S&P 500 Index Inclusion Effect: Does It Still Exist?

The S&P 500 index inclusion effect is the abnormal price move a stock makes when it is announced as an addition to the index (or a deletion from it). The effect was large and well documented in the 1980s and 1990s, but the most recent evidence says it has almost vanished: the Journal of Finance version of Greenwood and Sammon (2025) reports that the abnormal return on additions fell from an average of 7.4% in the 1990s to less than 1% over the past decade. This article walks through the research, explains why the move happens, and shows what it implies for anyone tempted to trade index announcements.

The S&P 500 Index Inclusion Effect by the Numbers

Why an index change should not move a price

Joining the S&P 500 changes nothing about a company’s cash flows. The firm sells the same products to the same customers the day after the announcement as the day before. In a textbook market, where a stock has many close substitutes and demand for it is perfectly elastic, the price should not move at all.

Yet funds that track the index must own the stocks in it. When a company is added, those funds have to buy it, and they have to buy it from other holders. If those holders will sell only at a higher price, the demand curve for the stock slopes down rather than being flat. That is the idea behind the early studies of the question, and it is why the index effect became a standard test case for how elastic stock demand really is. The pension fund rebalancing flows article covers another source of predictable, non-informational buying and selling.

The research arc, 1986 to 2004

Shleifer (1986) asked, in the title of his paper, whether demand curves for stocks slope down. He found that stocks newly included in the S&P 500 since September 1976 earned a significant positive abnormal return at the announcement, and that the return did not disappear for at least ten days after inclusion. The returns were positively related to measures of buying by index funds, which he read as consistent with downward-sloping demand curves. He also found them unrelated to S&P bond ratings, which argued against a pure “certification of quality” explanation.

Harris and Gurel (1986) studied the same kind of event and chose it on purpose: they argued that because S&P 500 changes are unlikely to convey information about a firm’s future prospects, they offer an excellent opportunity to study price pressure. They reported that prices rise by more than 3% immediately after an addition is announced, and that this increase is nearly fully reversed after two weeks. A reversal is the signature of temporary price pressure rather than new information.

Lynch and Mendenhall (1997) exploited a change in S&P’s practice. Since October 1989, S&P has (when possible) announced index changes one week before they take effect, which separates the announcement date from the effective date. The authors found significantly positive (negative) post-announcement abnormal returns for additions (deletions) that were only partially reversed. They interpreted this as temporary price pressure combined with downward-sloping long-run demand curves.

Chen, Noronha and Singal (2004) found an asymmetry: a permanent price increase for added firms, but no permanent decline for deleted firms. That is hard to square with explanations that predict symmetric responses. Their explanation is that changes in investor awareness contribute to the asymmetric price effects of additions and deletions.

Taken together, the early literature shows a real announcement-day jump, evidence that at least part of it faded, and a debate over whether the rest was permanent. The studies differ in how much reverses, so treat the percentages above as headline findings from the abstracts, not as a settled estimate.

The disappearing index effect

The most important recent paper is “The Disappearing Index Effect” by Greenwood and Sammon, published in the Journal of Finance (volume 80, issue 2, pages 657–698; online December 2024). The journal version reports that the abnormal return on S&P 500 additions fell from an average of 7.4% in the 1990s to less than 1% over the past decade. This happened despite a significant increase in the share of stock market assets linked to the index.

Deletions tell the same story. The paper reports large negative abnormal returns in the 1990s but an average return of only 0.1% between 2010 and 2020, and a similar decline for other index families.

The result is counterintuitive. More money is linked to the index than ever, yet the announcement premium has shrunk. One reading consistent with the evidence is that the market has become better at absorbing the predictable demand. That reading is our inference, not a mechanism this article can attribute to the paper. What the numbers do establish is that the old rule of thumb, “buy what is about to join the index”, can no longer lean on the 1990s magnitudes. The event windows the authors use are also specific to their paper, so the figures should not be compared with other studies’ windows without checking.

