Zachary’s karate club

Can you give me an example of a network?

You can, of course. Your Facebook friends, for example, and the ways they are connected to each other. Or the transport network of the city you live in. Or, if you’re being really original, the World Wide Web.

Funnily enough when it comes to classic examples in the field, these are all high up on the list but not quite as high as “friends in a karate club”. Bizarre as it may seem, this example is such a cliché that you can win an award for being the first one to mention it during a networks conference.

Talking about a network of friends seems like a perfectly reasonable thing to do, but what’s with the karate? The answer dates back to a 1977 paper by Wayne Zachary, An Information Flow Model for Conflict and Fission in Small Groups. Zachary studied the change in social dynamics of one particular university karate club over the course of three years.

Zachary’s karate club

The club had 34 socially active members, i.e. members who also interacted outside of club meetings. They formed various friendships among each other, without necessarily being aware of distinct social circles within the club. Zachary, however, noticed that each member would either mostly have friends in common with the karate instructor (node 1 above), or with the club president (node 34 above), but would rarely be equally well-connected with both. Long story short, he identified two disjoint social groups within the club using network analysis and when the president and the instructor fell out and the club split, this happened pretty much exactly the way the model predicted. Pretty neat, huh? And he didn’t even have that much data to begin with. Imagine what you could do with the stuff Facebook knows!


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