{
  "title": "S&P 500 Concentration History: How the Top 10 Changed From 1965 to 2025",
  "path": "/research/historical-sp500-concentration",
  "publishedAt": "2026-09-20",
  "updatedAt": null,
  "dataAsOf": "2025-06-30",
  "version": "1.0",
  "endpoints": [
    { "date": "1965-06-30", "topTenWeightPct": 38.24 },
    { "date": "1975-06-30", "topTenWeightPct": 29.08 },
    { "date": "1985-06-30", "topTenWeightPct": 19.47 },
    { "date": "1995-06-30", "topTenWeightPct": 17.67 },
    { "date": "2005-06-30", "topTenWeightPct": 20.92 },
    { "date": "2015-06-30", "topTenWeightPct": 17.81 },
    { "date": "2025-06-30", "topTenWeightPct": 38.15 }
  ],
  "decades": [
    {
      "startDate": "1965-06-30",
      "endDate": "1975-06-30",
      "annualizedSp500PriceReturnPct": 1.17,
      "concentrationChangePercentagePoints": -9.16,
      "startTopTenWeightPct": 38.24,
      "endTopTenWeightPct": 29.08
    },
    {
      "startDate": "1975-06-30",
      "endDate": "1985-06-30",
      "annualizedSp500PriceReturnPct": 10.84,
      "concentrationChangePercentagePoints": -9.61,
      "startTopTenWeightPct": 29.08,
      "endTopTenWeightPct": 19.47
    },
    {
      "startDate": "1985-06-30",
      "endDate": "1995-06-30",
      "annualizedSp500PriceReturnPct": 12.53,
      "concentrationChangePercentagePoints": -1.80,
      "startTopTenWeightPct": 19.47,
      "endTopTenWeightPct": 17.67
    },
    {
      "startDate": "1995-06-30",
      "endDate": "2005-06-30",
      "annualizedSp500PriceReturnPct": 10.00,
      "concentrationChangePercentagePoints": 3.25,
      "startTopTenWeightPct": 17.67,
      "endTopTenWeightPct": 20.92
    },
    {
      "startDate": "2005-06-30",
      "endDate": "2015-06-30",
      "annualizedSp500PriceReturnPct": 5.46,
      "concentrationChangePercentagePoints": -3.11,
      "startTopTenWeightPct": 20.92,
      "endTopTenWeightPct": 17.81
    },
    {
      "startDate": "2015-06-30",
      "endDate": "2025-06-30",
      "annualizedSp500PriceReturnPct": 11.66,
      "concentrationChangePercentagePoints": 20.34,
      "startTopTenWeightPct": 17.81,
      "endTopTenWeightPct": 38.15
    }
  ],
  "june1965TopTen": [
    { "company": "AT&T", "weightPct": 9.11 },
    { "company": "General Motors", "weightPct": 7.06 },
    { "company": "Standard Oil of New Jersey (Exxon)", "weightPct": 4.36 },
    { "company": "IBM", "weightPct": 4.15 },
    { "company": "Du Pont", "weightPct": 2.80 },
    { "company": "Texaco", "weightPct": 2.71 },
    { "company": "Sears, Roebuck & Co.", "weightPct": 2.68 },
    { "company": "General Electric", "weightPct": 2.24 },
    { "company": "Eastman Kodak", "weightPct": 1.66 },
    { "company": "Gulf Oil (Chevron)", "weightPct": 1.47 }
  ],
  "june2025TopTenCohort": [
    { "company": "JPMorgan Chase", "entryDateLabel": "June 1973", "startingWeightPct": 0.04, "maximumWeightPct": 1.90 },
    { "company": "NVIDIA", "entryDateLabel": "November 2001", "startingWeightPct": 0.07, "maximumWeightPct": 7.30 },
    { "company": "Amazon", "entryDateLabel": "November 2005", "startingWeightPct": 0.13, "maximumWeightPct": 5.00 },
    { "company": "Apple", "entryDateLabel": "November 1982", "startingWeightPct": 0.18, "maximumWeightPct": 7.70 },
    { "company": "Broadcom", "entryDateLabel": "July 2000", "startingWeightPct": 0.37, "maximumWeightPct": 2.50 },
    { "company": "Meta", "entryDateLabel": "December 2013", "startingWeightPct": 0.58, "maximumWeightPct": 3.10 },
    { "company": "Alphabet", "entryDateLabel": "April 2006", "startingWeightPct": 0.67, "maximumWeightPct": 4.40 },
    { "company": "Microsoft", "entryDateLabel": "June 1994", "startingWeightPct": 0.94, "maximumWeightPct": 7.30 },
    { "company": "Berkshire Hathaway", "entryDateLabel": "February 2010", "startingWeightPct": 1.28, "maximumWeightPct": 2.10 },
    { "company": "Tesla", "entryDateLabel": "December 2020", "startingWeightPct": 1.58, "maximumWeightPct": 2.40 }
  ],
  "june2025TopTenCohortAverages": {
    "startingWeightPct": 0.58,
    "maximumWeightPct": 4.37
  },
  "fullPeriod": {
    "startDate": "1965-06-30",
    "endDate": "2025-06-30",
    "annualizedSp500PriceReturnPct": 7.42,
    "concentrationChangePercentagePoints": -0.09
  },
  "selectedFindings": {
    "june1965TopTenWeightPct": 38.24,
    "june2015TopTenWeightPct": 17.81,
    "june2025TopTenWeightPct": 38.15,
    "lowestDecadeEndpointDate": "1995-06-30",
    "lowestDecadeEndpointWeightPct": 17.67,
    "fiftyYearDeclinePercentagePoints": 20.43,
    "finalDecadeIncreasePercentagePoints": 20.34,
    "priorDeclineReversedPct": 99.55947136563876,
    "june2025VsJune2015Multiple": 2.1420550252667043,
    "fullPeriodConcentrationChangePercentagePoints": -0.09,
    "fullPeriodAnnualizedSp500PriceReturnPct": 7.42,
    "decadesWithRisingConcentration": 2,
    "decadesWithFallingConcentration": 4
  },
  "sources": {
    "historicalStudy": "https://www.spglobal.com/spdji/en/documents/research/research-in-the-shadows-of-giants.pdf",
    "methodology": "https://www.spglobal.com/spdji/en/methodology/article/sp-us-indices-methodology/"
  },
  "methodology": {
    "endpointReconstruction": "Start with the published June 1965 top-10 company weights and add S&P DJI's published decade-level changes in top-10 weight.",
    "frequencyBoundary": "The seven public concentration values are decade endpoints. They are not digitized estimates of the proprietary quarterly series shown in the source report.",
    "returnBoundary": "S&P 500 returns are annualized price returns in USD and exclude dividends, matching the source table.",
    "inferenceBoundary": "The six decade blocks are descriptive. They do not establish that concentration changes cause returns or provide a market-timing rule."
  }
}