The booms and busts in U.S. stock prices over the post-war period can to a large extent be explained by fluctuations in investors' subjective capital gains expectations. Survey measures of these expectations display excessive optimism at market peaks and excessive pessimism at market throughs....

We study a standard consumption based asset pricing model with rational investors who entertain subjective prior beliefs about price behavior. Optimal behavior then dictates that investors learn about price behavior from past price observations. We show that this imparts momentum and mean...