Eugene Fama and the Efficient Market Hypothesis
In 1970, Eugene Fama published "Efficient Capital Markets" in the Journal of Finance, synthesising decades of research into a single coherent framework. The core idea is deceptively simple: in a competitive market with many well-resourced participants, prices rapidly incorporate all available information. Trying to beat such a market is, on average, a losing game once costs are accounted for.
Fama organised the theory into three distinct forms, each defined by what information is assumed to be already reflected in prices.
Weak Form Efficiency
Weak form efficiency holds that current prices already reflect all past price and trading volume data. If this is true, technical analysis — the practice of reading charts, identifying patterns, and predicting future price movements from historical price sequences — cannot generate consistent excess returns. Any pattern that becomes widely known is immediately traded away. The implication is that price history alone contains no exploitable information.
Semi-Strong Form Efficiency
Semi-strong form efficiency is the more demanding claim: prices reflect not just past prices but all publicly available information. This includes earnings reports, balance sheets, dividend announcements, analyst forecasts, and macroeconomic data. If semi-strong efficiency holds, fundamental analysis — carefully studying a company's financials to find undervalued stocks — cannot consistently generate alpha above a passive benchmark after costs. The moment a piece of public information is released, the market rapidly incorporates it.
Strong Form Efficiency
Strong form efficiency is the most extreme claim: prices reflect all information, including private information not yet released to the public. Under this form, even insider traders cannot systematically profit. In practice, this form is generally rejected — there is empirical evidence that corporate insiders do earn abnormal returns, which is precisely why insider trading is illegal in most jurisdictions.
What the EMH Does and Does Not Claim
A common misreading is that the EMH claims markets are always rational or that prices are always correct. Fama's claim is more modest: once an inefficiency is widely identified, rational arbitrageurs will trade it away. The EMH does not rule out individual instances of mispricing — it claims those mispricings are unpredictable and therefore unexploitable after costs.
Fama shared the 2013 Nobel Prize in Economics for this work — notably alongside Robert Shiller, whose research documented asset price bubbles. The Nobel Committee's decision to award both simultaneously was a deliberate acknowledgement that both rational efficiency and behavioural departures from it are real phenomena.