Stability under learning of equilibria in financial markets with supply information

In a recent paper Ganguli/Yang (2009) demonstrate, that there can exist multiple equilibria in a financial market model a' la Grossman/Stiglitz (1980) if traders possess private information regarding the supply of the risky asset. The additional equilibria differ in some important respects from the usual equilibrium of the Grossman-Stiglitz type which still exists in this model. This note shows that these additional equilibria are always unstable under eductive learning (cf. Guesnerie (2002)) and adaptive learning via least-squares estimation (cf. Marcet/Sargent (1988) or Evans/Honkapohja (2001)). Regarding the original Grossman-Stiglitz type equilibrium, the stability results are less clear cut, since this equilibrium might be unstable under eductive learning while it is always stable under adaptive learning.