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because of the people we expect to meet in the course of doing them. The whole point of “social networking” events in the business world, for example, is to put yourself in a situation where you might meet interesting people. Likewise, the determination of some parents to get their children into the “right” schools has less to do with the quality of education they will receive than the classmates they will have. That said, of course, it is not equally easy for everyone to get into Harvard, or to get invited to the most desirable social gatherings. On a longer timescale again, therefore, your position in the social structure constrains not only whom you can get to know now but also the choices that will determine your future position in the social structure. Arguments about the relative importance of individual preferences and social structure invariably get bogged down in this chicken-and-egg tangle, and so tend to get resolved by ideology rather than by data. Those who believe in the power of individual choice can always contend that structure is simply the consequence of choices that individuals have made, while those who believe in the power of structure can always contend that the appearance of choice is illusory.

18. A similar finding has subsequently been reported in another study of homophily using data collected from Facebook (Wimmer and Lewis, 2010).

19. Some studies have found that polarization is increasing (Abramowitz and Saunders 2008; Bishop 2008), whereas others have found that Americans agree more than they disagree, and that views on one issue, say abortion, turn out to be surprisingly uncorrelated with views on other matters, like gun ownership, or immigration (Baldassari and Gelman 2008; Gelman et al. 2008; DiMaggio et al. 1996; Fiorina et al. 2005).

20. See Baldassari and Bearman (2007) for a discussion of real versus perceived agreement. In spite of the practical difficulties, some pioneering studies of precisely this kind have been conducted, first by Laumann (1969) and later by Huckfeldt and colleagues (Huckfeldt et al. 2004; Huckfeldt and Sprague 1987).

21. Clearly Facebook is an imperfect representation of everyone’s friendship network: Not everyone is on Facebook, so some close friends may be missing, while many “friends” are barely acquainted in real life. Counting mutual friends can help differentiate between genuine and illusory friendships, but this method is also imperfect, as even casual acquaintances on Facebook may share many mutual friends. A better approach would be to observe how frequently friends communicate or perform other kinds of relational acts (e.g., clicking on a newsfeed item, commenting, liking, etc.); however, this data is not yet available to third-party developers.

22. For details of the Friend Sense study, see Goel, Mason, and Watts (2010).

23. Projection is a well-studied phenomenon in psychology, but it has been difficult to measure in social networks, for much the same reasons that have stymied network research in general. For a review of the projection literature, see Krueger and Clement (1994), Krueger (2007), and Robbins and Krueger (2005).

24. See Aral, Muchnik, and Sundararajan (2009) for a recent study of influence in viral marketing.

25. For other recent work using e-mail data see, Tyler et al. (2005), Cortes et al. (2003), Kossinets and Watts (2006), Malmgren et al. (2009), De Choudhury et al. (2010), and Clauset and Eagle (2007). For related work using cell-phone data, see Eagle et al. (2007) and Onnela et al. (2007); and for work using instant messaging data, see Leskovec and Horvitz (2008).

26. For information on the progress on cancer see an excellent series of articles, “The Forty Years War” published in the New York Times. Search “forty years war cancer” or go to http://bit.ly/c4bsc9. For a similar account of the genomics revolution, see recent articles by Wade (2010) and Pollack (2010).

27. I have made a similar argument elsewhere (Watts 2007), as have a number of other authors (Shneiderman 2008; Lazer et al. 2009).

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