The efficient market hypothesis holds that asset prices reflect all available information, so that it is not possible to consistently beat the market except by luck or by taking on more risk. It is one of the most influential ideas in modern finance and one of the most disputed. In 2013 the Nobel Prize in Economics was shared by a principal architect of the theory and one of its most effective critics, in the same year, for work that points in opposite directions.
Formulated in its modern form by Eugene Fama in the 1960s, the hypothesis comes in three strengths. The weak form says prices already reflect all past price data, so technical analysis of charts cannot produce excess returns. The semi-strong form says prices reflect all publicly available information, so analysing published accounts cannot either. The strong form says prices reflect all information including private information, which almost nobody defends, since insider trading demonstrably pays.
The underlying logic is competitive rather than mystical. If information predicting a price rise were freely available and unused, someone would trade on it, and that trading would move the price until the opportunity disappeared.
The practical record is strong. The large majority of actively managed funds underperform simple index funds over long periods, after fees, and the minority that outperform in one period rarely repeat it at rates distinguishable from chance. This finding has been replicated across decades and markets, and it is the basis of the index fund industry, which has moved trillions of dollars on the strength of it. On this evidence, the hypothesis is at least a very good approximation for the ordinary investor.
Critics point to phenomena the theory struggles to explain. Robert Shiller showed that stock prices are far more volatile than the underlying stream of dividends can justify, which is difficult to reconcile with prices tracking fundamental value. Asset bubbles, from tulips to dot-com stocks to housing, appear to be episodes in which prices detached from any reasonable estimate of worth, and the crash of 2008 was widely read as a failure of the assumption that markets price risk correctly.
Behavioural finance argues that investors are systematically, not randomly, irrational, so their errors do not cancel out. Persistent anomalies such as momentum and the small-firm effect appear to offer returns the theory says should not exist. And the observation that some investors, notably certain long-horizon value investors, have outperformed for decades is either evidence against the hypothesis or evidence that with enough participants some will win repeatedly by chance, depending on who is arguing.
Defenders respond that anomalies tend to shrink once published, which is what the theory predicts, and that bubbles are obvious only afterwards. A partial synthesis, sometimes called the adaptive markets hypothesis, treats efficiency as a variable that rises and falls with competition and conditions rather than as a fixed property. There is no consensus, and the disagreement is substantive rather than terminological: it concerns whether prices are usually right, and what follows for regulation if they are not.
