Let me tell you about one of the most frightening moments of my life. It came when I was at university, spending a week in Blackpool as a delegate to the National Union of Students conference.

As the meeting progressed, the mood became distinctly menacing. There was an air of suppressed violence. Some students complained that a few Conservatives had been distributing “Hang Nelson Mandela” leaflets. They correctly said that these were stupid and grossly offensive. They incorrectly demanded that something be done.

So the NUS president announced that he was expelling these Conservatives from the conference. The entire audience, perhaps 2,000 people or so, stood up, turned to face the offending Tories — who were watching proceedings from a balcony at the back of the hall — and began stamping their feet. As the pace of the stamping quickened, arms began to rise pointing at the offenders. Softly at first, then louder and still louder, the group began to chant. “Out! Out! Out! Out! Out!”

At which point a vanguard of members of the Socialist Workers Party went running towards the fleeing students to see if they could administer some justice. I watched it all, silent, mouth agape, head down. I wasn’t frightened because I felt personally threatened. I was a member of the SDP back then. But I hope that as long as I live I never experience another political meeting like that one. For within a few days of meeting to discuss the student grant and preaching peace and love to the world, the NUS conference had become a little like the Nuremberg rally. Arms raised, feet stamping, Out! Out! Out!

In a new book, Going to Extremes, Cass Sunstein provides an academic term for the experience I had in Blackpool. It is called group polarisation. Here, in a nutshell, is Professor Sunstein’s theory. “Groups go to extremes. More precisely, members of a deliberating group usually end up at a more extreme position, in the same general direction as their inclinations before deliberation began.”

The professor provides an example from a study he conducted in Colorado. He divided a panel of Americans into two groups based on their views, liberal and conservative. Then he sought their opinion, individually, on a range of political questions. He asked, for instance, their view on allowing same-sex civil unions, and what they thought of an international treaty on climate change. He then let the two groups have separate meetings to discuss the issues. When the meetings were concluded, Professor Sunstein surveyed the members again.

He found that almost every member of either group ended up with a stronger, more extreme, position after they spoke with one another. The liberals became more passionately liberal, the conservatives more conservative. The groups were also significantly more homogeneous in their view after deliberation. And, on average, on each issue they were farther away from each other. The groups had polarised.

This is just one example. There are plenty of others. Group discussion among racially prejudiced people made them more prejudiced; the punitive damage awards of mock juries are higher than the median of individual jurors; a group of chess players was more inclined to a risky strategy than the individuals; a group of burglars became more cautious about the ease of breaking into a house than they would individually; protesters against police brutality became more supportive of violent action after group debate.

And students, gathered in Blackpool, became slowly more angry and extreme until their liberalism turned into chanting and expulsions. Not to mention the Tories who, getting together in little groups, somehow managed to persuade themselves that Nelson Mandela should be hanged.

Last week Cass Sunstein was confirmed as director of the Office of Information and Regulatory Affairs, a senior role in the Obama Administration. And the reaction has followed the course the professor’s group polarisation theory would have predicted. Perhaps best known here as one of the co-authors of Nudge, Professor Sunstein is the most free-market appointment President Obama could have made to the office. And when his nomination was first announced, he garnered quite a few individual conservative endorsements. But as the confirmation process continued, the attacks started. The “outspoken” (a polite word for loudmouth) conservative television commentator, Glenn Beck, laid into Professor Sunstein for being, of all things, “an animal rights nut”. Other conservatives joined in. In the end almost every Republican senator voted against his confirmation. A classic example of group polarisation.

I believe that the work, and now the life, of Cass Sunstein provide a lesson to political moderates. The views of individuals change when they are part of groups. Certainty becomes greater as theories are corroborated, people gather information that confirms their earlier views, and group members seek approval from others. All this pushes individuals farther in the direction they were already inclined to go. Professor Sunstein calls his book Going to Extremes, but group polarisation means that a mildly centrist group would become more resolutely centrist.

And this is true of virtual groups, people identifying with a cause, not only people meeting in a room.

So the right strategy for converting a group, for changing the views of those in it, is to change the boundaries of those who belong to it. James Purnell and other Blairites, for instance, want to move Labour in their direction, towards the centre. They would be as well advised to seek an organisational solution. They should seek to change the rules of the party to broaden the range of people taking part in the group debate. Tony Blair had this idea with his discussions with Paddy Ashdown, but he didn’t see it through.

And David Cameron, if he wants to keep his party with him, should change it by creating a broad church. He should change the group taking part in the Conservative discussion in order to moderate the views of Conservatives. It wouldn’t drown out the Right, it would change it from the inside.

There are, you see, two sides to group polarisation.

daniel.finkelstein@thetimes.co.uk

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