Efficient Markets Hypothesis (EMH)

EMH Definition and Forms

investor analyzing stocks - EMH
What is the Efficient Market Hypothesis? Can it help you become a better investor?. Getty Images

You may not have heard of the Efficient Market Hypothesis, also known as EMH, but you've probably wondered why even the most experienced mutual fund portfolio managers often lose to the major market indexes (or indices if you prefer) such as the S&P 500 Index.

EMH helps explain this investing phenomenon.

Efficient Market Hypothesis: EMH Definition

The Efficient Market Hypothesis (EMH) essentially says that all known information about investment securities, such as stocks, is already factored into the prices of those securities.

Therefore no amount of analysis can give an investor an edge over other investors. EMH does not require that investors be rational; it says that individual investors will act randomly but, as a whole, the market is always "right." In simple terms, "efficient" implies "normal." For example, an unusual reaction to unusual information is normal.

Defining the Forms of EMH

There are three forms of EMH: Weak, Semi-strong and Strong. Here's what each says about the market.

  • Weak Form EMH: Suggests that all past information is priced into securities. Fundamental analysis of securities can provide an investor with information to produce returns above market averages in the short term but there are no "patterns" that exist. Therefore fundamental analysis does not provide long-term advantage and technical analysis will not work.
  • Semi-Strong Form EMH: Implies that neither fundamental analysis nor technical analysis can provide an advantage for an investor and that new information is instantly priced in to securities.
  • Strong Form EMH: Says that all information, both public and private, is priced into stocks and that no investor can gain advantage over the market as a whole. Strong Form EMH does not say some investors or money managers are incapable of capturing abnormally high returns but that there are always outliers included in the averages.

    Proponents of EMH often invest in index funds or certain ETFs because they are passively managed (these funds simply attempt to match, not beat, overall market returns). Index investors might say they are adhering to the common saying, "If you can't beat 'em, join 'em." Instead of trying to beat the market, they will buy an index fund that invests in the same securities as the underlying benchmark index.

    With that said, there are certainly a handful of investors who can beat the market. However, these investors are in the minority and it is arguable that at least some part of their success can be attributable to luck.

    For more on EMH, including arguments against it, see this Efficient Market Hypothesis paper from legendary economist Burton G Malkiel.

    Disclaimer: The information on this site is provided for discussion purposes only, and should not be misconstrued as investment advice. Under no circumstances does this information represent a recommendation to buy or sell securities.

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