Efficient Market Hypothesis (EMH)

With the Capital Asset Pricing Model (CAPM) explaining how assets are priced and Modern Portfolio Theory (MPT) showing how to build optimal portfolios, the final question is: can you beat the market at all? The Efficient Market Hypothesis (EMH) addresses whether analysis and stock-picking can consistently produce above-market returns.

The Core Idea

• The Efficient Market Hypothesis states that security prices fully reflect all available information
• If markets are truly efficient, it is impossible to consistently "beat the market" through analysis or market timing
• Developed by economist Eugene Fama in the 1960s
• The practical implication: if markets are efficient, passive investing (index funds) is the most rational strategy

Think of it this way: If every piece of available information is already baked into a stock's price, no amount of research can find a "hidden bargain." The price you see is the fair price. That is why EMH supporters favor index funds over stock-picking.

The Three Forms of EMH

EMH comes in three versions, each reflecting a different level of information incorporated into prices:

Weak Form

• Prices reflect all historical market data (past prices, trading volume, patterns)
• Technical analysis cannot produce excess returns because all past price patterns are already reflected in current prices
• Fundamental analysis may still be useful because financial statements and other public data may not be fully reflected in prices

Exam Tip: Gotchas

• Weak form does NOT invalidate fundamental analysis. It only makes technical analysis useless. Fundamental analysis (analyzing financial statements, earnings) may still produce excess returns under weak-form efficiency.

Semi-Strong Form

• Prices reflect all publicly available information (includes everything in weak form plus financial statements, earnings reports, news, analyst research)
• Neither technical nor fundamental analysis can produce excess returns
• Only insider (nonpublic) information could provide an edge, but trading on it is illegal
• Most academics and practitioners accept semi-strong efficiency as a reasonable approximation of developed markets

Exam Tip: Gotchas

• Semi-strong form kills BOTH technical and fundamental analysis. The progression matters: Weak kills technical only. Semi-strong kills both. Remember that semi-strong includes everything in weak form plus additional public information.
• If markets are semi-strong efficient, active managers cannot consistently beat the market using publicly available research.

Strong Form

• Prices reflect all information, both public and private/insider
• No one can consistently earn excess returns, not even corporate insiders
• This is the most extreme form and is generally NOT supported by empirical evidence (insiders do appear to earn excess returns in practice)

Exam Tip: Gotchas

• Strong form means even insider information cannot help. This is the most extreme version and is generally NOT supported by real-world evidence.
• Strong form is theoretical, not practical. The exam may test that strong form is NOT empirically supported.

Market Anomalies

Certain patterns appear to challenge EMH but do not necessarily disprove it:

• January effect: Stocks (especially small-caps) tend to outperform in January
• Small-firm effect: Small-company stocks have historically outperformed large-company stocks on a risk-adjusted basis
• Value effect: Stocks with low price-to-book ratios tend to outperform growth stocks
• Momentum: Stocks that have performed well recently tend to continue performing well in the short term

These anomalies may reflect compensation for additional risk, data mining artifacts, or temporary inefficiencies that get corrected over time.

Exam Tip: Gotchas

• Market anomalies challenge EMH but do not disprove it. The exam may present anomalies as evidence against EMH; the correct answer is that they are inconsistencies, not proof that markets are inefficient.

Investment Implications

Memory Aid: "WeSS"

• Weak = historical data only (kills Technical)
• Semi-Strong = all public info (kills Technical + Fundamental)
• Strong = all info including insider (kills everything)