The internet currently has two very different models of social networking. There is, of course, Facebook - a massive sprawl of friends and acquaintances that allows us to keep track of people we know in real life. I'm "friends" with my grandmother, a bunch of second cousins and it seems like most of my high school class. The defining feature of this network is its focus on "social closeness" - I want to keep track of these people because I have some kind of connection to them. We are all part of the same "clan".

The second model of online social networking is Twitter. The interesting thing about Twitter is that it encourages us to follow people who we have no social connection to at all. If I follow Ashton Kutcher or LeBron James or Margaret Atwood it's not because I know them from high school, or because I met Ashton at a party last week. Instead, it's because we share a set of common interests and beliefs. In other words, our connection is abstract - it's rooted in perceptions of social similarity, and not literal social closeness.

What's interesting is that the differences between Twitter and Facebook mirror a longstanding debate about how the human brain processes other people. Do we assess others based on similarity, on their devotion to the same political beliefs, TV shows and sports teams? Or do we focus instead on their closeness to us, on whether or not they're a sibling or a third cousin or a distant friend? This might seem like a minor mystery, but it has some major implications for social cognition. Are we clannish creatures, obsessed with bloodlines and kinship? Or are we more interested in finding a group that shares our peculiar passions and opinions?

These are the lofty questions addressed in a new paper in The Journal of Neuroscience by Fenna Krienen, a grad student in the Harvard lab of Randy Buckner. The main experiment involved sixty-six different participants that provided personality information about themselves and two friends. (Family members and significant others could not be selected.) One of these friends was supposed to have similar preferences and interests, while other friend was supposed to be dissimilar. For instance, a liberal Democrat might choose a disimilar friend who was a moderate Republican, or a person obsessed with punk rock might select an acquaintance who only listened to Chopin. Then, the scientists made up biographies of similar and dissimilar strangers for each volunteer based on their personality profiles. While the subjects were in the brain scanner, they played a version of “The Newlywed Game,” in which they were asked to predict how another person would answer a specific question. Would they want an aisle or window seat on a plane? Would they vote for Obama in the next election? Did they have a pet?

The purpose of the experiment was to figure out what, exactly, the brain was interested in. Did it care about similarity? Or was it more interested in social closeness? Interestingly, Krienen and Buckner found that the medial prefrontal cortex (MPFC) - a brain area associated with the perception of value and the regulation of social behavior - only increased in activity when people answered questions about actual friends, regardless of their similarity. (Damage to the MPFC typically leads to severe social deficits, so that people are unable to follow social norms or anticipate the consequences of their actions.) Interestingly, the MPFC showed no increase in activity when subjects contemplated a stranger with common interests. Here is a relevant paragraph from the paper:

There is considerable evidence that social species develop mechanisms to aid in identifying and evaluating kin, in-group members, and individuals otherwise relevant to one’s survival. Considering social cognition from this perspective, neural mechanisms may have evolved for distinguishing personally significant social information. Across a series of studies we consistently found that regions within the MPFC respond strongly to judgments about personally relevant individuals, exhibiting a dissociated response profile when making judgments about close others (friends) relative to unfamiliar others (strangers).

On the one hand, there is something slightly deflating about this data. It would be nice if we weren't such clannish creatures, if we didn't assess others in terms of whether or not they were part of the same tribe or came from the same place or were a member of the same family. But Krienen and Buckner argue that this is a legacy of our primate past. While similarity is nice - it makes cocktail conversation a little bit easier - it's ultimately less important than mere familiarity. Of course, social cognition is an incredibly complex process, and can't be neatly reduced to the blood flow of a single brain area. But this study helps us better understand why closeness matters, and why Facebook has more than 500 million active users, 50 percent of whom check their Facebook page every single day. We care about people we know, even if they vote for the other party.