EMH: Efficient Market Hypothesis
The efficient market hypothesis says stock prices quickly reflect available information. If news spreads fast, prices jump before most people can trade on it, so beating the market is hard. EMH comes in three forms depending on how much information prices are assumed to include.
What you will learn
- Why good news pushes a stock price up and bad news pushes it down
- The idea that prices already reflect available information
- Weak, semi-strong and strong forms of market efficiency
- Why insiders can profit before news goes public
- What EMH means for stock pickers and index investors
Worked example
A company strikes oil. A friend of the CEO hears first; the public hears through the news a day later. Who can profit, and how does EMH change the answer?
- Before the news is public, the price has not moved, so whoever knows first can buy cheap
- Once the news spreads, buyers rush in and the price rises
- If news spreads almost instantly (for example through social media), the public price adjusts before anyone can profit from public news
- Semi-strong efficiency says public news is priced in fast; only private (insider) information could still earn an edge, and trading on it is usually illegal
Answer: The faster information spreads, the more efficient the market and the smaller the window to profit. Under strong-form efficiency, not even insiders would have an edge.
Common questions
What is the efficient market hypothesis in simple terms?
It is the idea that stock prices already reflect the information available to investors. Because new information is priced in quickly, you cannot consistently earn above-average returns, after adjusting for risk, just by analyzing news or past prices.
What are the three forms of market efficiency?
Weak form: prices reflect all past price and volume data, so chart reading should not work. Semi-strong form: prices reflect all public information, so analyzing public news should not work. Strong form: prices reflect all information, public and private, so even insiders cannot win.
Does the efficient market hypothesis mean you can't beat the market?
It means beating the market consistently, after risk and costs, is very hard, not that it never happens. Some investors beat it by luck in any year. EMH is one reason many investors choose low-cost index funds.
Is the stock market really efficient?
Evidence is mixed. Large markets look broadly efficient, and most active funds fail to beat their benchmarks over time. But anomalies, bubbles and behavioral biases suggest prices are not always perfectly right. Many experts see EMH as a useful approximation.