Efficient Market Hypothesis
high confidence — compiled with source citation from the investment brain.
Efficient Market Hypothesis
Efficient Market Hypothesis
The efficient market hypothesis (EMH) is the academic claim that asset prices already reflect all available information, so that analyzing stocks is useless. It is the doctrine the two central investors in this corpus — warren-buffett and howard-marks — spend their careers arguing against, and their disagreement is precise rather than dismissive.
The frequent-versus-always distinction
Buffett's 1988 letter gives the cleanest formulation of the error. Recounting how the doctrine "became highly fashionable - indeed, almost holy scripture in academic circles during the 1970s", he writes: "Observing correctly that the market was frequently efficient, they went on to conclude incorrectly that it was always efficient. The difference between these propositions is night and day." Markets are often efficient — that part is granted — but not always, and the gap between the two is where active investing lives. Marks states the corollary from the practitioner side: exceptional risk-adjusted returns can only be "achieved in inefficient markets, and even then not all the time or by everyone."
The evidence against always-efficiency
Buffett's counter-evidence is the 63-year arbitrage record across Graham-Newman, Buffett Partnership, and Berkshire, where unleveraged returns averaged 20% per year against a market return of just under 10% — a differential he calls "a statistically-significant differential" The point is not that stock-picking beats darts; it is that a clearly identified, hindsight-free universe of positions produced results the hypothesis cannot accommodate.
The investor's stance
Marks states the operational consequence directly: "If we thought the market was always right – the efficient market hypothesis – we wouldn't spend our lives as active investors." To be an active investor is, by definition, to believe you know more than the consensus on at least some occasions. His bottom line is that "the market does not have above average insight, but it often is above average in emotionality", which is why contrarianism is built on doing the opposite of the crowd at extremes. See mr-market and second-level-thinking.
Reflexivity: why markets shape the fundamentals they price
Marks adds a theoretical objection beyond mere inefficiency, borrowing George Soros's reflexivity: financial markets "cannot possibly discount the future correctly because they do not merely discount the future; they help to shape it." When investor behavior changes the fundamentals — as the demand for mortgage products weakened lending standards and bid up home prices — markets enter dynamic disequilibrium, and boom/bust sequences arise that the theory cannot produce.
A tool-free doctrine
Buffett's 1996 letter goes further than rebuttal: the hypothesis and its apparatus are not just wrong but unnecessary. "To invest successfully, you need not understand beta, efficient markets, modern portfolio theory, option pricing or emerging markets." What matters is the circle-of-competence and the margin-of-safety — not a theory of price formation. The hypothesis's practical failure is what keynes-beauty-contest and risk-vs-uncertainty also circle: prices are set by fallible, emotional, interdependent minds, not by an omniscient aggregator.
Sources
berkshire-hathaway/Letters-Markdown/1988 - Letter to Shareholders.mdhoward-marks/Memos-Markdown/2006-03-27 - It Is What It Is.mdberkshire-hathaway/Letters-Markdown/1988 - Letter to Shareholders.mdhoward-marks/Memos-Markdown/What Does the Market Know.mdhoward-marks/Memos-Markdown/2009-11-10 - Touchstones.mdberkshire-hathaway/Letters-Markdown/1996 - Letter to Shareholders.md