is a social phenomenon where a group of like-minded individuals will make a more extreme choice than any of the individuals would have done on their own. This phenomena presents a very real problem in American politics because it becomes severe when we quarantine ourselves along ideological lines and only talk to those who agree with us. Some examples of this are the Republican Primary, college campuses, and internet political sites.
A quick trip through my student union to look at the fliers (not to mention talking to people) will show that social liberalism is the expected point of view. And everyone sees some sort of need to outdo each other on who can be the most radical. The idea of a social conservative is either unthinkable (not even considered) or hated (backward, bigoted, etc). I have trouble beliving that all of these individuals would see the world in such a way if they were not surrounded by likeminded others.
For the opposite thought, take a look at the Republican Primary. We've gotten to the point where every single candidate must outdo all the others or else is called a RINO. This happens because we have a collection of Republicans throughout the country who meet together and try to find who's the most like them. Except "like them" slowly turns into most extreme because everyone identifies as a conservative and sees a need to display himself as the most conservative.
We've forgotten what it's like to meet with people on the other side of the political spectrum. In our balkanized, polarized world, most conservatives never have much meaningful conversation with liberals and most liberals never have much meaningful conversation with conservatives. John Edwards turned out to be right about one thing: the are Two Americas, who've schizmed so far apart we no longer understand why the other side belives what it does and have no respect for our political opponents.