ETFs, Learning, and Information in Stock Prices

74 Pages Posted: 4 May 2020 Last revised: 4 Aug 2020

Date Written: August 3, 2020


This paper studies how the introduction of ETFs, and the growth of ETF ownership, can change investors' learning behavior. I develop a rational-expectations model where agents decide (1) whether they want to become informed or not and (2) if informed, how to allocate their limited attention between learning about individual stocks and a systematic risk-factor. Introducing an ETF does not universally increase or decrease learning about systematic risk. If the volatility of the systematic risk-factor is large, risk aversion is high, or the cost of becoming informed is high, introducing the ETF leads investors to devote more attention to the systematic risk-factor. Otherwise, the ETF may lead investors investors to learn more about the individual stocks. I decompose the effect of introducing the ETF into 2 channels: (1) Changes in the share of agents who decide to become informed (2) Re-allocation of attention among informed investors. I then extend the model, allowing an intermediary to buy the underlying stocks and create more shares of the ETF. Finally, I link the model's predictions to empirical evidence on the growth of ETF ownership and less informative stock prices.

Keywords: Asset Pricing, Information Choice

Suggested Citation

Sammon, Marco, ETFs, Learning, and Information in Stock Prices (August 3, 2020). Available at SSRN: or

Marco Sammon (Contact Author)

Harvard Business School ( email )

Boston, MA 02163
United States

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