Physics, because of its astonishing success at predicting the future behavior of material objects from their present state, has inspired most financial modeling. Physicists study the world by repeating the same experiments over and over again to discover forces and their almost magical mathematical laws. Galileo dropped balls off the leaning tower, giant teams in Geneva collide protons on protons, over and over again. If a law is proposed and its predictions contradict experiments, it’s back to the drawing board. The method works. The laws of atomic physics are accurate to more than ten decimal places.
It’s a different story with finance and economics, which are concerned with the mental world of monetary value. Financial theory has tried hard to emulate the style and elegance of physics in order to discover its own laws. But markets are made of people, who are influenced by events, by their ephemeral feelings about events and by their expectations of other people’s feelings. The truth is that there are no fundamental laws in finance. And even if there were, there is no way to run repeatable experiments to verify them.
Like all physicians, Derman and Wilmott want all modelers to swear by The Modelers’ Hippocratic Oath:
MODELERS OF ALL MARKETS, UNITE! You have nothing to lose but your illusions.
- ~ I will remember that I didn’t make the world, and it doesn’t satisfy my equations.
- ~ Though I will use models boldly to estimate value, I will not be overly impressed by mathematics.
- ~ I will never sacrifice reality for elegance without explaining why I have done so.
- ~ Nor will I give the people who use my model false comfort about its accuracy. Instead, I will make explicit its assumptions and oversights.
- ~ I understand that my work may have enormous effects on society and the economy, many of them beyond my comprehension.
Given Derman’s background as an academic it is not surprising that he advocates an indexing approach in his newest book Models Behaving Badly. The Wall Street Journal provides a short review:
The basic problem, according to Mr. Derman, is that “in physics you’re playing against God, and He doesn’t change His laws very often. In finance, you’re playing against God’s creatures.” And God’s creatures use “their ephemeral opinions” to value assets. Moreover, most financial models “fail to reflect the complex reality of the world around them.”
It is hard to argue with this basic thesis. Nevertheless, Mr. Derman is perhaps a bit too harsh when he describes EMM—the so-called Efficient Market Model. EMM does not, as he claims, imply that prices are always correct and that price always equals value. Prices are always wrong. What EMM says is that we can never be sure if prices are too high or too low.
The Efficient Market Model does not suggest that any particular model of valuation—such as the Capital Asset Pricing Model—fully accounts for risk and uncertainty or that we should rely on it to predict security returns. EMM does not, as Mr. Derman says, “stubbornly assume that all uncertainty about the future is quantifiable.”
The basic lesson of EMM is that it is very difficult—well nigh impossible—to beat the market consistently. This lesson, or “model,” behaves very well when investors follow it. It says that most investors would be better off simply buying a low-cost index fund that holds all the securities in the market rather than using either quantitative models or intuition in an attempt to beat the market. The idea that significant arbitrage opportunities are unlikely to exist (and certainly do not persist) is precisely the mechanism behind the Black-Scholes option-pricing model that Mr. Derman admires as a financial model behaving pretty well.