The behavior of the stock market

From experience we know that investors may 'temporarily' move financial prices away from their long term aggregate price 'trends'. (Positive or up trends are referred to as bull markets; negative or down trends are referred to as bear markets.) Over-reactions may occur—so that excessive optimism (euphoria) may drive prices unduly high or excessive pessimism may drive prices unduly low. New theoretical and empirical arguments have since been put forward against the notion that financial markets are 'generally' efficient (i.e., in the sense that stock prices in the aggregate tend to follow a Gaussian distribution).