Fight disinformation: Sign up for the free Mother Jones Daily newsletter and follow the news that matters.

I’ve been reading about Wall Street’s fabulous Gaussian copula function for some time, but aside from a vague notion that it was a rocket science method of measuring risk, I’ve never had the slightest idea what it was actually all about.  In this month’s Wired, Felix Salmon explains.  Basically, it’s a clever way of figuring out whether the odds of two different bonds defaulting are correlated.  If they are, then bundling them together into a single security is risky since there’s a good chance they’ll both go into the toilet at the same time. If they aren’t, bundling them together is fairly safe since even if one of them defaults, at least the other one is still safe.  Its inventor was a math guru named David X. Li:

In 2000, while working at JPMorgan Chase, Li published a paper in The Journal of Fixed Income titled “On Default Correlation: A Copula Function Approach.” (In statistics, a copula is used to couple the behavior of two or more variables.) Using some relatively simple math — by Wall Street standards, anyway — Li came up with an ingenious way to model default correlation without even looking at historical default data. Instead, he used market data about the prices of instruments known as credit default swaps.

….When the price of a credit default swap goes up, that indicates that default risk has risen. Li’s breakthrough was that instead of waiting to assemble enough historical data about actual defaults, which are rare in the real world, he used historical prices from the CDS market…..Li wrote a model that used price rather than real-world default data as a shortcut (making an implicit assumption that financial markets in general, and CDS markets in particular, can price default risk correctly).

Hmmm.  The implication here is that the fundamental problem with the Gaussian copula — which was the mathematical basis behind the proliferation of CDOs, CLOs, and all the other shiny new investment vehicles that imploded so spectacularly last year — is that it was based on the relatively brief historical record of credit default swaps.  I don’t have any doubt that that’s true, but a few paragraphs later the real villain turns out to be a familiar one:

“Everyone was pinning their hopes on house prices continuing to rise,” says Kai Gilkes of the credit research firm CreditSights, who spent 10 years working at ratings agencies. “When they stopped rising, pretty much everyone was caught on the wrong side, because the sensitivity to house prices was huge. And there was just no getting around it. Why didn’t rating agencies build in some cushion for this sensitivity to a house-price-depreciation scenario? Because if they had, they would have never rated a single mortgage-backed CDO.”

There’s just no getting around it: there might have been technical problems with the Gaussian copula function, but even if it had worked the way people thought it did it wouldn’t have mattered.  The rating agencies and the sell-side BSDs were just using it as an excuse to pretend that house prices would rise forever anyway.  That was a far more fundamental problem than the statistical shortcomings of the formulae they used.

Still, it’s an intriguing piece that’s worth reading.  You can put it alongside Joe Nocera’s piece last month on Value at Risk, yet another quant model developed at JPMorgan whose wide misuse contributed mightily to our current economic meltdown.  When common sense takes a holiday, it turns out, all the math in the world can’t save you.

AN IMPORTANT UPDATE

We’re falling behind our online fundraising goals and we can’t sustain coming up short on donations month after month. Perhaps you’ve heard? It is impossibly hard in the news business right now, with layoffs intensifying and fancy new startups and funding going kaput.

The crisis facing journalism and democracy isn’t going away anytime soon. And neither is Mother Jones, our readers, or our unique way of doing in-depth reporting that exists to bring about change.

Which is exactly why, despite the challenges we face, we just took a big gulp and joined forces with the Center for Investigative Reporting, a team of ace journalists who create the amazing podcast and public radio show Reveal.

If you can part with even just a few bucks, please help us pick up the pace of donations. We simply can’t afford to keep falling behind on our fundraising targets month after month.

Editor-in-Chief Clara Jeffery said it well to our team recently, and that team 100 percent includes readers like you who make it all possible: “This is a year to prove that we can pull off this merger, grow our audiences and impact, attract more funding and keep growing. More broadly, it’s a year when the very future of both journalism and democracy is on the line. We have to go for every important story, every reader/listener/viewer, and leave it all on the field. I’m very proud of all the hard work that’s gotten us to this moment, and confident that we can meet it.”

Let’s do this. If you can right now, please support Mother Jones and investigative journalism with an urgently needed donation today.

payment methods

AN IMPORTANT UPDATE

We’re falling behind our online fundraising goals and we can’t sustain coming up short on donations month after month. Perhaps you’ve heard? It is impossibly hard in the news business right now, with layoffs intensifying and fancy new startups and funding going kaput.

The crisis facing journalism and democracy isn’t going away anytime soon. And neither is Mother Jones, our readers, or our unique way of doing in-depth reporting that exists to bring about change.

Which is exactly why, despite the challenges we face, we just took a big gulp and joined forces with the Center for Investigative Reporting, a team of ace journalists who create the amazing podcast and public radio show Reveal.

If you can part with even just a few bucks, please help us pick up the pace of donations. We simply can’t afford to keep falling behind on our fundraising targets month after month.

Editor-in-Chief Clara Jeffery said it well to our team recently, and that team 100 percent includes readers like you who make it all possible: “This is a year to prove that we can pull off this merger, grow our audiences and impact, attract more funding and keep growing. More broadly, it’s a year when the very future of both journalism and democracy is on the line. We have to go for every important story, every reader/listener/viewer, and leave it all on the field. I’m very proud of all the hard work that’s gotten us to this moment, and confident that we can meet it.”

Let’s do this. If you can right now, please support Mother Jones and investigative journalism with an urgently needed donation today.

payment methods

We Recommend

Latest

Sign up for our free newsletter

Subscribe to the Mother Jones Daily to have our top stories delivered directly to your inbox.

Get our award-winning magazine

Save big on a full year of investigations, ideas, and insights.

Subscribe

Support our journalism

Help Mother Jones' reporters dig deep with a tax-deductible donation.

Donate