Efficient Market HypothesisEMH
The efficient market hypothesis says asset prices already reflect available information, so investors can't consistently beat the market except by luck or by taking more risk. It comes in weak, semi-strong and strong forms depending on how much information prices are said to include.
If thousands of smart, well-funded investors analyse every stock, can anyone keep finding bargains? The efficient market hypothesis says mostly no: prices adjust so fast to new information that consistent outperformance is nearly impossible. The idea underpins index investing — and is hotly debated by active traders. This entry explains EMH, its three forms and the evidence against it.
- Category
- Fundamentals and Macroeconomics
- Reading time
- 3 min
- Also called
- Efficient markets hypothesis, Market efficiency, Weak, semi-strong and strong form efficiency
- Updated
EMH in plain words
The efficient markets hypothesis comes largely from Eugene Fama's work, summarised in his 1970 review of efficient capital markets.1
Fama argued that an investor may get lucky short term, but cannot realistically expect to beat the market average substantially over the long run.1
Behavioral finance challenges this by documenting biases such as herding and confirmation bias that can push prices away from fair value.2
The three forms of EMH, step by step
Weak form
Prices reflect all past prices; technical analysis can't win consistently.
Semi-strong form
Prices reflect all public information; fundamental analysis can't either.
Strong form
Prices reflect even private information.
Implication
Low-cost index funds make sense. See Index Fund.
Test it
Do active funds beat benchmarks after costs? See Alpha.
Look for anomalies
Patterns that seem to violate it. See January Effect.
Worked examples
Example 1 — News priced in
Fast adjustmentA company reports earnings 10% above forecasts; the stock jumps 6% within minutes.
| Reaction time | Minutes |
| Chance to profit after reading the news | Small |
Under semi-strong efficiency, public news is reflected in prices almost immediately. See Earnings Surprise.
Example 2 — Active vs passive
Fund lagsAn active fund returns 8.5% before costs; the index returns 8%; the fund charges 1%.
| Fund after fees | 8.5% − 1% = 7.5% |
| Vs index | −0.5 points |
Consistent with EMH, beating the market after costs over the long run is difficult.1
Example 3 — A bubble
Inefficiency?A stock rises 300% on hype, then falls 80%.
| Peak vs start | 4× |
| Final vs start | 4 × 0.2 = 0.8× |
Episodes like this support critics who argue that biases such as herding move prices.2 See Market Bubble.
EMH forms
| Aspect | Information in prices | What can't beat the market |
|---|---|---|
| Weak | Past prices and volume | Technical analysis |
| Semi-strong | All public information | Fundamental analysis |
| Strong | Public and private | Even insider information |
| See also | Random walk | Random Walk Theory |
Why EMH matters to investors
Passive investing
The case for index funds. See Active and Passive Management.
Judging managers
Alpha should be rare. See Alpha.
Testing strategies
Beware of data mining. See Data Snooping.
Understanding anomalies
Momentum, value and seasonal effects. See Factor Investing.
Advantages and disadvantages
Advantages
- Explains why beating the market is hard.
- Supports low-cost investing.
- Clear framework for testing strategies.
Disadvantages
- Bubbles and crashes challenge it.
- Anomalies persist.
- Assumes rational investors.
Going deeper
Read as far as you need. Each level builds on the one before it.
Luck vs skill
Beginner
EMH accepts that some investors win, but attributes most short-term success to luck rather than repeatable skill.1 See Sample Size and Statistical Significance.
Same message as random walk
Intermediate
Fama's theory carries essentially the same implication for investors as the random walk theory.1
Behavioral critique
Advanced
Behavioral finance documents systematic biases — herding, framing, confirmation — that can make prices deviate from fundamentals.2 See Behavioral Finance.
Common mistakes
Thinking EMH means prices are always right
It says they're hard to beat, not perfect.
Confusing efficiency with stability
Efficient prices can still be volatile.
Ignoring costs
Fees and taxes decide many tests.
Mistaking luck for skill
Short records prove little.
Check your understanding
Test yourself before moving on. Each answer comes with an explanation.
Key points to remember
- EMH says prices reflect available information.
- It has weak, semi-strong and strong forms.
- Beating the market consistently is very hard.
- It supports index investing.
- Behavioral finance challenges it.
Frequently asked questions
Is the market efficient?
Does EMH mean technical analysis is useless?
Did Fama win a Nobel Prize?
Further learning
- Random Walk Theory
A closely related theory.
- Behavioral Finance
The main challenge to EMH.
- Index Fund
The practical response.
- Alpha
What EMH says is rare.
Sources
This article is written in our own words. The sources below support the specific facts, rules and figures marked with a number in the text. Links open the original publisher’s page.
- Corporate Finance Institute. Efficient Markets Hypothesis (opens in a new tab) Checked .
- Corporate Finance Institute. Behavioral Finance Glossary (opens in a new tab) Checked .
Educational content, not financial advice. Examples use hypothetical prices and simplified costs; actual fees, margin rules and regulations vary by broker, market and country. We are not affiliated with any publisher linked above. Last reviewed . Report an error or request a source credit.