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S&P 500 Market-Cap Concentration

Name: S&P 500 Market-Cap Concentration
Creator: LENSE Analytics

S&P 500 Market-Cap Concentration — Top 10 vs. Bottom 490

Source: Company filings + daily price data

Data updated quarterly

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LENSE Analysis — S&P 500 Market-Cap Concentration

Updated Jun 16, 2026

The S&P 500's top-10 concentration data is unavailable in this bundle, preventing a grounded analysis of that specific indicator. However, the cross-metric snapshot and constituent earnings reveal a market heavily skewed toward mega-cap technology and AI-adjacent names—NVDA, GOOGL, AAPL, MSFT, and AMZN collectively represent roughly 44% of the index by weight—with earnings growth concentrated in semiconductor and cloud infrastructure plays posting triple-digit EPS gains (NVDA +108.9%, MU +410.9%, AVGO +125.4%) while traditional sectors lag. The equity risk premium has turned negative at -1.03%, a structural warning that valuations have decoupled from risk compensation, leaving the index vulnerable to any disappointment in the AI capex cycle or a shift in geopolitical sentiment that could unwind the concentration premium currently embedded in mega-cap tech multiples.

What it measures

The S&P 500 concentration index measures the share of total index market capitalization held by the top 10 largest constituents versus the remaining 490, alongside two rebased performance indexes tracking each cohort independently since Q1 1999. A top-10 weight of 35% means the ten largest companies account for more than a third of the entire index's market cap.

Why it matters

Market-cap concentration determines how closely the index return tracks any individual stock and how much diversification the index actually provides. At high concentration, the index return is dominated by a handful of mega-caps; the average constituent's experience diverges significantly from the index return. Historically, extended periods of high concentration have preceded mean-reversion as prior cycle winners cede leadership. Comparing the performance of the top-10 versus bottom-490 cohorts reveals whether concentration is driven by superior earnings growth (sustainable) or multiple expansion (more vulnerable to reversal). The current AI-era concentration is the highest since the technology bubble peak of 2000.

How it is calculated

Top-10 Weight (%) = Σ Market Cap (top 10 constituents) ÷ Σ Market Cap (all constituents) × 100

LENSE computes the S&P 500 concentration metric at quarterly frequency using quarter-end market capitalizations sourced from daily price and share count data. The top-10 constituents are ranked by market cap as of each quarter-end. Index constituency at each quarter is resolved point-in-time using point-in-time index constituency, ensuring the composition reflects actual historical membership. Performance indexes for each cohort are computed as rebased total-return series starting at 100 in Q1 1999, weighted by market cap within each cohort.

Recent (quarterly)

Recent data unavailable.

Data source: Company filings + daily price data. Computed and published by LENSE Analytics.