WHAT caused Friday's stock plunge? What does it mean for the future? Nobody knows, and not much.

Attempts to explain daily stock movements are usually foolish: a real-time survey of the 1987 stock crash found no evidence for any of the rationalisations economists and journalists offered after the fact, finding instead that people were selling because, you guessed it, prices were falling. And the stock market is a terrible guide to the economic future: Paul Samuelson once quipped that the market had predicted nine of the last five recessions, and nothing has changed on that front.

Still, investors are clearly jittery - with good reason. US economic news has been good though not great lately, but the world as a whole still seems remarkably accident-prone. For seven years and counting we've lived in a global economy that lurches from crisis to crisis: Every time one part of the world finally seems to get back on its feet, another part stumbles. And America can't insulate itself completely from these global woes. But why does the world economy keep stumbling?