How Much Does Joining an Index Make a Stock Rise? A Popular Myth Worth Dropping
Every time word gets out that a big company is about to join the S&P 500, someone asks the same question right away: so how much does it go up? Behind the question sits a popular assumption. Index funds are "forced" by the rules to buy, the buying floods in, and the stock naturally floats higher.
The assumption is half right. The mechanical buying the rules compel is real, and it can be quantified down to a specific percentage. But the "inclusion premium" that buying was supposed to push up has all but disappeared over the past two decades. The Harvard Business School study by Greenwood and Sammon, "The Disappearing Index Effect," supplies the first-hand data: the abnormal return from announcement to effective date for an S&P 500 inclusion has fallen from an average of 7.4% in the 1990s to 0.3% over the last decade, and the latter is statistically no different from zero (HBS / NBER w30748, published in the Journal of Finance 2025).
The passive buying is real; the inclusion premium no longer is. This piece pulls the two apart, answers what a spot in an index is actually worth, and lands on the most extreme sample on the board right now: SpaceX, which went public on 2026-06-12 with a float of just 4.25%.
The First Gate: Why SpaceX Can Join the Nasdaq-100 but Not the S&P 500
Before weighing what inclusion is worth, look at what gets a stock in the door at all. The three big U.S. index families hand out tickets on very different terms, and SpaceX happens to sit right in the gap between them.
| Dimension | S&P 500 | Nasdaq-100 | Russell 1000/2000 |
|---|---|---|---|
| Profit hurdle | Yes: GAAP earnings screen | None | None |
| Market cap / eligibility | Cap floor + liquidity + ~12-month listing seasoning | Pure market-cap ranking (top 100 non-financials) | Ranked by total market cap into tiers (rank 1,000 splits 1000/2000) |
| Fastest inclusion for a new listing | Must clear every hurdle first | ~15 trading days (fast track) | 5th trading day (IPO into Russell Top 500) |
| Decision method | Index committee, with discretion | Rules-based | Rules-based |
The S&P 500 sets a hard profit hurdle: under the S&P Dow Jones Indices methodology, a candidate must show positive GAAP earnings both in its most recent quarter and over the most recent four consecutive quarters before it can be added. SpaceX is still deep in losses, so that gate stays shut in the near term. More to the point is an official statement on 2026-06-04. After a public consultation, S&P Dow Jones Indices flatly declined to relax any standard for mega-cap companies, saying it would not change any eligibility criteria, including the financial-viability screen, the listing seasoning, or the minimum public float, and it laid down the principle that no exemption would be granted on market cap alone (S&P DJI official announcement). In short, no amount of market cap buys a fast ticket into the S&P 500.
The Nasdaq-100 runs on different logic. Its methodology states plainly that there is "no minimum or maximum market-capitalization requirement," and nowhere in the eligibility list is there any profitability test (Nasdaq-100 methodology). An unprofitable but enormous company can join the Nasdaq-100 and not the S&P 500, and the root of that split is whether a profit hurdle exists at all.
The Nasdaq-100 also rolled out a "fast track" on 2026-05-01. A newly listed company whose total market cap ranks among the top 40 of existing constituents and that meets eligibility (subject to a 3x float cap) is ranked as of its 7th trading day, announced after the close on the 10th trading day, and typically added on the 15th trading day, compressing what used to be roughly a three-month wait down to 15 trading days (Nasdaq 2026-05 rule FAQ). That is the rule that puts SpaceX about 15 trading days from a possible Nasdaq-100 entry once it lists. The Russell system ranks by total market cap into tiers, and an IPO large enough to make the Russell Top 500 takes a fast track and is added after the close of its 5th trading day (FTSE Russell methodology).
How Big Is the Passive Flood: $20 Trillion and 7–8% of Mechanical Buying
Once a stock is in, how big is the passive buying really? On this side, the numbers are concrete. According to S&P Dow Jones Indices' 2024 annual asset survey, as of year-end 2024 roughly $20 trillion tracked the S&P 500, of which about $13 trillion was pure indexing (passively managed) and about $7 trillion was active benchmark tracking; assets tied to all of the firm's indices came to $27.7 trillion (S&P Dow Jones Indices, "2024 Annual Survey of Indexed Assets"). That is a reservoir large enough to move prices.
