Efficient Market Hypothesis
What is Efficient Market Hypothesis?
The efficient market hypothesis says asset prices already reflect all available information, so no one can reliably beat the market by using it.
If prices already contain everything known about an asset, only genuinely new information can move them, and new information by definition cannot be predicted. Price changes therefore look close to random, and a strategy built on facts the market already has should not produce dependable outperformance. Economists distinguish three versions that differ only in how much information the price is assumed to hold: past trading data alone (weak), all public information (semi-strong), and private information as well (strong). Be careful not to confuse this informational efficiency with allocative or productive efficiency from microeconomics. The hypothesis is a claim about how quickly information enters prices, not a claim that markets deliver the socially best outcome.
Efficient Market Hypothesis: a worked example
A company is expected to report earnings of $2.00 per share and instead reports $2.50. Within seconds of the release the share price jumps from $40 to $48, a rise of 20 percent ($8 divided by $40). A student who reads the headline an hour later and buys at $48 gains nothing from the news, because the good result is already inside the price paid. Under the hypothesis, that trade earns an unusual profit only if the crowd that moved the price to $48 got it wrong, and the headline itself gives no way to know.
The mistake students make with efficient market hypothesis
Students take the hypothesis to mean prices are always correct and that crashes or bubbles cannot happen. The claim is narrower: prices reflect available information, so a mispricing cannot be identified in advance using that same information. Prices can still look badly wrong in hindsight, and they move violently when the information changes. The hypothesis also does not say investing is pointless, only that publicly known facts are already in the price.
Efficient Market Hypothesis questions
What are the three forms of the efficient market hypothesis?
The three forms are weak, semi-strong and strong, and each assumes prices contain more information than the last. Under the weak form, studying past price charts cannot help, because that history is already priced in. The semi-strong form adds every public disclosure, so reacting to news is too late, and the strong form adds private information, meaning even an insider could not gain.
Does the efficient market hypothesis mean bubbles are impossible?
No, the hypothesis does not rule out enormous price swings; it says a mispricing cannot be spotted in advance with information the market already has. Defenders argue that what looks obviously like a bubble is usually obvious only after prices fall. Behavioral economists disagree and point to episodes where prices ran far past any plausible fundamental value.
Is the efficient market hypothesis true?
The evidence is mixed, and the answer depends on which form is being tested. The weak form holds up reasonably well, since past price patterns are a poor guide to future ones, while the strong form is rejected, because people trading on private information do make unusual profits, which is one reason insider trading is illegal. The semi-strong form is the contested middle ground.
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