Posted by Lubos Pastor, Taisiya Sikorskaya, and Jinrui Wang (University of Chicago Booth School of Business), on
Tuesday, July 7, 2026
Lubos Pastor is the Charles P. McQuaid Distinguished Service Professor of Finance, Taisiya Sikorskaya is an Assistant Professor of Finance and a Fama Faculty Fellow, and Jinrui Wang is a Research Professional, all at the University of Chicago Booth School of Business. This post is based on their recent paper.
Market Concentration and Fund Regulation
The U.S. stock market has become highly concentrated. Between 2015 and 2024, the share of the ten largest stocks in total U.S. market capitalization rose from 13% to 31%. In large-cap growth, it rose from 30% to 48%. By the end of 2024, the “Magnificent 7” stocks alone represented roughly one-third of the S&P 500 and more than half of the Russell 1000 Growth Index.
This rise in concentration has received substantial attention, often in connection with market power and the rise of a handful of dominant firms. We examine a different implication. As a few stocks have come to dominate major benchmarks, an old diversification rule has begun to bind, forcing funds to trim the largest stocks, with consequences for fund performance and stock prices.
Figure 2 illustrates why this interaction matters today: stock market concentration has risen rapidly just as mutual funds and exchange-traded funds (ETFs) have become much larger holders of U.S. equities.
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Comment Letter on the SEC’s Proposal to Replace Quarterly Reporting with Semiannual Reporting
More from: Jeff Mahoney, Council of Institutional Investors
Jeffrey P. Mahoney is General Counsel at the Council of Institutional Investors. This post is based on his SEC comment letter.
The Council of Institutional Investors (CII)[1] respectfully submits this letter in response to the Securities and Exchange Commission’s (SEC or Commission) request for comments on its proposed “amendments to allow companies to file semiannual reports on Form 10-S in lieu of quarterly reports on Form 10-Q to meet their interim reporting obligations under the Securities Exchange Act of 1934 (‘Exchange Act’)” (Proposed Rule).[2] CII opposes the Proposed Rule. CII is a nonprofit, nonpartisan association of U.S. public, corporate and union employee benefit funds, other employee benefit plans, state and local entities charged with investing public assets, and foundations and endowments with combined assets under management of approximately $5.2 trillion. Our member funds include major long-term shareowners with a duty to protect the retirement savings of millions of workers and their families, including public pension funds with more than 15 million participants – true and real “Main Street” investors through their pension funds. Our associate members include non-U.S. asset owners with about $5.8 trillion in assets, and a range of asset managers with more than $74 trillion in assets under management.
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