In the interest of avoiding confusion and generally making clear what we are talking about, it would be better to frame this debate as being about passive investing, instead of the efficient-market hypothesis.
What the EMH means traditionally is that for some market, conditioned on some set of available information, the expected value of the future price is equal to the current price times one plus the expected discount rate; in other words: current prices reflect available information. This doesn’t need to be taken literally, but markets being efficient should mean that it is at least approximately true. What “the expected discount rate” means is unclear because, if nothing else, it depends on the preferences of the traders, but in any case we can probably agree that the discount rate on the typical large cap stock from 2010 to 2020 was not greater than 20%.
Market efficiency implies that no possible trading system can be expected to perform better than equilibrium returns.
Everyone on LessWrong knows that a sufficiently good superintelligence could have made much greater than 20% returns trading on a single large cap stock from 2010 to 2020. So for many markets for large cap stocks, the equality hasn’t been even very approximately true.
And it doesn’t matter whether the information set is just publicly available past stock prices, or all publicly available information. Also it doesn’t matter whether the AI has access only to information that is at least one year old; it can probably still make much greater than 20% returns.
So markets aren’t even close to weakly efficient in the traditional sense.
The traditional EMH (“prices reflect available information”) is still what most of the world thinks of as the efficient-market hypothesis, even if there are many people (not at all just rationalists) using the term to refer to the thesis of passive investing (something vaguely like: “it is hard to make risk-adjusted returns, after fees, that beat the market benchmark”).
It’s hard to exclude the traditional EMH on grounds that it’s meaningless, that it doesn’t have practical implications, or that nobody ever thought it was true, because all three of these things are false. If markets were superintelligent, that would have far greater practical implications than any thesis about how to invest our savings: financial markets would become the most important thing in the world, we would use them as an oracle to answer all our questions, solve all our scientific problems, etc. Knowing that they aren’t able to do that is important.
Very important, for three reasons:
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