Friday, October 7, 2011

What moves the markets? Part I

It's a key assertion of the Efficient Markets Hypothesis that markets move because of news and information. When new information becomes available, investors quickly respond by buying and selling to register their views on the implications of that information. This is obviously partially true -- new information, or what seems like information, does impact markets.

Yesterday, for example, US Treasury Secretary Timothy Geithner said publicly that, despite the ominous economic and financial climate, there is “absolutely” no chance that another United States financial institution will fail. (At least that's what the New York Times says he said; they don't give a link to the speech.) That was around 10 a.m. Pretty much immediately (see the figure below) the value of Morgan Stanley stock jumped upwards by about 4% as, presumably, investors piled into this stock, now believing that the government would step in the prevent any possible Morgan Stanley collapse in the near future. A clear case of information driving the market:

Of course, this just one example and one can find further examples, hundreds every day. Information moves markets. Academics in finance have made careers by documenting this fact in so-called "event studies" -- looking at the consequences for stock prices of mergers, for example.

But I'm not sure how widely it is appreciated that the Efficient Markets Hypothesis doesn't only say that information moves markets. It also requires that markets should ONLY move when new information becomes available. If rational investors have already taken all available information into account and settled on their portfolios, then there's no reason to change in the absence of new information. Is this true? The evidence -- and there is quite a lot of it -- suggests very strongly that it is not. Markets move all the time, and sometimes quite violently, even in the total absence of any new information.

This is important because it suggests that markets have rich internal dynamics -- they move on their own without any need for external shocks. Theories which have been developed to model such dynamics give markets with realistic statistical fluctuations, including abrupt rallies or crashes. I'm going to explore some of these models in detail at some point, but I wanted first to explore a little of evidence which really does nail the case against the EMH as an adequate picture of markets "in efficient equilibrium."

Anyone watching markets might guess that they fluctuate rather more strongly than any news or information could possibly explain. But research has made this case in quantitative terms as well, beginning with a famous paper by Robert Shiller back in 1981. If you believe the efficient markets idea, then the value of a stock ought to remain roughly equal to the present value of all the future dividends a stock owner can anticipate getting from it. Or, a little more technically, real stock prices should, in Shiller's words, "equal the present value of rationally expected or optimally forecasted future real dividends discounted by a constant real discount rate." Data he studied suggest this isn't close to being true.

For example, the two figures below from his paper plot the real price P of the S&P Index (with the upward growth trend removed) and of the Dow Jones Index versus the actual discounted value P* of dividends those stocks later paid out. The solid lines for the real prices bounce up and down quite wildly while the "rational" prices based on dividends stay fairly smooth (dividends don't fluctuate so strongly, and calculating P* involves taking a moving average over many years, smoothing fluctuations even further).

These figures show what has come to be known as "excess volatility" -- excess movement in markets over and above what you should expect on the basis of markets moving on information alone.

Further evidence that it's more than information driving markets comes from studies specifically looking for correlations between new events and market movements. On Monday, October 19, 1987, the Dow Jones Industrial Average fell by more than 22% in one day. Given a conspicuous lack of any major news on that day, economists David Cutler, James Poterba and Larry Summers (yes, that Larry Summers) were moved soon after to wonder if this was a one-off weird event or if violent movements in the absence of any plausible news might have been common in history. They found that they are. Their study from 1989 looked at news and price movements in a variety of ways, but the most interesting results concern news on the days of the 50 largest singe day movements since the Second World War. A section of their table below shows the date of the event, how much the market moved, and the principle reasons given in the press for why it moved so much:

Within this list, you find some events that seem to fit the EMH idea of information as the driving force. The market fell 6.62 percent on the day in 1955 on which Eisenhower had a heart attack. The outbreak of the Korean War knocked 5.38% off the market. But for many of the events the press struggled mightily to find any plausible causal news. When markets fell 6.73% on September 3, 1946, the press even admitted that there was "No basic reason for the assault on prices."

[Curiously, I seem to have found what looks like a tiny error in this table. It lists the outbreak of the Korean War (25 June, 1950) as explaining the big movement one day later on June 26, 1950. But then it lists "Korean war continues" as an explanation for a movement on June 19, 1950, five days before the war even started!]

Cutler and colleagues ultimately concluded that the arrival of news or information could only explain about one half of the actual observed variation in stock prices. In other words, the EMH leaves out of the picture something which is roughly of equal importance as investors' response to new information.

More recently in 2000, economist Ray Fair of Yale University undertook a similar study which found quite similar conclusions. His abstract explains what he found quite succinctly:
Tick data on the S&P 500 futures contract and newswire searches are used to match events to large five minute stock price changes. 58 events that led to large stock price changes are identified between 1982 and 1999, 41 of which are directly or indirectly related to monetary policy. Many large five minute stock price changes have no events associated with them.
 All in all, not a lot of evidence supporting the EMH view on the exclusive role of information in driving markets. Admittedly, these studies all have a semi-qualitative character based on history, linear regressions and other fairly crude techniques. Still, they make a fairly convincing case.

In the past few years, some physicists have taken this all a bit further using modern news feeds. More on that in the second part of this post. The conclusion doesn't change, however -- the markets appear to have a rich world of internal dynamics even in the absence of any new information arriving from outside.


  1. However, outside a crude characterisation, surely the EMH implies that prices reflect some average interpretation of the information available. Therefore, this interpretation can change even without any apparent news. As such, it may be perfectly possible for market to react before the outbreak of the Korean War if it is believed to be certain to happen tomorrow.

    In any case, I would not look at this as a binary. We know that markets cannot be efficient (as Grossman and Stiglitz showed) but there is a spectrum of relative efficiency in incorporating information. All those internal dynamics that you will speak about next interfere to a greater or lesser extent.

  2. Isn't EMH is an assumption implemented for the sake of simplifying theories? It also contains the assumption that everyone has the same investment horizon (I think usually assumed to be 1-day!).

    Reading some of the Manias and Panics type of books (juxtaposing with my school's legendary index investing philosophy) I have often run across the thought that the valuation assessment is only from those trading, and not really everybody's. Therefore, isn't it subject to some bias by short-termers, who are more likely to jump in and out and be more reactive to everything than maybe a "normal" person would?

    So two folks get together today and decide GE is worth 20 x earnings, and tomorrow two separate folks decide it is only 10 x earnings, neither really makes a difference to someone who will not be selling it for 30 years.