Rational Markets: Yes or No? The Affirmative Case

This paper presents the logic behind the increasingly neglected proposition that prices set in developed financial markets are determined as if all investors are rational. It contends that realistically, market rationality needs to be defined so as to allow investors to be uncertain about the characteristics of other investors in the market. It also argues that investor irrationality, to the extent it affects prices, is particularly likely to be manifest through overconfidence, which in turn is likely to make the market in an important sense too efficient, rather than less efficient, in reflecting information. To illustrate, the paper ends by re-examining some of the most serious evidence against market rationality: excess volatility, the risk premium puzzle, the size anomaly, calendar effects and the 1987 stock market crash