Cass Sunstein — co-author of the hugely influential Nudge and an 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?

Here is a clue. Some years ago, a number of citizens of France were assembled into small groups to exchange views about their president and about the intentions of the United States with respect to foreign aid. Before they started to talk, the participants tended to like their president and to distrust the intentions of the United States. After they talked, some strange things happened. Those who began by liking their president ended up liking their president significantly more. And those who expressed mild distrust toward the United States moved in the direction of far greater distrust. The small groups of French citizens became more extreme. As a result of their discussions, they were more enthusiastic about their leader, and far more sceptical of the United States, than similar people in France who had not been brought together to speak with one another.

This tale reveals a general fact of social life: much of the time groups of people end up thinking and doing things that group members would never think or do on their own. This is true for groups of teenagers, who are willing to run risks that individuals would avoid. It is certainly true for those prone to violence, including terrorists and those who commit genocide. It is true for investors and corporate executives. It is true for government officials, neighbourhood groups, social reformers, political protestors, police officers, student organisations, labour unions and juries. Some of the best and worst developments in social life are a product of group dynamics, in which members of organisations, both small and large, move one another in new directions.

Of course, the best explanations of fascism are not adequate to explain student rebellions, and even if we understand both of these, we will not be able to explain ethnic conflict in Iraq, the Rwandan genocide, abuse and brutality at Abu Ghraib, conspiracy theories involving Israel, or the subprime crisis. For particular events, general explanations can uncover only parts of the picture. But there are striking similarities among a wide range of social phenomena. The unifying theme is simple: when people find themselves in groups of like-minded types, they are especially likely to move to extremes. And when such groups include authorities who tell group members what to do, or who put them into certain social roles, very bad things can happen.

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In exploring why this is so, I hope to see what might be done about unjustified extremism, which is a threat to security, to peace, to economic development and to sensible decisions in all sorts of domains. My emphasis throughout is on the phenomenon of group polarisation. This phenomenon offers important lessons about the behaviour of the market, religious organisations, political parties, liberation movements, executive agencies, legislatures, racists, judicial panels, those who make peace, those who make war, and even nations as a whole.

Political extremism is often a product of group polarisation and social segregation is a useful tool for producing polarisation. In fact, a good way to create an extremist group, or a cult of any kind, is to separate members from the rest of society. The separation can occur physically or psychologically, by creating a sense of suspicion about non-members. With such separation, the information and views of those outside the group can be discredited, and hence nothing will disturb the process of polarisation as group members continue to talk. Deliberating enclaves of like-minded people are often a breeding ground for extreme movements. Terrorists are made, not born, and terrorist networks often operate in just this way. As a result, they can move otherwise ordinary people to violent acts. But the point goes well beyond such domains. Group polarisation occurs in our daily lives; it involves our economic decisions, our evaluations of our neighbours, even our decisions about what to eat, what to drink and where to live.

So why do like-minded people go to extremes? The most important reason for group polarisation, which is key to extremism in all its forms, involves the exchange of new information. Group polarisation often occurs because people are telling one another what they know, and what they know is skewed in a predictable direction. When they listen to each other, they move.

Suppose that you are in a group of people whose members tend to think that Israel is the real aggressor in the Middle East conflict, that eating beef is unhealthy, or that same-sex unions are a good idea. In such a group, you will hear many arguments to that effect. Because of the initial distribution of views, you will hear relatively fewer opposing views. It is highly likely that you will have heard some, but not all, of the arguments that emerge from the discussion. After you have heard all of what is said, you will probably shift further in the direction of thinking that Israel is the real aggressor, opposing eating beef, and favouring civil unions. And even if you do not shift — even if you are impervious to what others think — most group members will probably be affected.

