by John Paulos
First, let me note that the hypothesis comes in various strengths, depending on what information is assumed to be reflected in the stock price. The weakest form maintains that all information about past market prices is already reflected in the stock price. A consequence of this is that all of the rules and charts of technical analysis are useless. A stronger version maintains that all publicly available information about a company is already reflected in its stock price. A consequence of this version is that the earnings, interest and other elements of fundamental analysis are useless. The strongest version maintains that all information of all sorts is already reflected in the stock price. A consequence of this is that even inside information is useless.
So to what degree is the hypothesis true? The answer is surprising. The hypothesis, it turns out, has a rather anomalous logical status reminiscent of Epimenides the Cretan, who exclaimed, “All Cretans are liars.” More specifically, the Efficient Market Hypothesis is true to the extent that a sufficient number (sometimes relatively small) of investors believe it to be false.
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