S&P 500 index inclusion is no longer a payday. On September 4, 2026, S&P Dow Jones Indices added Bloom Energy, Illumina and Everpure and cut Molson Coors, The Trade Desk and Builders FirstSource, effective September 21. Five of those six moves are migrations inside the S&P Composite 1500 — offsetting flows that cancel each other out. S&P’s own research puts the median add-side excess return at -0.04%.
The index-inclusion trade has a great story and terrible math. This quarter’s rebalance is the cleanest illustration of that gap in years.
What changed in the S&P 500 on September 4, 2026?
S&P Dow Jones Indices announced its September quarterly rebalance after the close on September 4, 2026. Three companies join the S&P 500 and three leave, all effective before the open on September 21. The stated reason is that “the changes ensure that each index is more representative of its market capitalization range.”
The headline names are Bloom Energy (BE), Illumina (ILMN) and Everpure (P) on the way in. Molson Coors (TAP), The Trade Desk (TTD) and Builders FirstSource (BLDR) go out, all three landing in the S&P SmallCap 600.
The six S&P 500 changes, with the numbers that matter
Prices and volumes below are as of the September 4, 2026 close, per StockAnalysis.com. Weight estimates use full market cap against the $63.2 trillion S&P 500 market value GlobalData reported for January 2026, so treat them as approximations — the index has moved since, and S&P weights by float, not full cap.
| Ticker | Action | Sector | Price (Sep 4) | Market cap | Avg daily volume | Est. index weight | Moving from / to |
|---|---|---|---|---|---|---|---|
| BE | Add | Industrials | $252.87 | $74.5B | 16.4M sh | ~0.118% | Outside the S&P 1500 |
| P (Everpure) | Add | Info Tech | $99.51 | $33.2B | 2.2M sh | ~0.053% | From MidCap 400 |
| ILMN | Add | Health Care | $218.22 | $33.0B | 1.7M sh | ~0.052% | From MidCap 400 |
| TAP | Delete | Consumer Staples | $40.50 | $7.6B | 2.0M sh | ~0.012% | To SmallCap 600 |
| TTD | Delete | Comm. Services | $14.43 | $6.8B | 25.5M sh | ~0.011% | To SmallCap 600 |
| BLDR | Delete | Industrials | $65.79 | $7.1B | 2.4M sh | ~0.011% | To SmallCap 600 |
The asymmetry jumps out. The three additions carry $140.6 billion of market cap. The three deletions carry $21.4 billion. That is a 6.6-to-1 ratio, and it means the rebalance is not a swap — it is a net purchase funded by trimming every other holding.
Why is S&P 500 index inclusion no longer a payday?
Because the demand shock is no longer a surprise, and because most of it is offset. S&P Dow Jones Indices measured the median excess return of S&P 500 additions between announcement and effective date at 8.32% in 1995-1999, 3.64% in 2000-2010, and -0.04% in 2011-2021. The edge did not shrink. It vanished.
The research paper, written by Hamish Preston and Aye Soe and published in September 2021, tracks the collapse alongside an explosion in indexed money — from $577 billion tracking the S&P 500 at the end of 1996 to $5.427 trillion at the end of 2020.
That is the counterintuitive part. Ten times more passive money chasing the same index changes produced less price impact, not more.
What the academic work adds
Robin Greenwood and Marco Sammon of Harvard Business School reach the same conclusion in “The Disappearing Index Effect,” revised November 2023. Their measured abnormal returns for additions run 3.4% in the 1980s, 7.4% in the 1990s, 5.2% in the 2000s, and 1.0% in the 2010s — statistically insignificant by the end.
Their top explanation is the one this rebalance demonstrates: index migrations. When a stock moves from the MidCap 400 up into the S&P 500, the mid-cap trackers must sell it on exactly the day the large-cap trackers buy it. The demand shocks cancel.
Greenwood and Sammon also estimate that the multiplier measuring price impact per unit of demand fell roughly twenty-fold between the 1990s and the 2010s. Liquidity providers learned the trade and priced it away.
