Investopedia: What is the 'Efficient Market Hypothesis - EMH'?

Efficient Market Hypothesis - EMH

What is the 'Efficient Market Hypothesis - EMH'

The efficient market hypothesis (EMH) is an investment theory that states it is impossible to "beat the market" because stock market efficiency causes existing share prices to always incorporate and reflect all relevant information. According to the EMH, stocks always trade at their fair value on stock exchanges, making it impossible for investors to either purchase undervalued stocks or sell stocks for inflated prices. As such, it should be impossible to outperform the overall market through expert stock selection or market timing, and the only way an investor can possibly obtain higher returns is by purchasing riskier investments.
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BREAKING DOWN 'Efficient Market Hypothesis - EMH'

Although it is a cornerstone of modern financial theory, the EMH is highly controversial and often disputed. Believers argue it is pointless to search for undervalued stocks or to try to predict trends in the market through either fundamental or technical analysis.

While academics point to a large body of evidence in support of EMH, an equal amount of dissension also exists. For example, investors such as Warren Buffett have consistently beaten the market over long periods of time, which by definition is impossible according to the EMH. Detractors of the EMH also point to events such as the 1987 stock market crash, when the Dow Jones Industrial Average (DJIA) fell by over 20% in a single day, as evidence that stock prices can seriously deviate from their fair values.
What EMH Means for Investors

Proponents of the EMH conclude that, because of the randomness of the market, investors could do better by investing in a low-cost, passive portfolio. Data compiled by Morningstar Inc. through its June 2015 Active/Passive Barometer study supports the conclusion. Morningstar compared active managers’ returns in all categories against a composite made of related index funds and exchange-traded funds (ETFs). The study found that year-over-year, only two groups of active managers successfully outperformed passive funds more than 50% of the time. These were U.S. small growth funds and diversified emerging markets funds.

In all of the other categories, including U.S. large blend, U.S. large value and U.S. large growth, among others, investors would have fared better by investing in low-cost index funds or ETFs. While a percentage of active managers do outperform passive funds at some point, the challenge for investors is being able to identify which ones will do so. Less than 25% of the top-performing active managers are able to consistently outperform their passive manager counterparts.
Next Up Market Efficiency

    Efficient Market Hypothesis - ...
    Market Efficiency
    Passive Management
    Semi-Strong Form Efficiency
    Management Fee
    Discounting Mechanism
    Fractal Markets Hypothesis (FMH)
    Benchmark
    Market Timing
    Passive Investing

Market Efficiency
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Market efficiency refers to the degree to which stock prices and other securities prices reflect all available, relevant information. Market efficiency was developed in 1970 by economist Eugene Fama, whose theory of efficient market hypothesis (EMH) stated it is not possible for an investor to outperform the market because all available information is already built into all stock prices. Investors who agree with this statement tend to buy index funds that track overall market performance and are proponents of passive portfolio management.
BREAKING DOWN 'Market Efficiency'
At its core, market efficiency measures the availability of market information that provides the maximum amount of opportunities to purchasers and sellers of securities to effect transactions without increasing transaction costs.

Differing Beliefs of an Efficient Market

Investors and academics have a wide range of viewpoints on the actual efficiency of the market, as reflected in the strong, semi-strong and weak versions of the EMH. Believers that the market is strong are those who agree with Fama, that is, passive investors. Practitioners of the weak version of the EMH believe active trading can generate abnormal profits, while semi-strong believers fall somewhere in the middle.
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