What We Talk About When We Talk About the Efficient Market Hypothesis

Via The Curious Capitalist:

Eugene Fama said that in order to test whether markets were efficient in this sense you needed an economic theory of how prices were determined. He chose the Capital Asset Pricing Model, devised a few years before by Jack Treynor, Bill Sharpe and John Lintner. It said risky stocks would outperform less-risky ones, with the risk that mattered being something called beta—the correlation of a stock’s movements to those of the overall market.

Roll started pointing out issues with CAPM in the 1970s, and Fama and French concluded in 1992 that the conjunction of CAPM and the EMH simply didn’t match the data. They chose to jettison CAPM, not the EMH (Fischer Black made more or less the opposite choice). But without an economic theory of how stock prices should move, there’s no way of testing the claim that markets are efficient in the “price is right” sense. Pricing models like the arbitrage pricing theory or the Fama-French factor models simply assume that prices are right, then extrapolate from that what the relevant risk factors must be that determine prices. But this assumption that prices are right is now based on no empirical evidence at all. In fact, both Fama and Roll have said that there’s just no way to tell whether prices are right or not.

That leaves us with an efficient market hypothesis that merely claims, as John Cochrane puts it, that “nobody can tell where markets are going.” This is an okay theory, and one that has held up reasonably well—although there are well-documented exceptions such as the value and momentum effects. But if “we can’t tell where the markets are going” was all the finance professors had to offer, they wouldn’t have had much influence.

The price-is-right combo of EMH and CAPM allowed finance professors to say much more than “we dunno.” They may not have known exactly where a stock’s price was headed, but thanks to CAPM they could confidently predict the bounds within which it would move. Thus armed they went on to conquer the world, eventually transforming MBA curricula, legal thinking, corporate governance, financial regulation and many aspects of investment practice. It’s admirable that finance scholars—especially Fama, since it was his theory in the first place—kept sniffing around and eventually concluded that the EMH/CAPM combo didn’t match the evidence. It’s not so great that some of them now pretend that the price-is-right version of the efficient market hypothesis never existed, and fail to fully confront what its demise means for a lot of the other things taught in finance and investment classes.

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