Rational Exuberance

Abstract

This article reviews the theory of speculative bubbles. Bubbles are a promising candidate as an explanation for the stock price run-up and collapse of the 1990s in the United States. The theory considers both irrational and rational bubbles, with emphasis on the latter. Rational bubbles, defined as the excess of security or portfolio prices over present values, can occur under conditions that are well understood. One argument relies on the assumed Pareto-optimality of equilibrium that rules out rational bubbles, but it is suggested that this argument is implausible.