Index inclusion or removal
📈 MarketThe stock is added to or dropped from a major index such as the S&P 500.
Typical size
3-10% on announcement for additions
Timeframe
Announcement to effective date, then often fades
Evidence
3 logged events
Why it moves the price
Purely mechanical: index funds must buy (or sell) the stock regardless of price or fundamentals, creating enormous non-discretionary flow on a known date.
What separates a good call from a naive one
One of the few genuinely predictable flow events — but it is well known, so much of the move happens on the announcement and pre-positioning has weakened the effect over the years. The post-inclusion drift is frequently negative.
Measured impact
Every logged event is measured against real closing prices: the move from the close before the event, compared with the S&P 500 over the same window.
Next day
+16.95%
- Median
- +18.65%
- Moved up
- 100% of 3
- vs S&P 500
- +16.94%
After 1 week
+27.06%
- Median
- +27.40%
- Moved up
- 100% of 3
- vs S&P 500
- +26.92%
After 1 month
+28.37%
- Median
- +20.97%
- Moved up
- 100% of 3
- vs S&P 500
- +26.23%
The logged evidence agrees with the textbook expectation. Based on 3 measured events.
The evidence (3)
Every row is a real event, measured against actual closing prices.
| Date | Stock | What happened | Next day | 1 week | 1 month | vs S&P |
|---|---|---|---|---|---|---|
| May 13, 2025 | COIN | Named as an S&P 500 addition Announced after the close on May 12. | +23.97% | +27.40% | +20.97% | +23.31% |
| Mar 4, 2024 | SMCI | Named as an S&P 500 addition Announced Friday Mar 1; this is the first session after. | +18.65% | +25.90% | +11.53% | +18.76% |
| Nov 17, 2020 | TSLA | S&P announces Tesla will join the S&P 500 Announced after the close on Nov 16. | +8.21% | +27.88% | +52.61% | +8.75% |
Measured from historical closing prices. Past reactions do not guarantee future ones. Not financial advice.