Money & Finance › Stocks & markets › Index funds and passive investing
Efficient market hypothesis
Fama 1970· Theory or school
Experts disagreeEconomists or investors genuinely disagree.
The theory that market prices already reflect available information, so beating the market consistently, after costs, is very hard.
See it on the Money & Finance map →How it works
Eugene Fama's 1970 review set out three forms. Weak: past prices cannot predict future prices. Semi-strong: public news is priced in within minutes. Strong: even insider information is priced in, which almost no one believes. The logic is competition: thousands of analysts chase every scrap of news, so obvious bargains get bought away fast. The theory does not claim prices are always right, only that predictable, repeatable profits are scarce.
Why it matters
It is the intellectual case for index funds: if picking winners rarely pays after fees, owning the whole market cheaply becomes the logical alternative.
Think of it like
Like a busy submarket where every broker sees the same comps: a mispriced building rarely stays mispriced for long.
Facts, rules and data
S&P's SPIVA scorecards show roughly 85–90% of US large-cap stock funds trailing the S&P 500 over 15 years. Fama shared the 2013 Nobel Prize in economics.
Debates and risks
Bubbles and patterns like momentum suggest prices can stray for years. Robert Shiller, who shared that 2013 Nobel, argues markets are often irrational.
Inside this (1)
Connections (2)
▸Efficient market hypothesis is the theory behind Index funds and passive investingMainstream view
The connection
If prices already reflect what is knowable, stock pickers as a group cannot reliably beat the market after costs, so the practical answer is to own the market cheaply. Burton Malkiel urged a no-frills index fund in 1973, and Paul Samuelson in 1974. William Sharpe later showed the case holds by arithmetic alone.
When and why
Fama (1970), Malkiel's A Random Walk Down Wall Street (1973), Samuelson (1974), Vanguard (1976), Sharpe's 'The Arithmetic of Active Management' (1991).
How solid is the link?
SPIVA data show most active funds trail their indexes; Sharpe's arithmetic holds whether or not markets are fully efficient.
Debates and risks
The Grossman-Stiglitz paradox (1980): if everyone indexed, no one would research prices, and markets could not stay efficient.
▸Behavioral finance: how investors really behave challenges Efficient market hypothesisExperts disagree
The connection
Behavioral finance argues that biased investors push prices away from fair value, and that professionals cannot always correct this because betting against a crowd is risky and costly ('limits to arbitrage'). Robert Shiller showed in 1981 that stock prices swing far more than later dividends justify.
When and why
Shiller (1981) and De Bondt and Thaler's 'Does the Stock Market Overreact?' (1985); Fama and Shiller shared the 2013 Nobel Prize.
How solid is the link?
Documented patterns such as momentum and the value effect, plus bubble history; but each side reads the same evidence differently.
Debates and risks
Whether anomalies are mispricing or hidden risk is unresolved; McLean and Pontiff (2016) found many shrink sharply once published.