What can rational investors do about excessive volatility and sentiment fluctuations?

Bernard Dumas

"Our objective is to understand the trading strategy that would allow an investor to take advantage of "excessive" stock price volatility and "sentiment" fluctuations. We construct a general equilibrium model of sentiment. In it, there are two classes of agents and stock prices are excessively volatile because one class is overconfident about a public signal. This class of irrational agents changes its expectations too often, sometimes being excessively optimistic, sometimes being excessively pessimistic. We find that because irrational traders introduce an additional source of risk, rational investors reduce the proportion of wealth invested into equity except when they are extremely optimistic about future growth. Moreover, their optimal portfolio strategy is based not just on a current price divergence but also on a prediction concerning the speed of convergence. Thus, the...