Model Uncertainty, Ambiguity Aversion, and Market Participation

Ambiguity aversion alone does not explain the market nonparticipation puzzle. We show that in a rational expectations equilibrium model with a fund offering the risk-adjusted market portfolio (RAMP), ambiguity averse investors hold the fund and an information-based portfolio, and thus participate in all asset markets, directly or indirectly. This result follows from a new separation theorem which states that an investor's equilibrium portfolio can be decomposed into components, each matching the optimal portfolio based on only one information source (price versus private signal). Asset risk premia satisfy the CAPM with the fund as the pricing portfolio