S&P 500 Concentration History: How the Top 10 Changed From 1965 to 2025
One decade nearly reversed 50 years of S&P 500 deconcentration.
S&P Dow Jones Indices publishes a June 1965 top-10 starting weight and six decade-by-decade changes through June 2025. Reconstructing those endpoints shows top-10 concentration falling from 38.24% in 1965 to 17.81% in 2015, then rising +20.34 pp in the next decade to 38.15%.
The decade endpoints form a U-shaped concentration history
The concentration measure fell for most of the first five decades, reached its lowest reconstructed decade endpoint in June 1995, then returned almost exactly to its 1965 level by June 2025.
S&P 500 top-10 concentration at seven decade endpoints
Top-10 concentration starts at 38.24% in June 1965, falls to a lowest reconstructed decade endpoint of 17.67% in June 1995, and rises to 38.15% in June 2025.
2015–2025 almost erased the previous 50-year decline
From June 1965 to June 2015, the reconstructed top-10 weight fell 20.43 percentage points. In the next decade it rose 20.34 percentage points.
That one decade reversed 99.6% of the prior 50-year decline. The June 2025 endpoint was 2.14× the June 2015 level.
Strong returns occurred while concentration was both falling and rising
S&P DJI publishes annualized S&P 500 price returns for the same six decade blocks. The table is descriptive, not a causal test.
| Period | Start top 10 | End top 10 | Change | Annualized S&P 500 price return |
|---|---|---|---|---|
| 1965–1975 | 38.24% | 29.08% | -9.16 pp | 1.17% |
| 1975–1985 | 29.08% | 19.47% | -9.61 pp | 10.84% |
| 1985–1995 | 19.47% | 17.67% | -1.80 pp | 12.53% |
| 1995–2005 | 17.67% | 20.92% | +3.25 pp | 10.00% |
| 2005–2015 | 20.92% | 17.81% | -3.11 pp | 5.46% |
| 2015–2025 | 17.81% | 38.15% | +20.34 pp | 11.66% |
The highest price return in these six decade blocks was 12.53% annualized from 1985 to 1995, when top-10 concentration fell 1.80 percentage points. The 2015–2025 decade returned 11.66% annualized while concentration rose 20.34 points.
The concentration can persist even when the giants change
The June 1965 top 10 were a very different set of companies. S&P DJI's June 2025 cohort table also shows that the later leaders generally entered the index at much smaller weights than the maximum weights they eventually reached.
| June 1965 company | Index weight |
|---|---|
| AT&T | 9.11% |
| General Motors | 7.06% |
| Standard Oil of New Jersey (Exxon) | 4.36% |
| IBM | 4.15% |
| Du Pont | 2.80% |
| Texaco | 2.71% |
| Sears, Roebuck & Co. | 2.68% |
| General Electric | 2.24% |
| Eastman Kodak | 1.66% |
| Gulf Oil (Chevron) | 1.47% |
S&P DJI reports that the June 2025 top-10 cohort had an average starting index weight of only 0.58%, compared with an average maximum quarter-end weight of 4.37%.
| June 2025 top-10 cohort | S&P 500 entry | Starting weight | Maximum quarter-end weight |
|---|---|---|---|
| JPMorgan Chase | June 1973 | 0.04% | 1.90% |
| NVIDIA | November 2001 | 0.07% | 7.30% |
| Amazon | November 2005 | 0.13% | 5.00% |
| Apple | November 1982 | 0.18% | 7.70% |
| Broadcom | July 2000 | 0.37% | 2.50% |
| Meta | December 2013 | 0.58% | 3.10% |
| Alphabet | April 2006 | 0.67% | 4.40% |
| Microsoft | June 1994 | 0.94% | 7.30% |
| Berkshire Hathaway | February 2010 | 1.28% | 2.10% |
| Tesla | December 2020 | 1.58% | 2.40% |
This is the mechanical case for capitalization weighting, not a prediction about today's leaders. If a future winner is already in the index, its weight can grow with its float-adjusted market value without an investor having to identify the company in advance.
What this history does not prove
The seven concentration points also should not be read as yearly or quarterly observations. The source report contains a richer quarter-by-quarter history. This study deliberately publishes only the endpoints that can be reconstructed exactly from S&P DJI's tables without digitizing its chart.
For the current concentration and company-level AI exposure question, see the separate S&P 500 AI concentration study.
Methodology
- Sum the 10 June 1965 company weights published in S&P DJI's Exhibit 3. The result is 38.24%.
- Starting from that value, add each decade's published change in top-10 weight from Exhibit 5 and round each endpoint to two decimals.
- Pair each reconstructed concentration interval with S&P DJI's annualized S&P 500 price return for the same dates. Those returns exclude dividends.
- Use Exhibit 7 only for the June 2025 top-10 cohort's entry dates, starting weights, and maximum quarter-end weights.
- Do not digitize the quarterly concentration chart, interpolate missing years, or infer a causal relationship from the six decade observations.
Primary sources
Download the reconstructed history
The CSV contains the six matched decade periods with start/end concentration and price returns. The JSON adds all seven endpoints, the 1965 top-10 weights, the June 2025 cohort entry statistics, methodology notes, and source URLs.
Research data
- Download CSVCSVCSV downloadTabular public study data for spreadsheet analysis, independent checks, and new charts.Data snapshot June 30, 2025 · Reuse with attribution to the canonical study.
- Download JSONJSONJSON downloadStructured public study data for programmatic verification while preserving the published field names and research context.Data snapshot June 30, 2025 · Reuse with attribution to the canonical study.
Citation and reuse
When citing the concentration history, describe the seven values as reconstructed decade endpoints from S&P DJI's published tables. Do not describe the CSV as the source's quarter-by-quarter concentration series.