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What is the efficient market hypothesis?

The efficient-market hypothesis is an essential economic theory that helps investors devise a profitable investing strategy. Formulated in 1960 by Eugene Fama, it stresses that it is impossible to beat the market. 

This article will not only define this theory but also discuss the implications of efficient market hypothesis.

What is efficient market hypothesis?

The Efficient Market Hypothesis (EMH) states that you cannot consistently outperform the overall market because stock prices already include all the vital information that affects volatility. 

EMH suggests stocks are always priced fairly, making it futile for investors to search for undervalued stocks or predict market trends through fundamental or technical analysis.

Different forms of efficient market hypothesis

Efficient market hypothesis theory can be of the following three types: 

1. Weak Form Efficiency

The assertion made in this form is that the current price of a stock reflects its entire price history. You cannot use technical analysis — which uses past stock prices to predict future stock price movements — to achieve superior returns. 

However, you can utilise fundamental analysis to identify undervalued or overvalued stocks. 

2. Semi-Strong Form Efficiency

This EMH suggests that all publicly available information, including financial statements, news articles, and other public disclosures, is already incorporated into stock prices, not just historical prices. 

Under semi-strong form efficiency, neither technical nor fundamental analysis can assist in achieving consistent returns that outperform the market.  

3. Strong Form Efficiency

This form claims that stock prices reflect all information, public and private. That means even insider information is factored into the current stock prices. This makes it impossible for you or any other investor to have an advantage in the market.

How to test efficient market hypothesis?

The importance of an efficient market hypothesis to investors cannot be overstated. Therefore, knowing the following ways to test EMH becomes crucial.

1. Return predictability

This method tests whether variables known at time ( t ) can predict returns at time ( t+1 ). If you can use historical data to forecast future returns, it indicates that the market has not fully absorbed the information, contradicting the EMH.

2. Behavioural biases

Behavioural biases include overconfidence, anchoring, loss aversion, and herd mentality. They can cause systematic blunders in judgment and decision-making. These biases cause you to deviate from rationality, leading to mispriced assets and market anomalies. 

The presence of behavioural biases challenges the core efficient market hypothesis assumptions. If investors are not always rational, then markets cannot be fully efficient.

3. Market Anomalies

Anomalies such as the January effect, where small-cap stocks outperform the market in the first month of the year, or the weekend effect, where stock returns on Mondays are statistically lower than on other days, suggest that investor behaviour does not always align with the rational expectations of EMH. 

4. Event Studies

An event study would analyse the stock price before and after the announcement. It would determine whether and how quickly the market assimilated the information.

If prices adjust rapidly without any predictable patterns post-event, it supports the semi-strong form of EMH. Conversely, if the study finds that abnormal returns persist, investors could exploit this to achieve above-average returns.

5. Comparative Studies

Researchers may compare the stock performance of companies within the same industry but in different countries. They do so to assess how quickly and accurately information is incorporated into stock prices. 

Another approach is comparing the performance of various asset classes during different economic cycles.

Argument supporting the efficient market hypothesis

  • The hypothesis suggests that it is impossible to consistently achieve risk-adjusted excess returns (alpha) as stocks are fairly priced based on all known information.
  • EMH advocates investing in low-cost, passive portfolios. Data showing that active managers often fail to outperform index funds supports this.
  • There is a substantial body of evidence, including the performance of index funds, that supports the theory of market efficiency.
  • The regulatory environment in many financial markets is designed to promote transparency and fairness. This supports the principles of EMH.

Argument against efficient market hypothesis

  • Not all market participants have access to the same information, which can create advantages for some and lead to inefficiencies.
  • The existence of insider trading indicates that not all information is reflected in stock prices. This contradicts EMH.
  • Cases of market manipulation demonstrate that prices can be artificially influenced. This questions the concept of market efficiency.
  • Some investors, like Warren Buffett, have consistently outperformed the market, suggesting that skilful stock selection can lead to superior returns.
  • The market microstructure, including trading mechanisms and liquidity, can lead to price inefficiencies.

When is the Efficient Market Hypothesis Used?

Here are the main uses of EMH.

  • Index Fund Investing: This involves purchasing index funds that chase a market index like the Nifty 50. It is based on the belief that it is hard to beat the market consistently. Hence, matching the market’s performance is a more viable strategy.
  • Diversification: By putting your capital in a broad range of assets, you can reduce risk without sacrificing potential returns. This aligns with the EMH’s assertion that higher returns come with higher risk.
  • Regulatory Framework: Regulators may use EMH to create policies that promote transparency and information dissemination. The goal is to ensure that all market participants have access to the same information, thus supporting the premise of market efficiency.
  • Corporate Finance: In corporate finance, EMH influences how companies approach activities like issuing new stock or setting executive compensation. The hypothesis assumes that the market will fairly value securities based on available information, guiding companies in their financial decisions.
  • Risk Assessment: Investors use EMH to understand the relationship between risk and return. The efficient market hypothesis in security analysis suggests that to achieve higher returns, one must accept higher levels of risk. 


To sum up, EMH suggests that beating the market via stock selection or market timing is not possible. However, you can still employ various strategies to manage risk, minimise costs, and align portfolios with your financial goals. These strategies are not about outperforming the market but about achieving market returns cost-effectively and risk-adjusted. To learn more, read StockGro blogs. 


 What is the Efficient Market Hypothesis (EMH)?

EMH, a financial theory, states that stock prices incorporate all relevant information, making it challenging to attain above-average returns consistently.

Does EMH assume all investors are rational?

The Efficient Market Hypothesis posits that while individual investors may not always be rational, the market as a collective is efficient. It assumes that any irrational behaviour is random and thus cancels out, leaving prices to reflect all available information accurately.

How does EMH relate to trading strategies?

According to the Efficient Market Hypothesis, stock prices already incorporate all available information, making it difficult for technical and fundamental analysis to consistently beat the market.

What are the criticisms of EMH?

Critics of the Efficient Market Hypothesis argue it fails to account for human psychology, which can lead to market anomalies. They also point to market manipulation and information asymmetry, suggesting that markets are not always as efficient as the hypothesis proposes.

What are the primary forms of EMH?

The Efficient Market Hypothesis is categorised into three forms: weak, which considers past price information; semi-strong, which includes all public information; and strong, which encompasses all information, public and private, suggesting no investor can consistently outperform the market.

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