Now if your kids ask you why they have to learn math, you can tell them, “Because if you don’t, you could ruin the global economy, you little beast.” Wired has just published an article that traces the entire clusterfrak back to a formula published in 2000 by a mathemetician working for JPMorgan Chase. Bankers loved its simplicity but completely misused it—despite warnings from academics that it was flawed—to turn pretty much every security into a triple-A, no-risk fabrication.
At the heart of it all was Li’s formula. When you talk to market participants, they use words like beautiful, simple, and, most commonly, tractable. It could be applied anywhere, for anything, and was quickly adopted not only by banks packaging new bonds but also by traders and hedge funds dreaming up complex trades between those bonds.
“The corporate CDO world relied almost exclusively on this copula-based correlation model,” says Darrell Duffie, a Stanford University finance professor who served on Moody’s Academic Advisory Research Committee. The Gaussian copula soon became such a universally accepted part of the world’s financial vocabulary that brokers started quoting prices for bond tranches based on their correlations. “Correlation trading has spread through the psyche of the financial markets like a highly infectious thought virus,” wrote derivatives guru Janet Tavakoli in 2006.
So why did banks keep following it? Because times were good, and because they didn’t understand the flaws in the formula in the first place:
During the boom years, everybody could reel off reasons why the Gaussian copula function wasn’t perfect. Li’s approach made no allowance for unpredictability: It assumed that correlation was a constant rather than something mercurial. Investment banks would regularly phone Stanford’s Duffie and ask him to come in and talk to them about exactly what Li’s copula was. Every time, he would warn them that it was not suitable for use in risk management or valuation.
In hindsight, ignoring those warnings looks foolhardy. But at the time, it was easy. Banks dismissed them, partly because the managers empowered to apply the brakes didn’t understand the arguments between various arms of the quant universe. Besides, they were making too much money to stop.
And then the housing market imploded.
“Recipe for Disaster: The Formula That Killed Wall Street” [Wired]
(Photo: David Paul Ohmer and Wired)