Measuring social influence is notoriously difficult in observational data.  If I like Tin Hat Trio and so do my friends is it because I influenced them or we just have similar tastes, as friends often do.  A controlled experiment is called for.  It’s hard to figure out how to do that.  How can an experimenter cause a subject to like something new and then study the effect on his friends?

Online social networks open up new possibilities.  And here is the first experiment I came across that uses Facebook to study social influence, by Johan Egebark and Mathias Ekstrom.  If one of your friends “likes” an item on Facebook, will it make you like it too?

Making use of five Swedish users’ actual accounts, we create 44 updates in total during a seven month period.1 For every new update, we randomly assign our user’s friends into either a treatment or a control group; hence, while both groups are exposed to identical status updates, treated individuals see the update after someone (controlled by us) has Liked it whereas individuals in the control group see it without anyone doing so. We separate between three different treatment conditions: (i) one unknown user Likes the update, (ii) three unknown users Like the update and (iii) one peer Likes the update. Our motivation for altering treatments is that it enables us to study whether the number of previous opinions as well as social proximity matters.2 The result from this exercise is striking: whereas the first treatment condition left subjects unaffected, both the second and the third more than doubled the probability of Liking an update, and these effects are statistically significant.

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