Explore the historical weight, earnings support, valuation, and forward-return evidence behind S&P 500 concentration in one research workspace.
Published September 21, 2026Latest component data June 30, 20264 research dimensions4 source studies
Concentration is not one statistic. This explorer keeps four different questions separate while making their evidence easy to compare. Each view preserves its own observation date, denominator, methodology boundary, source study, and downloadable data.
These are descriptive findings from different dates and methods. They are intentionally not collapsed into a single score.
History · June 30, 202538.15% top-10 index weight in June 2025
That was nearly the same as the reconstructed June 1965 endpoint of 38.24%, after a 20.34-point increase from June 2015.
Open source study →Earnings · March 31, 20264.7 percentage-point cap-minus-earnings gap in March 2026
The top 10 represented 37.9% of market cap and 33.2% of trailing earnings. The gap narrowed by 3.3 points from March 2024.
Open source study →Valuation · June 30, 20262.0x top-10 forward-P/E spread in June 2026
The top 10 traded at 21.6x forward earnings versus 19.6x for the remaining companies. The spread narrowed by 8.1x from March 2024.
Open source study →Forward returns · June 30, 2025-0.72 full-sample correlation across six decade starts
Excluding the June 1965 starting point changes the correlation to -0.04, showing that the small historical sample is highly sensitive to one observation.
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Explore history
How unusual is today's top-10 index weight in the long-run record?
38.15% top-10 index weight in June 2025. That was nearly the same as the reconstructed June 1965 endpoint of 38.24%, after a 20.34-point increase from June 2015.
Historical concentration and forward-return outcomes run through June 2025, earnings uses March 2026, and valuation uses June 2026.
Denominators stay separate
Index weight, earnings share, forward P/E, and historical return sensitivity answer different questions and are not one comparable unit.
Exact public observations only
The explorer does not trace or interpolate unlabeled observations from third-party charts. It uses the exact public records admitted by the underlying studies.
No market-timing score
The forward-return sample is small and sensitive to one starting point. The explorer does not convert concentration into a buy, sell, or fair-value signal.
