S&P 500 concentration: historical weights need a dated register

A dated correction preserves benchmark and index-level definitions while withdrawing unsupported historical weights, forward returns and allocation thresholds.

What changed in this article

Editorial correction · 9 September 2026. The original article was published on 19 February 2026 and said the top ten S&P 500 names were near 40% of index weight. It also presented three concentration episodes, 524 weekly observations through 13 February 2026 and +6.3 percentage points of next-12-month equal-weight outperformance above a 35% threshold. The page did not preserve historical constituent weights, the weekly universe, episode-selection rule, benchmark rows or calculations. Those weight, sample, forward-return, threshold and allocation claims are withdrawn. The original URL and publication date remain unchanged.

An index methodology and a current fund holdings page define a benchmark. They do not recreate a historical constituent-weight series.

What the dated records establish

Source	Dated fact	Boundary
State Street SPY benchmark page	State Street describes SPY as seeking the price and yield performance of the S&P 500 and describes the benchmark as a diversified, float-adjusted market-capitalization-weighted large-cap index.	The current product page is not a historical February 2026 top-ten-weight snapshot.
S&P 500 series at FRED	FRED identifies SP500 as a daily closing price index sourced to S&P Dow Jones Indices. A retrieved 13 February 2026 close was 6,836.17.	This is a price-index boundary. It does not supply constituent weights, a total-return RSP comparison or a forward return.
S&P U.S. Indices Methodology	S&P Dow Jones Indices publishes methodology material for U.S. indices.	The article did not preserve the historical constituent file and cannot be reconstructed from a methodology landing page alone.

What is withdrawn

The near-40% top-ten weight, high-30s February reading, three-episode sample, 524-week panel, 2016–2026 observation window as a verified dataset, 35% regime threshold, +6.3pp next-12-month result, breadth/volatility association and 0.50% incremental-risk rule are not verified findings. No replacement concentration level, forecast or allocation instruction is published.

The neutral question—how concentration changes through time and how cap-weighted and equal-weighted benchmarks differ—remains possible after a dated constituent-weight and forward-return register is frozen.

Minimum register for a future concentration study

Record the index and fund identifiers; historical constituent files and weights; reconstitution and corporate-action treatment; observation dates; price or total-return convention; cap- and equal-weight benchmark rows; regime and episode rules; forward horizons; missing data; and complete calculations.

Original publication: 19 February 2026. Correction: 9 September 2026. AI-assisted source checking and writing; no independent human expert review. This page is educational market research, not personalized investment advice.