The mechanism takes one sentence to state: when a new member with a weight of w% is added, passive funds tracking the index have to buy at that weight, and the demand comes to roughly "passive assets × w%." Greenwood and Sammon estimate that the mechanical buying by index trackers at inclusion runs to about 7–8% of the stock's float.
The benchmark case is Tesla. On 2020-12-21, Tesla entered the S&P 500 in a single full-weight step at 1.69%, making it the fifth-largest member of the index at the time. On the last trading day before inclusion (Friday, 2020-12-18), passive and active benchmark managers piled in together, pushing the stock up 5.96% to close at an all-time high of $695, with volume topping 200 million shares, more than 4 times the prior 30-day average (CNBC 2020-12-20). Tesla was one of the richest-valued inclusion candidates ever at that point: a forward P/E around 186, a full-year gain above 730%, and a market cap past $658 billion. No one doubts that the buying was real, and that it came in hard.
But the "Inclusion Premium" Has Disappeared
The flood of buying no longer buys a flood of excess return. This is the most counterintuitive section of the piece.
Greenwood and Sammon split four decades of the inclusion effect into ten-year slices, and the result is a clean decay curve:
| Decade | Inclusion abnormal return | Deletion abnormal return |
|---|---|---|
| 1980s | +3.4% | −4.6% |
| 1990s | +7.4% (peak) | −16.1% |
| 2000s | +5.2% | −12.4% |
| 2010s | +1.0%* | −0.6%* |
(Entries marked * are statistically indistinguishable from zero. Source as above, NBER w30748.)
The inclusion premium fell from its 1990s peak of 7.4% to 0.3% over the last decade, and the deletion effect narrowed from −16.1% to −0.6%. All of this happened while the share of assets tied to the index rose sharply: mutual funds plus ETFs tracking the S&P 500 went from near zero in the 1980s to around 7% of total U.S. market cap in recent years. The naïve expectation, "bigger passive, stronger premium," got the opposite from the data.
Why? The mechanism Greenwood and Sammon give is this: at inclusion, index trackers do mechanically buy about 7–8% of the float, but total institutional holdings barely move. Take the 2020 inclusions: index funds bought 7.53% of float on average, yet total institutional holdings rose only 0.65%. Active investors sold and provided the liquidity, absorbing the price impact. The other force is front-running. As "which stock gets added" becomes more predictable, arbitrageurs position ahead of the announcement date, and the excess return between announcement and effective date gets cashed out early, leaving little behind.
More rigorous event studies point the same way. Patel and Welch (2017) found that, into the 2000s, adding or removing a stock from the S&P 500 no longer produced a permanent shift in investor demand, and the related price effect tended to reverse afterward (Patel & Welch, 2017). A New York Fed study measured a cumulative abnormal return of just +3.34% over the 40 trading days after the effective date, with part of the initial gain given back about two months later for some included stocks (NY Fed Staff Report 484).
This conclusion needs some room left around it: the inclusion premium "disappearing" is not the same as "permanently at zero." Goldman Sachs estimates that several 2025 inclusion names (Block, Coinbase, DoorDash) beat an equal-weight S&P 500 by about 7.4 ppt (percentage points) on their announcement day, a sign of a "partial comeback" attributed to a revival in retail money. That is a recent industry observation, and this piece has not independently verified its causality; it bounds the structural decay in direction without overturning it. Hold one sense of proportion: treating inclusion as a sure-thing lottery ticket no longer holds.
So What About SpaceX: The Real Variable Is the 4.25% Float
If the inclusion premium is already gone, where is the story in SpaceX joining the Nasdaq-100? It is in the structure of its float, which is exactly what makes the stock unusual. SpaceX listed on the Nasdaq under the ticker SPCX on 2026-06-12, priced at $135 the day before for a valuation of roughly $1.77 trillion, with the public free float at only about 4.25% of total shares and founder stock under a 366-day lock-up (see this column's How Will SpaceX Trade After Its IPO?). A tiny float, plus the Nasdaq-100 squeezing the inclusion window down to 15 trading days, magnifies ordinary passive buying into a tail risk.
