With the top 10 US stocks today representing a bigger percentage of overall market value than they did at the height of the late 1990s dot-com Bubble, investors are increasingly worried about concentration risk. While not discounting this worry, Morningstar Indexes, with help from its new Center for Research in Security Prices (CRSP) US stock database, helps put the question in much broader historical context.
In his recent article―Stock Market Concentration Has Surpassed Its 1930s Peak. Should Investors Worry?―Morningstar Indexes strategist Dan Lefkovitz taps into the extensive US equity database from CRSP, acquired by Morningstar from the University of Chicago in 2026, to examine the nuances of US equity market concentration over the past century.
Notably, current concentration is not as unusual as some believe when considered in the context of 100 years of US stock market history. From the 1920s to the late 1960s, the weight of the top 10 US stocks regularly exceeded 25% of overall market value. The latest top-heavy US equity market trend actually began in 2020, when the top 10 US stocks breached 20% of market value, reaching 37.7% on October 31, 2025, according to CRSP—a level that exceeded its previous peak in 1932.
Alex Poukchanski, Director of Analytics, Morningstar Indexes & CRSP Research Data Products, said:
“Historical context is key for investors. The 100 years of US stock market data that CRSP has compiled covers market returns through major economic and global changes, enabling investors to observe a range of outcomes to make informed decisions. Market concentration is just one example where applying a historical lens can give investors better context to inform long-term decision making.”
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