When groups move, they do so in large part because of the impact of information. People tend to respond to the arguments made by other people — and the pool of arguments, in a group with a predisposition in a particular direction, will inevitably be skewed in the direction of the original predisposition. Certainly this can happen in a group whose members tend to support aggressive government regulation to combat climate change. Group members will hear a number of arguments in favour of aggressive government regulation and fewer arguments the other way. If people are listening, they will have a stronger conviction, in the same direction from which they began, as a result of deliberation. If people are worried about climate change, the arguments they offer will incline them toward greater worry. If people start with the belief that climate change is a hoax and a myth, their discussions will amplify and intensify that belief. And indeed, a form of ‘environmental tribalism’ is an important part of modern political life. Some groups are indifferent to environmental problems that greatly concern and even terrify others. The key reason is the information to which group members are exposed. If you hear that genetically modified food poses serious risks, and if that view is widespread in your community, you might end up frightened. If you hear nothing about the risks associated with genetically modified food, except perhaps that some zealots are frightened, you will probably ridicule their fear. And when groups move in dangerous directions — toward killing and destruction — it is usually because the flow of information supports that movement.

Those who lack confidence and who are unsure what they should think tend to moderate their views. Suppose that you are asked what you think about some question on which you lack information. You are likely to avoid extremes. It is for this reason that cautious people, not knowing what to do, tend to choose some midpoint between the extremes. But if other people seem to share their views, people become more confident that they are c
orrect.

As a result, they will probably move in a more extreme direction. What is especially noteworthy is that this process of increased confidence and increased extremism is often occurring simultaneously for all participants. Suppose that a group of four people is inclined to distrust the intentions of the United States with respect to foreign aid. Seeing their tentative view confirmed by three others, each member is likely to feel vindicated, to hold their view more confidently, and to move in a more extreme direction. At the same time, the very same internal movements are also occurring in other people (from corroboration to more confidence, and from more confidence to more extremism).

But those movements will not be highly visible to each participant. It will simply appear as if others ‘really’ hold their views without hesitation. As a result, our little group might conclude, after a day’s discussion, that the intentions of the United States, with respect to foreign aid, cannot be trusted at all. We have a clue here about the great importance of social networks, on the internet and in ordinary life, in creating movements of various sorts. Social networks can operate as polarisation machines because they help to confirm and thus amplify people’s antecedent views. Those who are inclined to support a cause or a candidate may become quite excited if support is widespread on their social network.

In 2008 Barack Obama greatly benefited from this process, in a way that created extreme enthusiasm for his candidacy. Some of this was planned; his campaign self-consciously promoted social networks that spread favourable information. But some of this was spontaneous. Obama supporters, especially young people, worked hard on their own to take advantage of existing networks and create new ones that would turn curiosity and tentative support into intense enthusiasm and active involvement.

A very different example is provided by Islamic terrorism, which is also fuelled by spontaneous social networks, in which like-minded people discuss grievances with potentially violent results. The terrorism specialist Marc Sageman explains that at certain stages, ‘the interactivity among a “bunch of guys” acted as an echo chamber, which progressively radicalised them collectively to the point where they were ready to collectively join a terrorist organisation. Now the same process is taking place online.’ The major force here is not websites, which people read passively; it consists of Listservs (which enable group emails), blogs and discussion forums, which are crucial in the process of radicalisation. Islamic terrorism is a product, in significant part, of group polarisation.

In the private sector, economic disasters, for individuals and large groups, are often a product of conversations among like-minded people, in which some investment or project seems to be a sure winner. The economic crisis that began in 2008 was a product, in significant part, of a form of group polarisation, in which sceptics about the real estate bubble, armed with statistical evidence, did not receive a fair hearing or were in a sense silenced. The best companies, and the best investors, benefit from internal checks and balances.

Of course, not all extreme movements are bad. Some extreme movements may be desirable even when they result from mechanisms of the sort traced here. And even when they are not desirable, extreme positions can do a great deal of good. Nothing said here is meant to deny these claims. But if extreme movements are to occur, it should be because they are sensible and right and not because of the predictable effects of interactions among the like-minded.

This is an edited extract from Going to Extremes: How Like Minds Unite and Divide by Cass R. Sunstein, published by Oxford University Press on 9 July.

This article first appeared in the print edition of The Spectator magazine, dated