Stock Market Concentration Has Surpassed Its 1930s Peak. Should Investors Worry?

Morningstar

Dan Lefkovitz

Research

8 Pages

Morningstar’s Dan Lefkovitz looks at the top-heavy nature of today’s US stock market and how it compares to the past 100 years.

Key Takeaways

Top Heavy: The top 10 reached 37.7% of the US stock market value on Oct. 31, 2025, surpassing the previous month-end peak of May 31, 1932, when the top 10 stocks collectively represented 37.3% of market value.
Bubble Indication?: Concentration doesn’t necessarily indicate a bubble, but It Is a risk factor.
Nondiversified: As for concentration, it’s worth noting that many funds have had to register as “nondiversified” with the Securities and Exchange Commission. Even broad market index funds may run afoul of diversification requirements defined under the Investment Company Act of 1940.
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