term for this phenomenon is homophily (from the Ancient Greek ‘homou’, meaning together, and Greek ‘philia’ meaning friendship). There is a huge amount of research showing homophily within demographic categories like gender, age, ethnicity and so on. For example, 25-year-old Jasmin is more likely to be friends with 24-year-old Jenny than with 53-year-old Jared. Now you might say “Obviously it is more likely that girls the same age are friends since they could have gone to school together, and unless Jared was their teacher, how in the world should they have met him?”. Good point. Of course, homophily within demographic groups could be biased by existing social structures like school, university, work etc., but the similarities based on which we pick our friends go way beyond our demographics.

Selfhout and his colleagues (2010) investigated the relationship between personality traits and new friendships. They used the well-known Big Five theory, which defines five main dimensions of personality (Openness to experiences, Conscientiousness, Extraversion, Agreeableness, Neuroticism) to describe an individual’s personality. In their study, the researchers found that late adolescents were more likely to make new friends with people who showed similar levels of Agreeableness, Extraversion and Openness to experiences. Kind of makes sense, right? Assuming that these personality dimensions reflect how we perceive the world around us, how we think about it, and how we react to it, people who are very similar to us in these personality traits might perceive the world and react to it in a similar way as we do. As a consequence, others’ thoughts and actions become easier to predict and therefore, interactions with these people are more effortless and fun. Ergo, we become friends. Furthermore, knowing that somebody shares the same interests, beliefs and values as we do makes us feel good about ourselves, reinforcing our own beliefs and values, and producing positive emotions as a result.

Similar neural responses predict friendship

You may have already heard about what has been mentioned above, or at least figured it out through your own experience. But what if someone told you that we have even more in common with our friends than we thought? That we connect on a much deeper level?

Parkinson and colleagues (2018) guessed that friends, who should perceive the world and react to it in the same way, might also be wired the same way. In other words, their perception should be based on similar cognitive processes when exposed to stimuli. In order to test this hypothesis, the researchers characterized the social network of an entire cohort of graduate students (see Figure 1). So basically, they figured out what we usually do through gossiping: who’s friends with whom, second-degree friends (friends of friends), or even third-degree friends (friends of friends of friends) and so on. Then, some of the students had a functional MRI brain scan, where they were shown the same collection of short video clips of various topics while their brain activity was measured. Since the researchers were interested in the natural and spontaneous neural response when perceiving these clips, the students were asked to do nothing while watching the clips. And what did they find? Well, it turned out that similarity in neural responses across students indeed correlated with their social distance! In other words, students who were friends showed the most similar neural response patterns, followed by second-degree friends, and last third-degree friends, who showed the least similarities. Even when controlling potentially confounding demographic variables the same effect was found. But it gets better, the researchers were able to create an algorithm that could successfully categorize which students were friends, who were second, and who were third-degree friends, relying only on their neural response patterns! Show me your brain and I’ll tell you who you’re friends with!

Figure 1. The social network of an entire cohort of first-year graduate students (N=279) reconstructed by the researchers. Nodes indicate students; lines indicate mutually reported social ties between them. The orange circles indicate the subset of students who participated in the fMRI study (N=42). Taken from Parkinson et al. (2018).

Chicken or egg?

Wow, apparently, we really are wired like our friends! No wonder it’s like we share a brain. But at this point we are facing kind of a chicken-or-egg-issue: What came first – our great friendship or our twin brains? Are our brains working in a similar fashion as a cause or consequence of our friendship? Well, both makes sense. People with similarly reacting brains might perceive the world similarly and have similar interests and beliefs, which makes them more likely to meet in contexts related to their interests/beliefs. Therefore, they become friends eventually. But on the other hand, everybody knows that as friends we also influence each other (like when your friend uses that weird word all the time and all of a sudden you find yourself using it as well). But is this influence really that powerful? Are we really able to shape our friends’ brains?

Brain-to-brain coupling

Actually, we just might. When we feel like we’re “on the same wavelength” with someone, it might be because our brains literally are. Hasson and colleagues postulate that, via a signal transmitted through the environment, neural processes in one brain might be coupling to neural processes in another brain, and the oscillations (brain waves) become synchronized between people. This phenomenon is known as interpersonal neural synchronization or brain-to-brain coupling and has been investigated in various settings:

Stephens et al. (2010) used functional MRI to examine brain-to-brain coupling in verbal communication. They found joint and temporally-coupled response patterns in a speaker’s and a listener’s brain, in a way that the speaker’s neural responses were mirrored (temporally delayed) by the listener’s neural responses. In other words: processes in the speaker’s brain might induce the neural responses in the listener’s brain. In 2012, Jiang and colleagues found an increased neural synchronization during face-to-face communication in the left inferior frontal cortex, an area associated with understanding actions, imitation, empathy and social processing (Farrow et al., 2001).  

In 2018, Goldstein and colleagues conducted an EEG study to investigate brain-to-brain coupling in empathy. Twenty-two heterosexual romantic couples participated in the experiment. The women were given slightly painful heat, while their partners comforted them. Measuring both of their brain activities revealed that brain waves became synchronized between the partners, suggesting that the men were empathizing with their “suffering” partners.

(Want to learn more about neural synchronization in teamwork? Check out this post!)

What does that have to do with friendship?

So, could it be possible that we are actually shaping our friends’ brains? It makes sense, considering how much time we spend with our friends, talking to them, sharing our stories and feelings with them. Their brains might also be empathizing and synchronizing with ours. But still, we don’t know how powerful brain-to-brain coupling really is. So far, it has only been investigated in settings in which two individuals were in the same situation. But in order to persistently shape others’ brains and modulate neural response patterns, brain-to-brain coupling would have to induce long-term effects, which has not been shown so far. We know that one of the brain’s greatest features is its plasticity, which is the ability to adapt and allocate new functions to brain areas that previously did not serve that function. One could argue that friends automatically interact more often, leading to more frequent brain-to-brain coupling. This, in turn, could lead to similar neural patterns being induced repeatedly among friends and therefore become learned and maintained in the long run through neural plasticity. This is only a speculation, but it could explain the similarities in neural response patterns Parkinson and her colleagues found in their study among close friends.
But then again: Why do we sometimes meet someone and feel like we are on the same wavelength right from the beginning (and that without even having coupled our brains before)?


Coming back to our earlier question: are our brains working in a similar fashion as a cause or consequence of our friendship? The truth is, to this point, not even science is able to provide us with a definite answer, and it is probable that our syncing brains and friendships are intermingled in some sort of complicated interplay. There is still a lot of research to be done, especially in the field of such fluffy, complex social constructs as friendship. But so far, let’s be happy with the assumption that apparently, we are even closer to our friends than we thought we were. So, next time you’re having a really good time with your friends, just remember this article and imagine all your brains lighting up in the same areas, swinging together at the same wavelength.

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