Apply that to September 2026. Five of the six S&P 500 changes are migrations within the S&P Composite 1500. Everpure and Illumina come up from the MidCap 400; TAP, TTD and BLDR drop into the SmallCap 600. Only Bloom Energy is a genuine outside entrant.
Is the add side or the delete side the better trade?
On the last decade of data, the delete side wins by a hair — and that is the whole point. S&P’s research puts additions at -0.04% median excess return over 2011-2021 and deletions at +0.06%. Both round to zero. Neither side is a trade; both are noise with transaction costs attached.
That inversion is historically strange. Deletions used to be brutal: -9.58% median excess return in 1995-1999 by S&P’s count.
Here is the honest read of the September 2026 tape. Bloom Energy rose roughly 7% in after-hours trading on the announcement, closing at $252.87 and trading at $266.14 by 7:59 p.m. ET. That pop is real. What the research says is that it does not persist to the effective date often enough to pay for the spread and the risk.
And Bloom Energy is the one name where a pop makes mechanical sense — it is the only true outside entrant, so nobody is forced to sell it into the index funds’ bid.
How much stock must index funds actually buy?
Less than the headlines imply. Bloomberg’s figure, cited widely ahead of the announcement, is that funds tracking nearly $27 trillion are affected. That number is benchmarked assets, not replicating assets, and the two are very different things.
Use a fund you can actually verify instead. Vanguard’s VOO held $1.07 trillion in assets as of September 5, 2026 at a 0.03% expense ratio, per StockAnalysis.com. Here is what one fund of that size has to do.
- Bloom Energy: roughly $1.26 billion to buy, against $4.16 billion of average daily dollar volume — about 0.3 days.
- Everpure: roughly $561 million against $214 million of daily dollar volume — about 2.6 days. This is the tight one.
- Illumina: roughly $558 million against $381 million daily — about 1.5 days.
- The three deletions combined: roughly $362 million to sell, spread across three names that trade $611 million a day between them.
Everpure is the name to watch. At 2.6 days of volume from a single fund, it is the only one of the six where the index bid is large relative to the float that trades. The MidCap 400 sellers absorb part of that, which is exactly the offsetting mechanic Greenwood and Sammon describe.
Worked example: what the rebalance does inside a $10,000 position
Take $10,000 in an S&P 500 fund. Apply the estimated weights from the table above and the arithmetic is small enough to be startling.
Bought on your behalf: $11.78 of Bloom Energy (0.118% × $10,000), $5.25 of Everpure, $5.21 of Illumina. Total purchases: $22.24.
Sold on your behalf: $1.19 of Molson Coors, $1.07 of The Trade Desk, $1.12 of Builders FirstSource. Total sales: $3.38.
Gross turnover is $25.62 — about 0.26% of your position. The $18.86 gap between buys and sells is funded by trimming every other holding fractionally.
Now price the cost. Assume five basis points of round-trip trading cost on that turnover. That is an illustrative assumption, not a published figure — funds do not disclose per-event execution cost. Five basis points on $25.62 is $0.0128. One and a third cents.
VOO’s 0.03% expense ratio on the same $10,000 costs $3.00 a year. The fee is roughly 234 times the modeled rebalance cost. If you care about what index investing costs you, the expense ratio is the number that matters, not the quarterly reshuffle — which is why the SPY, VOO and IVV fee gap deserves more of your attention than this announcement does.
What does The Trade Desk’s 430 days in the index tell you?
That inclusion is a lagging indicator of market cap, never a forward one. The Trade Desk joined the S&P 500 on July 18, 2025 at $80.21. It closed at $14.43 on September 4, 2026 — down 82.0% — with a market cap of $6.78 billion. From addition to effective removal is 430 days.
Every S&P 500 index fund bought TTD near $80 because a committee said so, and every one of them will sell it near $14 for the same reason. That is not a flaw in indexing. It is the mechanical cost of a rules-based, market-cap-weighted strategy, and it is priced into the 0.03% you pay.
The skeptical point worth holding onto: the same committee process that ejects TTD at a $6.8 billion market cap admitted Bloom Energy at $74.5 billion after a 506.8% increase in market cap. Index membership follows price. It does not lead it.
