The efficient market hypothesis (EMH) is a theory that suggests that financial markets quickly and efficiently incorporate all available information into asset prices. Therefore, it is impossible to consistently achieve returns above the average market return, unless it is due to chance or the acceptance of greater risk. The EMH is a cornerstone of modern financial economics and has implications for investment strategy, risk management, and financial regulation.
There are three forms of the EMH: weak, semi-strong, and strong.
The weak form of the EMH posits that current market prices fully reflect all historical prices and any publicly available trading information, such as trading volume and bid-ask spreads. Therefore, technical analysis or the study of historical prices cannot be used to consistently generate excess returns.
The semi-strong form of the EMH suggests that stock prices quickly and efficiently incorporate all public information, including company announcements, financial statements, and economic data. Therefore, fundamental analysis or the study of financial and economic data cannot be consistently used to generate excess returns.
The strong form of the EMH is the most demanding version and suggests that all information, both public and private, is already reflected in market prices. Therefore, insider trading or the use of non-public information cannot be used to generate excess returns.
The EMH has important implications for investors and financial professionals as it suggests that it is difficult or impossible to consistently generate above-market returns through either technical or fundamental analysis. However, some critics have argued that the EMH is not entirely accurate, as there are instances where inefficiencies and anomalies exist in financial markets, which can be exploited through various trading strategies. These anomalies include the momentum effect, value effect, and the low-volatility anomaly. Overall, the EMH remains a controversial topic in finance that has significant implications for investment strategy and financial decision-making.