Sunstein, a legal scholar and now Obama's regulation tsar, discusses the phenomenon of "group polarisation". Studies show that a group of people who all hold a certain opinion moderately will, after discussion, hold it more strongly. This, Sunstein posits, helps to explain terrorism, conspiriology, the decision to invade Iraq, global-warming denial, and various other contemporary ills.

Sunstein calls any strongly held view a kind of "extremism". But, of course, people often vehemently hold opinions that are true or morally praiseworthy. We thus get the rather comic formulations of "justified" or "good" extremism (eg, civil-rights activists). Bad extremists suffer from a "crippled epistemology" (they don't know much, and what they know is wrong), but good extremists are just "sensible and right". So group polarisation is not always a bad thing after all. What we need, Sunstein concludes, is a "second-order diversity" in which many polarised groups argue out the issues in the public sphere. How this differs from current American politics is left as an exercise for the reader.