The Friday Fillip

This Friday it’s about friends. No, I mean real meat friends, not the phrends on phacebook. And my overbold assertion is that I know something about your friends. Here’s what I know:

Your friends have more friends than you do.

Nothing personal. Just statistical. And, almost paradoxical, given that friendship is by definition a reciprocal relationship (which is part of the reason that “Our Mutual Friend” is as wrong as the phrase “mutual agreement”: “friend in common” for the first, simply “agreement” for the second. But I digress.).

My assertion is known as the friendship paradox, and the statistical truth was first articulated by sociology prof Scott L. Feld in 1991, who at one point in his article expressed it this way:

While it is not a mathematical necessity that each individual will have fewer friends than the mean of her or his own friends, it is likely that most people will find themselves in this situation.

The basic logic can be described simply. If there are some people with many friendship ties and others with few, those with many ties show up disproportionately in sets of friends.

Which is to say, I think, that you shouldn’t feel bad. It’s just that, as was the case in highschool, you’re still hanging out with the popular kids. (Well, most of you are. Statistically.)

All of which would be of slight amusement value, perhaps, if some smart folks hadn’t realized that this fact could be put to good use. One such is Nicholas Christakis, a medical researcher, who has figured out that using the friendship paradox can help predict the spread of epidemics. He’ll explain it to you in the video below, but essentially it comes down to this, that by asking a random selection of people in the target population to nominate their friends, we can identify the most popular people in a network, and, consequently, those who are most likely to become infected at the start of an outbreak of disease; watching these popular people gives us considerable warning of the impending epidemic.

Interestingly, Christakis points out that this technique can be used to anticipate the outbreak of any phenomenon that spreads by contagion, be it disease, the purchase of a consumer product, or an idea. What makes his observation useful is the ease with which it’s possible to find the trend setters, so to speak.

Below is the TED video in which Christakis explains the idea. If you start at about the 7.42 point, you’ll see him explain the friendship paradox and how he’s applied it to his problem.