The Hidden Cost of the Magnificent Seven
How market concentration can hurt fund performance.
- By
- July 27, 2026
- CBR - Finance
The $10 trillion that US investors have riding on the mutual funds and exchange-traded funds that own large-cap stocks has become a more concentrated bet. A decade ago, the 10 largest companies accounted for about 13 percent of total market capitalization, explain Chicago Booth’s Lubos Pastor and Taisiya Sikorskaya and former Booth research professional Jinrui Wang (now a PhD student at Duke). Today, they find, just seven tech companies account for almost one-third of the market’s total value and more than half of the Russell 1000 Growth Index.
This dominance is likely raising investor concerns about diversification, and the researchers reveal another, less obvious risk. They make a case that this concentration may force funds to trim their largest holdings, which could move stock prices and hurt fund performance.
The outsize footprint of the behemoth stocks, referred to by many in the markets as the Magnificent Seven, is pushing some funds and ETFs toward breaching a federal rule that has been on the books for 90 years. Most mutual funds and ETFs follow a diversification rule that limits how much of a given stock they can hold. The 50/5/10 regulation requires that, for at least half of a fund’s portfolio, no single stock position can make up more than 5 percent of the fund’s total assets (or 10 percent of the company’s voting stock, which is far less common).
For decades, this rule of the Internal Revenue Service was basically a formality that few large-cap managers had to worry about. But then came the rise of the Magnificent Seven, which at the time of the study were Alphabet, Amazon, Apple, Meta Platforms, Microsoft, Nvidia, and Tesla.
Pastor, Sikorskaya, and Wang analyzed 2019–24 data for nearly 5,000 US equity funds, focusing on how close each was to breaking the rule at any given moment.
They deemed any fund within 5 percentage points of breaching the 50 percent ceiling to be “constrained.” While actually crossing the 50 percent threshold is rare, the researchers argue that in practice, any manager who gets that close effectively sees a flashing yellow light that the portfolio needs to be tweaked to avoid an outright breach.
In the study, the peak of constrained concentration was in third quarter 2024, when 171 funds managing nearly $1.4 trillion, or about 8 percent of total fund assets, were in the yellow-light zone. Among large-cap growth funds, roughly one in three portfolios was in the danger zone, and those funds represented about half the category’s total assets.
A few stocks dominate the market
The largest companies accounted for nearly half the market value of large-cap growth stocks at the end of 2024, pushing funds closer to diversification limits.
Constrained funds can respond in two ways to stay compliant: The more common move is to trim the stocks that are triggering the warning light. Or the funds can dilute the problem away by investing new money flows in cash, other nonstock assets, or stocks besides the Magnificent Seven—all which reduce the overall footprint of any single stock as a share of the total portfolio.
Either way, performance takes a hit. For large-cap growth funds under these constraints, returns ran about 28 basis points lower, on average, over the three months after the trimming prompted by 50/5/10 concerns. During a period of strain in 2023–24—when the Magnificent Seven’s dominance was at and near its most extreme—the drag was more than 1 full percentage point.
There’s also an opportunity cost for funds forced to trim their biggest holdings. The researchers find a clear pattern: When enough large funds sell the same stocks at the same time to stay within regulatory limits, that selling pressure can push prices down—even though nothing has changed about the underlying value of those companies. The price drop appears to be a short-term effect linked to the wave of rule-induced selling. As that pressure fades and the selling stops, prices tend to recover.
During 2023–24, Pastor, Sikorskaya, and Wang find, stocks affected by the constraint went on to outperform unaffected stocks by 2.3 percent over the next six months. The constrained funds, having reduced the overall size of their investment in those stocks, effectively got less of that bounce than they otherwise would have. (While the researchers note this finding is statistically significant, they also caution it is based on a relatively short window of data.)
This pattern is a mirror image of another well-known market effect, they say. Just as rules that limit short selling can artificially inflate a stock’s price by keeping pessimists out of the market, rules that limit how much optimists can own can temporarily push prices down. In both cases, the rules prevent certain investors’ views from being fully reflected in stock prices, leading to either overpricing or underpricing until the pressure eases.
Importantly, the study finds that some index funds face the same constraint as actively managed funds, respond similarly, and share the hit to performance.
That may be news to index-fund investors who presume they are passively tracking a given benchmark. In reality, indexes can be pressured to change their weightings proactively to ensure they don’t run afoul of the 50/5/10 rule. In July 2023, the Nasdaq 100 was rebalanced to reduce the weighting of the Magnificent Seven, specifically to help tracking funds comply with the IRS rule.
While their research focused on the direct impact to constrained funds, Pastor, Sikorskaya, and Wang note that all investors are ultimately affected, given that the selling can contribute to a temporary dip in stock prices. And that most likely was not an intention of the 1936 regulation.
“A reassessment of regulatory limits or benchmark construction may be needed to ensure that diversification rules continue to protect investors without inadvertently impairing market efficiency,” the researchers conclude.
Lubos Pastor, Taisiya Sikorskaya, and Jinrui Wang, “The Hidden Cost of Stock Market Concentration: When Funds Hit Regulatory Limits,” Working paper, March 2026.
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