{
  "title": "S&P 500 Concentration and Forward Returns: Does a Top-Heavy Market Predict the Next Decade?",
  "path": "/research/sp500-concentration-forward-returns",
  "publishedAt": "2026-09-20",
  "updatedAt": null,
  "dataAsOf": "2025-06-30",
  "version": "1.0",
  "observations": [
    {
      "startDate": "1965-06-30",
      "endDate": "1975-06-30",
      "startingTopTenWeightPct": 38.24,
      "annualizedSp500PriceReturnPct": 1.17
    },
    {
      "startDate": "1975-06-30",
      "endDate": "1985-06-30",
      "startingTopTenWeightPct": 29.08,
      "annualizedSp500PriceReturnPct": 10.84
    },
    {
      "startDate": "1985-06-30",
      "endDate": "1995-06-30",
      "startingTopTenWeightPct": 19.47,
      "annualizedSp500PriceReturnPct": 12.53
    },
    {
      "startDate": "1995-06-30",
      "endDate": "2005-06-30",
      "startingTopTenWeightPct": 17.67,
      "annualizedSp500PriceReturnPct": 10
    },
    {
      "startDate": "2005-06-30",
      "endDate": "2015-06-30",
      "startingTopTenWeightPct": 20.92,
      "annualizedSp500PriceReturnPct": 5.46
    },
    {
      "startDate": "2015-06-30",
      "endDate": "2025-06-30",
      "startingTopTenWeightPct": 17.81,
      "annualizedSp500PriceReturnPct": 11.66
    }
  ],
  "selectedFindings": {
    "pearsonCorrelation": -0.72,
    "pearsonExcluding1965": -0.04,
    "leaveOneOutMinCorrelation": -0.87,
    "leaveOneOutMaxCorrelation": -0.04,
    "highestStartingConcentrationPct": 38.24,
    "highestStartingConcentrationForwardReturnPct": 1.17,
    "lowestStartingConcentrationPct": 17.67,
    "lowestStartingConcentrationForwardReturnPct": 10,
    "minimumForwardReturnPct": 1.17,
    "maximumForwardReturnPct": 12.53
  },
  "sources": {
    "historicalStudy": "https://www.spglobal.com/spdji/en/documents/research/research-in-the-shadows-of-giants.pdf"
  },
  "methodology": {
    "observationBoundary": "Six non-overlapping decade starts from June 1965 through June 2015 are paired with the following 10-year annualized S&P 500 price return.",
    "returnBoundary": "Returns are S&P 500 price returns in USD and exclude dividends, matching the source.",
    "timingBoundary": "The predictor is starting top-10 concentration, not the change in concentration during the return period.",
    "currentBoundary": "June 2025 concentration is not included because its subsequent 10-year return is not yet observable.",
    "frequencyBoundary": "Starting concentrations are exact decade endpoints reconstructed from published table values. The proprietary quarterly chart is not digitized.",
    "inferenceBoundary": "This study tests starting top-10 concentration against the following decade's annualized S&P 500 price return. It is different from the source paper's discussion of changes in concentration versus contemporaneous index performance."
  }
}