This research note explores the historical and structural drivers of equity market concentration in the U.S., examining how concentration affects market efficiency, capital allocation, and investor returns. The authors argue that while high concentration levels are not unprecedented, they raise important questions about diversification, active management, and long-term expectations.mo
Stock Market Concentration
Morgan Stanley
Michael Mauboussin, Dan Callahan
Research
18 Pages
Key Takeaways
Concentration isn't new: Large-cap dominance has historical precedent and is often tied to fundamental shifts, not just investor enthusiasm
Impacts on active managers: High concentration raises the hurdle for active strategies benchmarked to broad indices like the S&P 500
Diversification remains vital: Investors should evaluate concentration risk at both index and portfolio levels to avoid unintentional exposures
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