Joachim Andersen

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Efficient-Market Hypothesis: Why Easy Bargains Are So Hard to Find

Efficient-Market Hypothesis illustrated by a man in a camel overcoat and leather gloves walking past a golden banknote on wet cobblestones outside a columned stock exchange at night, smiling knowingly, while a young woman in a black coat and red scarf bends down behind him to pick it up, beneath a glowing ticker of golden lights.
Efficient-Market Hypothesis illustrated by the $100 bill on the sidewalk: “If it were real, someone would have picked it up already,” until someone does.


The Efficient-Market Hypothesis is the idea that market prices quickly reflect the information available, so it is very hard to consistently beat the market by finding mispriced bargains. It’s one of the mental models I find most useful, because it makes me humble about my own predictions and helps me ask what others already know, not only in investing, but also in business and everyday life.

The essence of the Efficient-Market Hypothesis is to ask: “If this opportunity is so obvious, why hasn’t someone else already taken it, and what do I know that the market doesn’t?“


Origin: Bachelier, Samuelson, Fama and the Random Walk

The roots go back to the French mathematician Louis Bachelier, whose 1900 doctoral thesis, Théorie de la spéculation, modeled price changes on the Paris Bourse as random. His work was largely forgotten for decades. In 1965, Paul Samuelson published “Proof That Properly Anticipated Prices Fluctuate Randomly“, showing that if prices already include everything investors expect, future price changes should be unpredictable.

The economist Eugene Fama gave the idea its name and structure in his 1970 review, “Efficient Capital Markets: A Review of Theory and Empirical Work“. In 1973, Burton Malkiel brought it to a wide audience with A Random Walk Down Wall Street, famously suggesting that a blindfolded monkey throwing darts at the stock pages could pick a portfolio as well as the experts. Fama shared the 2013 Nobel Memorial Prize in Economic Sciences with Lars Peter Hansen and Robert Shiller, one of the theory’s best-known critics.


Key Concepts

  • Weak, Semi-strong and Strong forms
    Fama described three versions.
    • The weak form says past prices can’t predict future prices.
    • The semi-strong form says all public information is already in the price.
    • The strong form says even private, insider information is reflected.
      Most evidence supports the weak and largely the semi-strong forms, but not the strong form.

  • Prices Reflect Information
    A market price is the combined judgment of many buyers and sellers, each acting on what they know. When new information appears, they trade on it, and the price adjusts. That is why prices move on news that is surprising, not on news that was expected.

  • The $100 Bill on the Sidewalk
    An old joke captures the idea: an economist and a friend see a $100 bill on the sidewalk. As the friend bends down to pick it up, the economist says: “Don’t bother. If it were real, someone would have picked it up already.” The point is that obvious free money rarely lasts, but the joke also shows the danger of taking the theory too literally.

  • The Grossman-Stiglitz Paradox
    In 1980, Sanford Grossman and Joseph Stiglitz pointed out that a perfectly efficient market is impossible. If prices already reflected all information, nobody would be paid to gather it, and then prices would stop reflecting it. Markets must therefore be inefficient enough to reward those who do the research.

  • Behavioral Finance, Bubbles and Shiller
    In 1981, Robert Shiller showed that stock prices move far more than changes in dividends can justify. Behavioral finance argues that people are subject to overconfidence, herding and fear, which can create bubbles and crashes. Shiller’s book Irrational Exuberance was published in March 2000, at the peak of the dot-com bubble.


Some Examples

  • Index Funds vs. Active Funds
    According to S&P’s SPIVA U.S. Scorecard for year-end 2024, 89.5% of actively managed US large-cap funds underperformed the S&P 500 over the previous 15 years. Beating the market after fees is hard, which is a major reason for the rise of low-cost index funds. See ETFs vs. Index Funds for more (education, not investment advice).

  • Buffett’s Hedge-fund Bet
    In December 2007, Warren Buffett bet that an S&P 500 index fund would beat a selection of hedge funds over the ten years from 2008 to 2017. Protégé Partners picked five funds-of-funds. The index fund gained 125.8%, while the five funds-of-funds gained between 2.8% and 87.7%, and the winnings went to Girls Inc. of Omaha.

  • News Moving Prices Within Minutes
    When the space shuttle Challenger exploded in 1986, it took a presidential commission months to identify the faulty part. A study by Michael Maloney and Harold Mulherin found that, within about an hour, the stock market had singled out Morton Thiokol, the maker of the faulty seals, whose shares fell about 12% that day, while the other contractors largely recovered.

  • Everyday Life
    The same logic applies outside finance. The obvious best table at a popular restaurant is usually taken. A house in a great location at a bargain price often has a hidden reason. A job that pays far more than similar jobs usually has a catch. When I see an apparent bargain, I ask what others know that I don’t.


Applying the Efficient-Market Hypothesis

When I want to use this mental model, this is the simple process I follow:

  1. Assume the Price Already Reflects What is Known
    Start from the idea that many smart people have already looked at the same opportunity.

  2. Ask What My Edge Is
    I’m honest about whether I really know something others don’t, or can act in a way others can’t.

  3. Look for the Hidden Reason
    When something looks too cheap or too good, I look for what I might be missing.

  4. Keep Costs Low
    If I can’t reliably beat the market, fees, taxes and trading costs matter more than clever predictions.

  5. Focus Where Markets Are Less Efficient
    Real edges are more likely in small, local, illiquid or specialized markets, and in areas where I have unique knowledge.

  6. Stay Humble and Patient
    Accept that much of what happens is noise, and avoid reacting to every piece of news.


Pitfalls and Limitations

  • Markets are not perfectly efficient. As the Grossman-Stiglitz paradox shows, some inefficiency must exist, and some investors have beaten the market over long periods.

  • Bubbles happen. The dot-com bubble around 2000 and the housing bubble before 2008 show that prices can drift far from fundamentals for years before they correct.

  • The theory fits large, liquid markets best. In small, illiquid or local markets, like a niche industry or my own neighborhood, there may be real edges that the theory makes me overlook.

  • Taken too far, it breeds cynicism. If I assume every opportunity is already taken, I may never pick up the $100 bill, start the business, or pursue the idea that others missed.


Related Mental Models



The Efficient-Market Hypothesis reminds me that the crowd is often smarter than I think, and that apparent bargains usually have a reason. By respecting what markets know, keeping costs low and looking for real edges only where I truly understand something others don’t, I make calmer and better decisions, while staying alert to the bubbles and blind spots the theory can’t explain.



To learn more about economics and how key parts of our world work,
check out: Wise: How to Attain a Deep Understanding of the World, History, Our Place in It, and How to Live Well



📚 Sources and References


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