ABSTRACT The abnormal return associated with a stock being added to the S&P 500 has fallen from an average of 7.4% in the 1990s to less than 1% over the past decade. This has occurred despite a significant increase in the share of stock market assets linked to the index. A similar pattern has occurred for index deletions, with large negative abnormal returns during the 1990s but an average return of only 0.1% between 2010 and 2020. We investigate the drivers of this phenomenon and discuss implications for market efficiency. We document a similar decline in the index effect among other families of indices.

Retail investors trade hard-to-value stocks. We document a large and persistent spread in the stock-level intensity of retail trading, even allowing for known biases in the attribution of retail trades. Stocks with a high share of retail-initiated trades exhibit higher shares of intangible capital, longer duration cash flows, and a higher likelihood of being mispriced. Consistent with retail-favored stocks being harder to value, we document that these stocks are less sensitive to earnings news and more sensitive to retail order imbalances. Such segmentation of trading intensity arises in a model where informed investors face a trade-off between the benefits of hiding their trades within noisy retail investor order flow and the costs of producing information about the fundamentals of hard-to-value stocks.