Book review: Models Behaving Badly

Bringing ethics into his analysis, Mr. Derman has no patience for coddling the folly of individuals and institutions who over-rely on faulty models and then seek to escape the consequences. He laments the aftermath of the 2008 financial meltdown, when banks rebounded “to record profits and bonuses” thanks to taxpayer bailouts. If you want to benefit from the seven fat years, he writes, “you must suffer the seven lean years too, even the catastrophically lean ones. We need free markets, but we need them to be principled.”

Excerpts from the review of Emanuel Derman’s book in the December 14 WSJ.  His comment about Maxwell’s equations reminded me of a great (nerd alert) t-shirt from my engineering days…

Mr. Derman’s particular thesis can be stated simply: Although financial models employ the mathematics and style of physics, they are fundamentally different from the models that science produces. Physical models can provide an accurate description of reality. Financial models, despite their mathematical sophistication, can at best provide a vast oversimplification of reality. In the universe of finance, the behavior of individuals determines value—and, as he says, “people change their minds.”

In short, beware of physics envy. When we make models involving human beings, Mr. Derman notes, “we are trying to force the ugly stepsister’s foot into Cinderella’s pretty glass slipper. It doesn’t fit without cutting off some of the essential parts.” As the collapse of the subprime collateralized debt market in 2008 made clear, it is a terrible mistake to put too much faith in models purporting to value financial instruments. “In crises,” Mr. Derman writes, “the behavior of people changes and normal models fail. While quantum electrodynamics is a genuine theory of all reality, financial models are only mediocre metaphors for a part of it.” …

He sums up his key points about how to keep models from going bad by quoting excerpts from his “Financial Modeler’s Manifesto” (written with Paul Wilmott), a paper he published a couple of years ago. Among its admonitions: “I will always look over my shoulder and never forget that the model is not the world”; “I will not be overly impressed with mathematics”; “I will never sacrifice reality for elegance”; “I will not give the people who use my models false comfort about their accuracy”; “I understand that my work may have enormous effects on society and the economy, many beyond my apprehension.”

Sampling from models that behave well, Mr. Derman gives an eloquent description of James Clerk Maxwell’s electromagnetic theory in a chapter titled “The Sublime.” He writes: “The electromagnetic field is not like Maxwell’s equations; it is Maxwell’s equations.” In another chapter, titled “The Absolute,” he outlines Spinoza’s “Theory of Emotions”—a description of the nature of emotions that did for man’s inner life, Mr. Derman says, “what Euclid did for geometry.” But then he turns to financial models—behaving badly.

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.” …

Bringing ethics into his analysis, Mr. Derman has no patience for coddling the folly of individuals and institutions who over-rely on faulty models and then seek to escape the consequences. He laments the aftermath of the 2008 financial meltdown, when banks rebounded “to record profits and bonuses” thanks to taxpayer bailouts. If you want to benefit from the seven fat years, he writes, “you must suffer the seven lean years too, even the catastrophically lean ones. We need free markets, but we need them to be principled.”

Advertisements
This entry was posted in Economics and tagged . Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s