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Small Cap Indices Agree to Disagree

All indices deviate from the market, but investors may be surprised how little US small cap indices have in common.

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July 11, 2025
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Asset class index funds are often considered a way to get passive exposure to a market segment. But indices are just representations of the market, composed according to the rules determined by the index. This is particularly evident in the US small cap stock space, where overlap statistics indicate substantial differences in holdings. For example, the Russell 2000 Index and CRSP US Small Cap Index have less than a third of their weight in common (32.7%).

All portfolios, even those that are indexed, require decisions about what stocks to include, how to weight them, and when to rebalance. Investors should evaluate how those decisions are made. For example, broad market and small cap index funds often hold small cap stocks with low profitability or high asset growth, segments shown by research to strongly underperform the market. Excluding these stocks can increase expected returns for strategies holding small caps.

EXHIBIT 1

Overlap in Weight Among US Small Cap Indices as of December 31, 202

Holdings are subject to change. CRSP data is provided by the Center for Research in Security Prices, University of Chicago. S&P data © 2025 S&P Dow Jones Indices LLC, a division of S&P Global. All rights reserved. MSCI data © MSCI 2025, all rights reserved. Frank Russell Company is the source and owner of the trademarks, service marks, and copyrights related to the Russell Indexes. This article originally appeared in Above the Fray, a weekly newsletter for Dimensional clients.

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