Of all our constitutional rights, free speech might well be the most important. It has often been called "the guardian of all other rights." So long as the channels of communication are open, people can complain to the authorities, and in that way protect themselves against oppression and abuse.

But free speech also has a downside, which is becoming clearer every day. The downside involves "group polarization," the process by which members of deliberating groups become more extreme.

A small example: After talking with one another, citizens of France become more suspicious of the United States and its intentions with respect to economic aid.

A few years ago, I helped to design a little experiment in Colorado to explore whether group polarization would emerge on some of the most pressing issues of the day.

The experiment was undertaken in both Boulder, a pretty liberal place, and Colorado Springs, which is far more conservative. Citizens from each city were assembled into small groups.

They were first asked to state their views, privately and anonymously, on climate change, affirmative action and same-sex civil unions. Then they were asked to discuss those issues with one another, to reach a group verdict and to state their views again, privately and anonymously.

The result? People from Boulder became significantly more liberal on every one of those three issues. By contrast, people from Colorado Springs became significantly more conservative. Group members also became far more unified. After exchanging opinions, people in both cities ended up far less diverse in their views.

Something like the Colorado experiment is occurring on the Internet every day. There is a clue here about the rise of extremism of all kinds — not only among liberals and conservatives but also among people, in the United States and abroad, who are led to violence and terrorism.

Why does group polarization occur? One reason involves information. If you are in a group of people who think that climate change is not occurring, or that the United States is evil, you will hear a lot of arguments to that effect, and far fewer on the other side. So long as people are listening to one another, they will become more extreme.

Most people also want the good opinion of others. If you find yourself out of step with the group's apparent consensus, you might well move in its direction. The result will be to press the group's position toward one or another extreme.

In the United States, sharp divergences between whites and African-Americans can be explained partly by reference to group polarization, which also certainly occurs within Israel and the Palestinian Authority. The answer to the perennial question "Why do they hate us?" often lies in the process of group polarization.

An understanding of this process also explains why a fragmented system of communications may create serious problems, including outrage, hatred, and even violence.

Suppose that some people watch only Fox News and that others watch only MSNBC. Or suppose that on Facebook or Twitter, people are reading and listening mostly or only to like-minded others. Or suppose that people read only those newspapers or columnists with whom they are inclined to agree. The consequence will probably be a significant increase in extremism — and grave difficulty in achieving mutual understanding across political lines.

One goal of a system of free speech is, of course, to achieve exactly the opposite. John Stuart Mill, one of history's greatest theorists of freedom, wrote, "It is hardly possible to overrate the value, in the present low state of human improvement, of placing human beings in contact with persons dissimilar to themselves, and with modes of thought and action unlike those with which they are familiar." In his view, "such communication has always been, and is peculiarly in the present age, one of the primary sources of progress."

But let's be realistic. That progress will not occur on its own. It requires people to break out of their echo chambers to listen to others.

An understanding of group polarization does not, of course, justify any form of censorship. But it does suggest the immense importance of ensuring that our system of communications places people "in contact with persons dissimilar to themselves." Our system of self-government, and even our lives, may depend on it.

— Cass R. Sunstein is a Harvard Law School professor and former administrator of the White House Office of Information and Regulatory Affairs.

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