Paul Krugman: A Moveable Glut

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 rationalizations 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.