| Week (Fri close) | Trigger drop | +1W | +4W | +12W | +26W | +52W |
|---|---|---|---|---|---|---|
| 2026-06-26 ● live | -2.54% | -0.02% | -0.18% | — | — | — |
| 2026-06-05 | -2.03% | -1.25% | -2.35% | -1.77% | — | — |
| 2026-02-06 | -2.26% | -1.58% | -0.62% | +4.06% | +2.90% | — |
| 2020-11-13 | -2.89% | -1.34% | -2.32% | -1.94% | -7.08% | -3.09% |
| 2020-07-17 | -2.31% | -0.62% | -0.15% | +0.47% | -5.81% | -4.96% |
| 2020-06-05 ⭐ Twin | -4.31% | +3.04% | +5.63% | +7.93% | +0.65% | -4.01% |
| 2020-05-22 | -2.20% | -1.50% | -2.37% | -1.07% | -4.07% | -9.44% |
| 2020-04-10 | -2.97% | +2.19% | +0.95% | +1.24% | +1.24% | -6.01% |
| 2009-05-08 | -2.46% | +3.90% | +0.71% | +1.25% | -0.92% | -5.52% |
| 2008-12-19 | -2.60% | +1.58% | -1.09% | +1.28% | -7.56% | -12.11% |
The SPY/RSP ratio prices the cap-weighted S&P 500 against its equal-weighted twin — the cleanest real-time read on market concentration. A rising ratio means a handful of mega-caps are carrying the index; a falling ratio means breadth is broadening and the average stock is winning. A weekly drop of 2% or more is rare and violent — it has happened just 10 times since 2003, and the live Jun 26, 2026 print sits in the top 27.5% of all weekly moves. Only the June-2020 "Twin" (−4.31%) was more extreme.
The forward path splits cleanly in two. Short term, the ratio snaps back: cap-weight tends to reassert as dip-buyers defend the mega-caps, with the cohort median peaking near week 12 at +1.2% (67% win rate) — and the 2020 Twin printed a dramatic +7.9%. Long term, the snap inverts: by one year the ratio has rolled over to a median of -5.5% with a 0% win rate — i.e. in every completed case, cap-weight was lower 52 weeks later. The shock is not a clearing event; it is the leading edge of a multi-quarter un-winding of concentration into equal-weight.
The win rate decays monotonically — a coin toss (~50%) through the first few months, then falling off a cliff: by week 20 it is below 50%, and by week 52 it is zero. That crossover, around weeks 20–39, is the pivot from "tactical long cap-weight" to "strategic rotate to equal-weight." The tail risk on the wrong side is severe: the worst 52-week outcomes ran to −9% to −12% (the 2008 and 2020 concentration peaks).
Why it works: extreme concentration is mean-reverting — when a few names dominate, valuation dispersion and crowding eventually unwind toward the average stock; the violent weekly drop is the market beginning to price that. But respect the sample. Only 10 events, heavily clustered in 2020 (six of ten), so the effective independent count is far smaller — treat the 0%/52-week figure as "every prior instance resolved this way," not a law. The numbers are in-sample (no walk-forward), the short-term snapback is noisy event-to-event (the Twin is the dramatic outlier; the median is milder), and a genuine new mega-cap leadership regime could break the pattern. Size the tactical leg as a trade and the strategic rotation as a tilt, not an all-in switch.