Americans have become familiar with the kind of high-decibel, no-compromise political warfare between Republican and Democratic officeholders that led to a government shutdown last fall and threatened default on the national debt. While bitter partisanship is nothing new in American history, some social scientists fear the current wave is dangerously undermining national unity and the country's democratic traditions. Even the two main political parties are embroiled in infighting, with Republicans increasingly engaged in conflicts between traditional conservatives and those further to the right, and some liberal Democrats trying to push their party further to the left. Polarization in America is not limited to politics, either. People are moving into neighborhoods populated by others with like-minded views. And market researchers, pollsters and political scientists are discovering left/right preferences about what to drink, where to shop, how to be entertained and whom to marry.