Index Investing and Asset Pricing under Information Asymmetry and Ambiguity Aversion David Hirshleifer Chong Huang Siew Hong Teoh∗ April 7, 2019 Abstract 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 port- folio (RAMP) whose weights depend on information precisions as well as market values, all investors hold the same portfolios as in the economy without model un- certainty and thus engage in index investing. So RAMP improves participation and risk sharing. Asset alphas are zero with RAMP as pricing portfolio. RAMP can be implemented by a fund of funds even if no single manager has sufficient knowledge to do so. ∗Paul Merage School of Business, University of California, Irvine. David Hirshleifer,
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[email protected]. We thank Hengjie Ai (discussant), David Dicks, David Easley (discussant), Bing Han, Jiasun Li (discussant), Stefan Nagel, Olivia Mitchell, Stijn Van Nieuwerburgh, Yajun Wang (discussant), Liyan Yang, and seminar and conference participants at UC Irvine, National University of Singapore, Hong Kong Polytechnic University, the 27th Annual Con- ference on Financial Economics and Accounting, the American Finance Association 2017 Annual Meeting, the 12th Annual Conference of the Financial Intermediation Research Society, 2018 Western Finance As- sociation Annual Meeting, and 2018 Stanford Institute