Shakeouts and Market Crashes*

This article provides a microfoundation for the rise in optimism that seems to precede market crashes. Small, young markets are more likely to experience stock‐price run‐ups and crashes. We use a Zeira–Rob type of model in which demand size is uncertain. Optimism then grows rationally if traders' prior distribution over market size has a decreasing hazard. Such prior beliefs are appropriate if most new markets are duds and only a few reach a large size. The crash occurs when capacity outstrips demand. As an illustration, for the period 1971–2001 we fit the model to the Telecom sector.