Economists who adhere to rational-expectations models of the world will never admit it, but a lot of what happens in markets is driven by pure stupidity - or, rather, inattention, misinformation about fundamentals, and an exaggerated focus on currently circulating stories.
Robert J. ShillerA major boom in real stock prices in the US after Black Tuesday brought them halfway back to 1929 levels by 1930. This was followed by a second crash, another boom from 1932 to 1937, and a third crash. Speculative bubbles do not end like a short story, novel, or play. There is no final denouement that brings all the strands of a narrative into an impressive final conclusion. In the real world, we never know when the story is over.
Robert J. ShillerStock prices are likely to be among the prices that are relatively vulnerable to purely social movements because there is no accepted theory by which to understand the worth of stocks....investors have no model or at best a very incomplete model of behavior of prices, dividend, or earnings, of speculative assets.
Robert J. Shiller