How Many Fools Does It Take to Make a Bubble? – By Jason Zweig

Where do market "bubbles" come from? A team of neuroscientists and economists has produced the first scientific evidence for what prudent investors have long believed: Paying attention to what others are doing is the easiest way for traders to get carried away.

This new research can't prevent the mass contagions that lead to bubbles. But it might help you step back before you get swept up in the next one.

Economists have struggled and failed to explain why markets turn into manias. Some have denied bubbles exist; others have argued bubbles must somehow be "rational." Often, the argument is that bubbles are caused by "uninformed" traders, or "dumb money," while the "smart money" sits on the sidelines.

The latest findings suggest, however, that bubbles might be caused not by traders who lack information but by those who have too much.