Everyone Owns the Same Market Now
How similar the biggest investors' portfolios have become. For a fixed cohort of fourteen giant managers — the same institutions at every date — the median pairwise overlap between portfolios roughly doubled since 2013. A second line tracks each quarter's fifty largest reporters, which rises faster partly because that group itself drifted toward index-style managers. Overlap is similarity, not proof of copying — but similar books mean correlated exits.
What does it show?
The doubling holds for the same fourteen institutions throughout, so it is behavior, not a changing cast: big money increasingly holds one portfolio.
Methodology
Pairwise overlap between two portfolios is the sum, across issuers, of the smaller of the two portfolio weights: 100% means identical weights, 0% means no common holdings. The signal line tracks a fixed cohort of 14 managers that ranked among the 50 largest stand-alone reporters throughout the period, so the same institutions are compared at every date. It is drawn only in the 30 quarters where every cohort member ranks in that quarter's 50 largest reporters; several fell below that line during the 2022-24 drawdown, and those quarters are left as gaps rather than plotted with a smaller cohort. The context line uses each quarter's current 50 largest reporters; it rises faster partly because that cohort itself drifted toward index-oriented managers, which is composition, not behavior — the reason the fixed cohort carries the claim. Overlap measures portfolio similarity, not copying or coordinated trading. 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.