Cass Sunstein — adviser to President Obama — unveils his new theory of ‘group polarisation’, and explains why, when like-minded people spend time with each other, their views become not only more confident but more extreme. What explains the rise of fascism in the 1930s? The emergence of student radicalism in the 1960s? The growth of Islamic terrorism in the 1990s? The Rwandan genocide of 1994? Ethnic conflict in the former Yugoslavia and in Iraq? Acts of torture and humiliation by American soldiers at Abu Ghraib prison? The American financial crisis of 2008? The widespread belief, in some parts of the world, that Israel or the United States was responsible for the attacks of September 11, 2001? And what, if anything, do these questions have to do with one another?
