The efficient market hypothesis
market efficiency · no free lunch in markets
That asset prices already reflect available information, so consistently beating the market on public information is not possible. The useful version is not a claim about finance but about where easy opportunities can exist at all.
In practice
If an opportunity is obvious, available to everyone and worth taking, the return on it has usually already been competed away. Remaining opportunities are hard, unattractive or not visible to most people.
The common mistake
Taking it as proved or as refuted. It is a useful approximation whose failures are interesting, and both the strong form and the flat denial explain less than the question of how efficient a particular market is.
Eugene Fama's formulation in the 1960s: asset prices incorporate the information available about them. If everyone can see that a company will do well, that is already in the price, and buying it earns no excess return.
Three strengths are usually distinguished — prices reflect past prices, or all public information, or all information including private. The middle version is the one that is argued about seriously.
Why the mechanism generalizes
The reasoning is not really about finance. It is about competition for a known opportunity.
If a way of making money is visible, legal and does not require anything scarce, people will do it until the return falls to the cost of doing it. Persistent easy returns imply something preventing entry.
That makes the hypothesis a diagnostic question rather than a doctrine: when you find an apparently excellent opportunity, ask why it is still available. There are good answers, and the answer is the actual thing you are relying on.
The good answers
It requires something scarce. Capability, a relationship, a license, accumulated knowledge. This is a real edge and it is what positioning and career capital are for.
It is unattractive. Boring, unglamorous, low-status work is systematically underserved, because the people who could do it would rather do something else. This is one of the most reliable sources of return available to a small business.
It is too small. Below the threshold larger competitors can profitably serve. Most of what a good small firm does lives here.
It is genuinely new. The window before competition arrives, which is real and short, and is the whole of creative destruction from the entrant's side.
You are early to information that is not yet widespread. Real, and rarer than people think.
The bad answer
Everybody else has missed it. Occasionally true and usually a sign that the reason it is available has not been found yet. The disciplined version of the question is not "is this a good opportunity" but "what do I have that the people who did not take this lack?"
Where markets are visibly not efficient
The hypothesis is an approximation and the exceptions are informative. Prices move more than information seems to warrant, momentum persists longer than it should, and bubbles occur and are visible in hindsight. The theoretical objection is neat: if prices already reflected everything, nobody would be paid to research them, and without that research the prices would not reflect anything.
So markets are efficient roughly to the degree that people are paid to make them so. Which is why the concept transfers usefully: an opportunity in a market nobody is studying is far likelier to be real, and a market with many well-resourced participants competing on the same public information is one where the easy returns have already been taken.
Concept web
Open the full webQuestions
What is the efficient market hypothesis?
That asset prices already incorporate available information, so consistently beating the market using public information is not possible. Eugene Fama formulated it in the 1960s and it is usually stated in weak, semi-strong and strong forms.
How does market efficiency apply outside finance?
As a question about competition for known opportunities. If something is visible, legal and does not require anything scarce, competition drives the return down, so a persistent easy return implies a barrier worth identifying.
Why is an unattractive opportunity often a good one?
Because boring, low-status or very small work is systematically underserved — the people capable of doing it would rather do something else. The lack of competition is the source of the return.