In a setting with information asymmetry and a tradable value-weighted market index, ambiguity averse investors hold undiversified portfolios, and assets have non-zero alphas. But when a passive fund offers the risk-adjusted market portfolio (RAMP) whose weights depend on information precisions...

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...

We derive a separation theorem: investors hold a common risk-adjusted market portfolio regardless of their information sets, and a portfolio based upon their private signals. This implies that investors have non-negligible holdings of assets they know little about, so nonparticipation remains a...