Index Investing and the Informational Efficiency of Stock Prices
This article studies whether index investing has implications for the informational efficiency of stock prices.
This article studies whether index investing has implications for the informational efficiency of stock prices.
Pastor, Stambaugh, and Taylor (2015) and Zhu (2018) provide significant evidence of decreasing returns to scale (DRS) at both the fund and industry levels. The authors examine the robustness of their inferences after Adams, Hayunga, and Mansi (2021) critique the above two studies.
We discuss the academic research about the causal effect of indexing on arbitrage conditions and price discovery.
This time is almost always different, it seems, but the data suggest that things are typically always the same: chaotic and volatile. Stock market investors should be prepared for large short-term moves in stocks and they should be skeptical of narratives suggesting a causal relationship between environmental variables and future volatility.
We estimate that passive investors held at least 37.8% of the US stock market in 2020. This estimate is based on the closing volumes of index additions and deletions on reconstitution days. 37.8% is more than double the widely accepted previous value of 15%, which represents the combined holdings of all index funds. What’s more, 37.8% is a lower bound. The true passive-ownership share for the US stock market must be higher. This result suggests that index membership is the single most important consideration when modeling investors’ portfolio choice. In addition, existing models studying the rise of passive investing give no hint that prior estimates for the passive-ownership share were 50% too small. The size of this oversight restricts how useful these models can be for policymakers.
The analysis above suggests that portfolios that include or exclude emerging allocations are roughly the same. For some readers, this may be a surprise, but for many readers, this may not be "news." That said, even if the data don't strictly justify an Emerging allocation, the first principle of "stay diversified" might be enough to make an allocation.
Of course, the assumptions always matter.
Market commentators sometimes suggest that the equity ETF market is just a bunch of "index funds" that all do essentially the same thing: deliver undifferentiated stock market exposure.
How true is that statement? Fortunately, we can test the hypothesis that the ETF market is roughly a few thousand different ways to capture the same basic risk/returns. To do so, we leverage our Portfolio Architect tool to quantify the active share of all US equity ETFs against the S&P 500 index (the king of indexes).
A few quick charts for our readers. As we all know, technology-related sectors and names have been crushed. But is blood in the streets? Not really.
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