Felix Salmon has a terrific cover story in this month’s Wired on the all-too-seductive Gaussian Copula function, which made it easy to calculate risk on Wall Street. Take a big basket of financial instruments, each with their own risks, through them into the Copula function, and presto: you get the net risk. It was so beautiful and apparently bulletproof that investors began treating it like a black box. Then it all went wrong.
This story profiles David X. Li, the inventor of the formula (now living in China), and how its adoption led to the chronic mispricing of risk that toppled some of the largest banks.
It’s a hell of a story, for students of Wall Street and mathematics alike. Here’s the function, annotated (yes, we’re aware that it seems to be missing a parenthesis, but that’s in the original paper, too):