Why the Efficient Market Hypothesis (Weak Version) Says Nothing about the Ability to Identify Bubbles

One answer we keep hearing to that entirely reasonable question, “Why didn’t economists predict the crash?”, is that economic theory, in the form of the Efficient Markets Hypothesis, proves that reliable prediction is impossible. Sure, it is argued, some people like Brad Schiller and Dean Baker were jumping up and down and pointing to a housing bubble, but there are always Cassandras, and it is purely coincidental that these particular Cassandras turned out to be right. No doubt there are a range of other economists saying all sorts of things today, and in retrospect a few of them will be right too. But no one outpredicts the market on a regular basis, so there is no reliable way to know whose predictions today will prove correct in the future. This, we are told, is the lesson we need to learn from the EMH.

The logical fallacy here is so obvious that I would not bother with this post if it were not for the persistence of the EMH defense. So here goes.

First, for those not already versed, there are two levels of the EMH. The strong level says that market prices reflect the fundamental forces acting on an economy: there is no better measure of the true opportunity cost of something than its market price, or prediction of the future supply and demand conditions than its appropriate future, etc. This is known to be false, due to systematic biases and anomalies like overshooting, calendar effects, etc. That is not at issue.

The discussion largely centers around the weak version which says that, while market prices may not always be a great guide to real economic forces, their movements are not systematically predictable. At every moment, prices reflect all the forecasts of all the market participants who, between them, have access to all potential information and ways of utilizing it. A price moves only when new information arises. But to be truly new, this information has to be unpredictable—otherwise it is simply an inference from information that already exists. Because the information is unpredictable, so is its effect on prices. The randomness of price movements in turn implies that no one can outperform the market in betting on where they will go.

I have no problem with this. The fallacy arises when this argument is invoked to deny the possibility that economists can identify bubbles in real time. If you’re so smart you can spot a bubble, why aren’t you rich? If people could spot bubbles with any predictability, then the EMH would be wrong—but we know it’s right.

Let’s put aside the possibility that even the weak EMH can be wrong from time to time. We don’t need to go there; the error is more basic than this.

Let’s put ourselves back in 2005. It is two years before the unraveling of the financial markets, but I don’t know this; all I know is what I can see in front of me, publicly available 2005 data. I can look at this and see that there is a housing bubble, that prices are rising far beyond historical experience or relative to rents. The “soft” warning signs are all around me, like the explosion of cheap credit, the popularity of credit terms predicated on ever-rising prices, and the talk of a new era in real estate. Based on my perceptions, I anticipate a collapse in this market. What can I do?

If I am an investor, I can short housing in some fashion. My problem is that I have no idea how long the bubble will go on, and if I take this position too soon I could lose a bundle. In fact, anyone who went short in 2005 and passed on the following two years are price frothery grossly underperformed relative to the market as a whole. Indeed, you might not have the liquidity to hold your position for two long years and could end up losing everything. Of course, it is also possible that the bubble could have burst a year or two early and your bets could have paid off. What the EMH tells us is that, as an investor, not even your prescient analysis of the fundamentals of the housing market would enable you to outperform more myopic investors or even a trading algorithm based on a random number generator.

The logical error lies in confusing the purposes of an investor with those of a policy analyst. Suppose I work for the Fed, and my goal is not to amass a personal stash but to formulate economic policies that will promote prosperity for the country as a whole. In that case, it doesn’t much matter whether the bubble bursts in 2006, 2007 or 2010. In fact, the longer the bubble goes on, the more damage will result from its deflation. At the policy level, the relevant question is whether trained analysts, assembling data and drawing on centuries of experience in financial manias, can outperform, say, tarot cards in identifying bubbles. The EMH does not defend tarot.

To profit from one’s knowledge of a market condition one needs to be able to outperform the mass of investors in predicting market turns, which the EMH says you can’t do. Good policy may have almost nothing to do with the timing of market turns, however.