A beginner's guide to the efficient market hypothesis (EMH)
Can anyone regularly beat the stock market? The efficient market hypothesis, or EMH, gives one famous answer. This page explains what the efficient market hypothesis says, its three forms and why many people question it.
Quick answer
The efficient market hypothesis (EMH) is a theory that share prices already reflect all available information. If that is true, no investor can regularly earn higher returns than the market after adjusting for risk, because any new information is priced in almost at once. The theory comes in three forms: weak, semi-strong and strong.
Key takeaways
- EMH says prices already reflect available information.
- It was developed by economist Eugene Fama in the 1960s and 1970.
- It has weak, semi-strong and strong forms.
- It supports the case for low-cost index investing.
- Behavioural finance points to cases where markets seem inefficient.
What is the efficient market hypothesis?
The efficient market hypothesis is the idea that markets process information quickly and fairly. When news about a company comes out, buyers and sellers react so fast that the share price adjusts almost at once.
So, by the time an ordinary investor reads the news, it is already in the price. According to EMH, this makes it very hard to earn higher returns than the market on a regular basis, after taking risk into account.
Who developed the theory?
The American economist Eugene Fama shaped the theory in the 1960s and set it out in a well-known 1970 paper. He shared the 2013 Nobel Prize in economics for his work on asset prices.
Fama defined an efficient market as one where prices 'fully reflect' available information. He also split the idea into three forms, based on what kind of information is priced in.
What are the three forms of EMH?
| Form | Information already in prices | What it implies |
|---|---|---|
| Weak form | Past prices and volumes | Chart patterns alone cannot give an edge |
| Semi-strong form | All public information | Reading news and results cannot give an edge |
| Strong form | Public and private information | Even insiders cannot gain an edge |
Each form is stricter than the one before. The weak form is the easiest to accept, while very few people believe the strong form fully holds, since insider trading laws exist for a reason.
What does EMH mean for investors?
If markets are efficient, trying to pick winning stocks or time the market may not pay off after costs. This idea supports investing through low-cost index funds that simply follow the market.
EMH does not say prices are always 'right'. It says that price changes are hard to predict, because they depend on new information that nobody knows yet.
Criticism and limits of EMH
Behavioural finance argues that people are not always rational. Fear and greed can push prices too high in bubbles and too low in crashes, which looks inefficient.
Some patterns, such as momentum and certain calendar effects, have also been studied as possible exceptions. In markets like India, smaller stocks with less research coverage may adjust to news more slowly than large, widely followed stocks.
The key point to remember
The efficient market hypothesis is a useful starting point for thinking about how prices form. It does not have to be fully true to be useful, but it is a reminder that beating the market regularly is hard.
Frequently Asked Questions
What is the efficient market hypothesis in simple words?
The efficient market hypothesis says share prices already include all available information, so it is very hard to beat the market regularly after adjusting for risk.
Who gave the efficient market hypothesis?
Economist Eugene Fama developed it, with a well-known paper in 1970. He shared the 2013 Nobel Prize in economics.
What are the three forms of EMH?
The weak form covers past prices, the semi-strong form covers all public information and the strong form covers both public and private information.
Is the Indian stock market efficient?
Studies give mixed results. Large, widely tracked stocks tend to react to news quickly, while smaller stocks with less coverage may adjust more slowly.
Does EMH mean stock prices are always correct?
No. It means price changes are hard to predict because they depend on new information. Prices can still be wrong at times.
Disclaimer: Investments in the securities market are subject to market risks. Please read all related documents carefully before investing. This article is intended for informational and knowledge purposes only and should not be considered tax, financial, or investment advice. Tax laws and deductions may vary based on individual circumstances and regulatory changes. Readers are advised to consult a qualified tax advisor or financial professional before making any investment or tax planning decisions.
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