Recent empirical research in finance has uncovered two families of pervasive regularities underreaction of stock prices to news such as earnings announcements, and overreaction of stock prices to a series of good or bad news. In this paper, we present a parsimonious model of investor sentiment, or of how investors form beliefs, which is consistent with the empirical findings. The model is based on psychological evidence and produces both underreaction and overreaction for a wide range of parameter values.
Reprinted in Richard Thaler, ed., Advances in Behavioral Finance Vol. II, Princeton University Press and Russell Sage Foundation, 2005.