"Fat Tails" and implications for risk management

Benoît Mandelbrot passed away last week at the ripe old age of 85. Mandelbrot was most famous for his seminal work in the field of fractal geometry, but is also considered by many (e.g., Nassim Nicholas Taleb, the author of Fooled by Randomness and The Black Swan) as the “intellectual father” behind critiques of efficient markets models. Mandelbrot’s critique of efficient market theory was centered on the notion that actual return distributions are more “fat tailed” than would be implied by the normal distribution. Taleb provocatively argues in chapter 15 of his book The Black Swan that the bell curve (normal distribution), when applied to financial markets, is a “great intellectual fraud”. Taleb has also recently argued that “… the Nobel Prize for Economics (specifically, the 1990 awards to Harry Markowitz, Merton Miller and William Sharpe for their work on portfolio theory and asset-pricing models and the 1997 awards to Myron Scholes and Robert Merton for their work on option pricing theory) has conferred legitimacy on risk models that caused investors’ losses and taxpayer-funded bailouts…”, and that “investors who lost money in the financial crisis should sue the Swedish Central Bank for awarding the Nobel Prize to economists whose theories he said brought down the global economy” (see “`Black Swan’ Author Says Investors Should Sue Nobel for Crisis“). While there is no question that Dr. Taleb’s narrative is brash and provocative, I am not convinced. Of course, he would argue that people like me who received their graduate training in finance during the past 2-3 decades have a vested interest in defending orthodoxy for its own sake. However, it’s only fair to also recognize that Dr. Taleb has a vested interested in defending heterodoxy for its own sake. It seems that Taleb seeks to discredit pretty much anyone who happens to disagree with him, not on the strength of the arguments that they marshall on behalf of “orthodoxy”, but rather on the basis of ad hominem arguments about how they can’t be taken seriously because they are intellectually biased a priori in favor of efficient markets orthodoxy. I couldn’t have explained the implications of Benoit Mandelbrot’s research for financial markets any better than Dr. Ewan Kirk, who is Chief Executive for Cantab Capital Partners in Cambridge, UK, so I quote directly from Dr. Kirk’s letter to the Financial Times entitled “How Mandelbrot Caused Confusion“: “It is true that markets are very difficult to model precisely. Indeed, even after this simple transformation, there continue to be significant non normal features to markets and of course there are always “unknown unknowns” and “black swan” events. However, these issues are considerably more subtle than just presenting the 100-year unscaled daily returns of the stock market and implying that foolish theoreticians and practitioners are modeling the returns as a stationary Gaussian or normal distribution.” Also, the essay by Bob Gillespie entitled “Black Swans and Absurdistan” is worth reading. In closing, I would like to point out two interesting videos from FT.com. The first video, “Inefficient markets and Mandelbrot“, features a debate concerning whether the impact of Mandelbrot’s legacy has been overstated. The other video, “Why ‘efficient markets’ collapse” is an interview with Mandelbrot recorded last year in which Mandelbrot explains his more than 40-year old critique of the “efficient markets” hypothesis and why new (i.e., Mandelbrotian) theories on price movement discontinuities are needed in light of the financial crisis of 2007-????.”]]>

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