The Market's Effective Breadth Collapsed
The number of stocks the market effectively is. Tens of thousands of issuers appear in institutional filings, but weighting by where the money sits collapses that to an effective count — the number of equal-sized stocks that would produce the same concentration. That count has fallen from roughly five hundred to roughly one hundred since 2013: the S&P 500 era ended statistically before anyone announced it.
What does it show?
Diversification is quietly disappearing from inside portfolios: an index holder owns thousands of tickers that behave, in value terms, like about a hundred.
Methodology
The effective number of issuers is the inverse Herfindahl-Hirschman index of issuer value shares across all reported stand-alone Form 13F long positions: the number of equally-weighted stocks that would produce the same concentration. Tens of thousands of issuers appear in filings each quarter, but the effective count weights them by where the money actually sits. Aggregation, amendment handling and unit conversion follow the capital-concentration chart. Form 13F covers reported long positions in U.S.-listed securities only: no short positions, cash, private assets, most foreign listings, or most derivatives. Reported value moves with prices as well as portfolio decisions. Issuer-name normalization cannot remove every filer abbreviation or corporate renaming. Obvious pooled products (index funds and ETFs) and option rows are excluded before aggregation.