Mark D. White

Mark D. White

In today's Wall Street Journal, there is a book review by Burton Malkiel of Models. Behaving. Badly., a critique of excessive reliance of mathematical modeling in finance and economics, written by Emanuel Derman, a key player in developing the very techniques he now criticizes. Judging from Malkiel's review, Derman's book echoes themes which are familiar by now, but are no less welcome for that. Some excerpts from the review:

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 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." …

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."

And my favorite:

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."…