Think about your friends—the people you spend a lot of time with, see movies with, those people you'd text to grab a drink or dinner after a long week. Now think back to why you first became friends and ask yourself: was it because you like them? Or because you are like them? A recent study, led by Carolyn Parkinson, a psychologist at the University of California, Los Angeles, suggests that the answer may involve a complex network of brain regions that gets to the root of how friendship exists in our brains.

When I spoke with her, Parkinson told me that a key focus of her research is learning how social networks might shape or be shaped by how our brains process information. Her previous work explored how the brain encodes one’s social standing, or where one sits in relation to another within a social hierarchy. She now wanted to understand how friendship itself was fleshed out in the brain.

Parkinson and her co-authors, Adam Kleinbaum and Thalia Wheatley of Dartmouth College, used a measure called social distance to define the friendship networks of 279 graduate students. Four months into their academic semester, Parkinson asked the students to consider an online list of their classmates and click on their friends. This Facebookian measure can be used to count how closely tied two individuals are based on their degree of social connection. Social distance, similar to six degrees of separation (or, alternatively, of Kevin Bacon), expresses how closely tied two individuals are within a larger social group.

Consider three people: Bill, Grace and Thomas. Bill and Grace are friends. Grace and Thomas are also friends. But Bill and Thomas have never met. In this scenario, Bill & Grace and Grace & Thomas are friends with one degree of separation while Bill & Thomas are two degrees of separation from each other (linked by their mutual friend Grace).

Parkinson used the questionnaire data to create a network that showed how far in social ties each of the students were from one another. These distances ranged from one degree of separation, meaning the students were friends with one another, to five degrees of separation, meaning that to draw a connection between two students in the friendship network, one would have to traverse a chain five friendships long.

Parkinson then showed 42 of the students a series of short video clips that resemble the way your TV would look if you were flipping through channels: three minutes of the earth from space, a few minutes of journalists debating, some slapstick comedy, a brief interlude watching a soccer match. Each student watched the same series of videos while their brain activity was recorded with functional MRI.

After the scan, Parkinson took the resulting MRI data and separated them based on where they originated in the brain. She then created what is known as a time series plot that represents how, on average, a brain region’s activity changed as each student viewed the video sequence. With each time series plot in hand, Parkinson could then determine whether an individual’s social relations were correlated with how their brain responded to viewing those videos.

Parkinson discovered that, indeed, the closer the social tie, the more similarly the student’s brains responded to the videos. And interestingly enough, the brain regions that were most similar across friends were those involved in attention and social cognition. The take-home: friends think alike.

These results fascinate me. If our engagement with social media is any indication, we spend an enormous amount of time thinking about our friends—about friends we have now, those we’ve had in the past, those we wish to have; the joys, the pains, the suspense of friendships. But I wager we don’t often think about how that all happens, how beneath our veneer of consciousness, neural assemblies are churning through sensory information, trying to make sense of the world and struggling to understand how to act within it. Yes, we are a social species, so friendships and social ties are extremely important—but what does that mean? And how does that happen at the level of those neural assemblies? It turns out that our brains appear, in a very real way, to synchronize with people we befriend, an incarnation of social unity. Perhaps it’s not simply that you feel close to your friends, but rather that you are experiencing the world more closely.

And of course I wondered which happens first. To (conversationally) binarize the question, I wondered about two chicken/egg scenarios: One, do we become friends with someone because their brain processes information like our own? Or two, does the act of befriending cause our brains to process information more similarly to our friends?

Parkinson was careful to remind me that because her study was cross-sectional—meaning she took a snapshot of the students and how their brains function—she can’t draw conclusions regarding cause and effect. In other words, she can’t say whether it was scenario one or two.

Either way, I see her results as an argument for some level of neural determinism. Consider the first scenario, wherein people with similar brains are drawn toward one another. This is an obvious case wherein your neurobiology has sculpted your social relationships. You may think you are choosing your friends, but your brain is really just responding to some neurophysiologic reflection; you see the world similarly, and so become friends.

The other scenario is a bit spooky. Say you somehow become friends with someone, perhaps by sitting next to them in class. As you get to know one another, you exchange some cognitive contagion that alters the way both of your brains perceive reality. By befriending, you become somehow not you.

It’s probably a little bit of both; nature births chickens and eggs simultaneously. And given the fact that collectively, humans have been befriending one another for thousands of years, no neurological danger was revealed here. But still I wonder whether “falling into the wrong crowd” or “marrying up” have some neurological correlate. And what of inter-species friendships—do cat people and dog people’s brains process information more like their pets? And vice-versa? (I’m imagining the urge to stick my head out my car window.)

Fortunately Parkinson told me she is hard at work conducting a longitudinal study, one that follows people (which is to say human brains) from before they meet until they form friendships. So hopefully she’ll give us an answer soon. In the meantime, choose your friends wisely. If you can.