Sometimes, the market seems to react irrationally to economic or financial news, even if that news is likely to have no real effect on the fundamental value of securities itself. But, this may be more apparent than real, since often such news has been anticipated, and a counterreaction may occur if the news is better (or worse) than expected. Therefore, the stock market may be swayed in either direction by press releases, rumors, euphoria and mass panic; but generally only briefly, as more experienced investors (especially the hedge funds) quickly rally to take advantage of even the slightest, momentary hysteria.
Over the short-term, stocks and other securities can be battered or buoyed by any number of fast market-changing events, making the stock market behavior difficult to predict. Emotions can drive prices up and down, people are generally not as rational as they think, and the reasons for buying and selling are generally obscure. Behaviorists argue that investors often behave 'irrationally' when making investment decisions thereby incorrectly pricing securities, which causes market inefficiencies, which, in turn, are opportunities to make money 1. However, the whole notion of EMH is that these non-rational reactions to information cancel out, leaving the prices of stocks rationally determined.
The Dow Jones Industrial Average biggest gain in one day was 936.42 points or 11 percent, this occurred on October 13, 2008 2.