Do Stocks Held by Concentrated 13F Managers Beat the Index? Evidence from SEC Form 13F Filings, 2013–2025

This paper measures the twelve-month performance, after publication, of stocks held by "concentrated managers": SEC Form 13F filers with 5 to 100 stocks, at least 45% in their top ten positions, and low turnover. The sample has 129,534 stock-quarter observations of 8,228 stocks over 48 quarters (2013–2025) and includes stocks that stopped trading. With equal weights, price returns without dividends, and prices implied by the filings themselves, the average stock returned 3.7 percentage points less than the S&P 500 (Newey-West t −1.56, not significant); 40% of cases beat the index. Across 52 groups, the most consistent result is negative: stocks no manager had been adding to lagged the rest by 0.5 points (t −3.60). The gap grows with consecutive quarters of accumulation (+0.5 to +2.2 points), but the group with four or more quarters misses the usual threshold, and no accumulation group beat the index. Copying a portfolio with its weights, costs, taxes and dividends are not measured. This record holds the working paper (English and Spanish), the study observations (129,534 rows for concentrated managers and 258,817 for all 13F filers, Parquet and CSV), their data dictionary, the published results table and a short script that rebuilds the headline figure. It is not investment advice.