Is Rational Speculation in the Presence of Positive Feedback Traders Destabilizing?

DeLong (1990a) et al. show that in the presence of positive feedback traders rational speculation can be destabilizing, in that it drives the price of a risky asset above its expected value. A generalization of their seminal model with additional trading dates and an additional informative signal yields further interesting insights: it helps clarify when prices overreact, underreact, or even move in the "wrong" direction; when rational speculation is destabilizing or stabilizing; and whether overreaction is a symptom of market inefficiency or a manifestation of informational efficiency.