Which investor should care about the September rebalance?
Most should not. The rebalance moves about 0.26% of an S&P 500 fund’s book and costs pennies per $10,000. It matters at the margins for three specific groups, and matters for a fourth in a way most people get backwards.
| Investor type | Does this rebalance matter? | What the numbers say |
|---|---|---|
| Buy-and-hold S&P 500 index fund holder | No | ~$0.013 modeled cost per $10,000 vs $3.00/yr in expense ratio |
| Event-driven trader buying additions | Only if you can beat -0.04% | S&P DJI median add excess return, 2011-2021, was negative |
| S&P MidCap 400 fund holder | Yes, mechanically | Your fund sells Everpure and Illumina, buys TAP-sized names |
| Direct holder of TTD, TAP or BLDR | Yes, on liquidity | All three move to SmallCap 600; different tracking base, different flows |
| Tax-loss harvester | Possibly | Forced index selling can create liquidity windows around Sep 21 |
If you hold a broad index fund, the correct action on September 21 is nothing. That is also true of most macro events — the same conclusion the last Fed move produced for bond ETF holders.
S&P 500 index inclusion: frequently asked questions
When does the September 2026 S&P 500 rebalance take effect?
Before the market open on September 21, 2026. S&P Dow Jones Indices announced the changes after the close on September 4, 2026, giving funds roughly two weeks of notice.
Does a stock reliably rise when it joins the S&P 500?
Not anymore. S&P DJI measured the median excess return from announcement to effective date at -0.04% over 2011-2021, down from 8.32% in 1995-1999. Announcement-day pops still happen; they do not reliably persist.
Why was The Trade Desk removed from the S&P 500?
S&P cited market-cap representativeness. TTD’s market cap had fallen to $6.78 billion as of September 4, 2026, from a share price of $80.21 when it joined on July 18, 2025 to $14.43. It moves to the S&P SmallCap 600.
What is an index migration and why does it matter?
It is a stock moving between S&P indices rather than in or out of the S&P family entirely. Greenwood and Sammon identify migrations as the leading cause of the disappearing index effect, because mid-cap funds sell exactly what large-cap funds buy.
How much does my index fund pay to execute the rebalance?
Funds do not disclose it per event. Modeling five basis points of round-trip cost on $25.62 of turnover per $10,000 gives about 1.3 cents — roughly 1/234th of VOO’s $3.00 annual expense ratio on the same position.
Does index inclusion change a company’s fundamentals?
No. It changes who owns the shares and adds a permanent block of price-insensitive holders. Bloom Energy’s revenue and Illumina’s earnings are unchanged by the September 4 announcement.
The bottom line
S&P 500 index inclusion stopped being a tradable edge somewhere around 2011, and the September 2026 rebalance shows exactly why. Five of six changes are migrations that cancel their own flows. The one true entrant, Bloom Energy, needs 0.3 days of volume from the largest S&P 500 fund on earth.
What it depends on is narrow and specific: whether the added name comes from outside the S&P Composite 1500, and whether the required buy is large against its daily volume. On those two tests, Everpure — not the headline-grabbing Bloom Energy — is the only name in this batch with a mechanically tight setup, and even there the MidCap 400 sellers are on the other side.
For everyone holding a broad index fund, the number that decides your outcome is the expense ratio, not the reshuffle. Three dollars a year on $10,000 versus roughly one cent for the rebalance. The same logic that makes fee and custody structure decide the staking-ETF question decides this one.
This article is journalism, not investment advice. Do your own research before investing.
Sources
- S&P Dow Jones Indices — “Bloom Energy, Illumina, and Everpure Set to Join S&P 500,” September 4, 2026
- Hamish Preston & Aye Soe, S&P Dow Jones Indices — “What Happened to the Index Effect?” September 2021 (PDF)
- Robin Greenwood & Marco Sammon, Harvard Business School — “The Disappearing Index Effect,” revised November 2023 (PDF)
- S&P Dow Jones Indices — S&P 500 index page
- StockAnalysis.com — VOO assets, expense ratio and volume data (as of September 5, 2026)