Index-rebalancing forecaster Intropic estimates that in the narrow window of the 15 trading days after listing, passive and index funds could absorb about 30% of SpaceX's tradable float, against only about 4% under the old, slower rules; Bloomberg reported the estimate on 6/9 under the headline "Index Funds Eye a Third of the Float, SpaceX IPO Risks a Feedback Loop" (Bloomberg 2026-06-09). A feedback loop here means passive buying pushes the price up, the price pushes the index weight up, and the weight forces funds to buy more. On a 4.25% float, that loop is unusually sensitive.
The number deserves a cool-down. The 30% figure is a "forecast," not a realized fact, and the Nasdaq-100 fast track carries a 3x float cap precisely to rein in this kind of loop. Nasdaq's own worked example shows that a newly listed trillion-dollar company with a 6% float and $600 billion in tracking assets would require the index side to buy about $6 billion, equal to 10% of float, sizable but well short of out of control. So the story in SpaceX is clear: watch the Nasdaq-100 inclusion announcement (about 15 trading days after listing at the earliest) and that 3x float cap, rather than bet on an inclusion premium that has already thinned out.
Three Conclusions for Investors
- Stop paying a separate premium for "joining the index." The historical excess return over the announcement-to-effective window has dropped to indistinguishable from zero in the last decade. Treating inclusion as a standalone catalyst for a move up is trading a 2020s market on a 1990s script.
- What you actually need to compute is float × passive share. The absolute size of passive buying genuinely exists, and the size of its impact depends on mechanical buying divided by tradable float. Large caps have ample float, so the impact gets diluted; ultra-low-float new listings like SpaceX are where passive buying can really stir the price.
- For SpaceX, watch two variables on the Nasdaq-100. One is the inclusion announcement and that 15-trading-day window, the other is how the 3x float cap constrains the feedback loop. These two settle the supply and demand of its first quarter as a public stock more than "will it make the S&P 500."
Evidence and Limits
This piece was written through one round of multi-source adversarial verification: 5 angles searched in parallel, 26 sources, 85 factual claims extracted and merged into 59, of which 29 went into three-lens adversarial verification and 24 survived; surviving claims were traced back to primary sources where possible, the methodologies and announcements of S&P Dow Jones Indices, Nasdaq, and FTSE Russell, plus the academic papers from Greenwood-Sammon, Patel-Welch, and the New York Fed.
The strength of evidence is uneven across topics, and it should be stated honestly. The rule hurdles, passive-asset size, and the decay of the index effect are the best-supported blocks, mostly from primary sources. The "long-run performance after inclusion" block is thinner: one study (cited via Morningstar) found that companies added to the S&P 500 between 1989 and 2019 significantly underperformed size- and industry-matched peers over the following one to five years, but the authors themselves note this is correlation, not causation, and the sample stops at 2019, so this piece uses it only as a directional reference, not a load-bearing conclusion. The 30% float absorption on the SpaceX side is a "forecast," not realized data, a snapshot of an event in progress that may be revised later. This piece takes public information as of 2026-06, mid-month, as its timeliness baseline. This content does not constitute investment advice.
Main Sources
- Rule hurdles: S&P DJI 2026-06-04 mega-cap consultation results (declines to relax) · Nasdaq-100 methodology · Nasdaq 2026-05 fast-track FAQ · FTSE Russell methodology
- Passive-asset size: S&P Dow Jones Indices, "2024 Annual Survey of Indexed Assets"
- Index effect: Greenwood & Sammon, "The Disappearing Index Effect," NBER w30748 (JF 2025) · Patel & Welch 2017 · NY Fed Staff Report 484
- Tesla case: CNBC: Tesla enters the S&P 500 at 1.69% weight
- SpaceX case: Bloomberg: index funds eye a third of the float · related reading in this column: How Will SpaceX Trade After Its IPO?