How S&P chooses new members

Joining the S&P 500 is not mechanical, and that matters for anyone trying to anticipate additions. A committee decides. The criteria below are taken from S&P Dow Jones Indices announcements and from the S&P 500 methodology as reproduced in a 2025 SEC filing, so the text may lag the live methodology.

• Size. For additions, a minimum market capitalization of US$22.7 billion or more, effective July 1, 2025 (raised from US$20.5 billion). The criteria apply to additions, not to continued membership. A security-level float-adjusted market cap of at least 50% of the index’s company-level minimum is also required.
• Financial viability. The sum of the most recent four consecutive quarters of GAAP earnings (net income excluding discontinued operations) must be positive, as must the most recent quarter. This replaced an older rule requiring four straight positive quarters, a change announced in 2014.
• Liquidity. The float-adjusted liquidity ratio (annual dollar value traded divided by float-adjusted market cap) must be at least 1.0, and the stock must trade at least 250,000 shares in each of the six months before the evaluation date. Current constituents have no such minimum.
• Seasoning. IPOs must have traded on an eligible exchange for at least 12 months. Spin-offs from existing constituents are exempt.
• Sector balance. A candidate is evaluated for its contribution to sector balance, comparing GICS sector weights with the S&P Total Market Index. Passing the numeric screens therefore does not guarantee inclusion.
• Committee discretion. Domicile is determined by the U.S. Index Committee. There is no scheduled reconstitution; changes can be made at any time in response to corporate actions and market developments, with at least three business days’ notice and less at the committee’s discretion.

In June 2026, S&P Dow Jones Indices published the results of a consultation on the treatment of megacap companies. It reaffirmed that the GAAP positivity requirements are unchanged, and concluded that exceptions to the financial viability, seasoning and investable weight factor (IWF) requirements should not be granted solely based on market capitalization. The 12-month seasoning rule and the 0.10 IWF minimum remain for the S&P 500.

One practical consequence: a large, profitable company can pass every quantitative screen and still wait, because the committee also weighs sector balance. Predicting which eligible company is chosen, and when, is itself uncertain.

Case study: Tesla, December 2020

Tesla’s addition is the best-known recent example, and it shows both the old effect and the new complications. On November 16, 2020, S&P Dow Jones Indices announced that Tesla would join the S&P 500, effective prior to the open on December 21, 2020, coinciding with the December quarterly rebalance. Citing the large size of the addition, S&P consulted the market on adding it all at once or in two separate tranches. S&P described Tesla as one of the largest companies added to the index in the past decade. According to the Associated Press, Tesla’s market value at that day’s close was $386.83 billion, which would rank it tenth-most valuable in the index.

On November 30, 2020, S&P decided to add Tesla in a single step at its full float-adjusted market capitalization weight on December 21, weighing its expected liquidity and the market’s ability to absorb the volume. On announcement day, Al Jazeera reported that Tesla shares surged 12 percent in extended trading.

What we do not claim is just as important. This article does not state Tesla’s return on the effective date or any later reversal, because we could not verify those figures from a primary source. We also do not quote estimates of how much index-fund buying the addition required. A single large, well-publicized addition is an anecdote; it cannot tell you whether the average effect has returned.

What this means for traders

If the effect averages under 1% on additions, the gap between announcement and effective date is a small prize relative to the costs of chasing it. In Tesla’s case the move showed up in extended trading right after the announcement, so a trader reacting the next morning would be buying after much of the jump. Spreads and market impact around a crowded event eat into what is left; see transaction costs in trading and market microstructure for how those costs arise, and liquidity risk for why exiting a crowded position can be harder than entering it.

The research also suggests where to be cautious. Harris and Gurel found that the early gain was nearly fully reversed within two weeks, so buying after the jump meant buying into the part that faded. Chen, Noronha and Singal found no permanent decline for deleted firms, so shorting deletions is not supported by their evidence either. And because the committee has discretion, trying to predict additions ahead of the announcement is a bet on judgment rather than a rule.

For long-term investors the lesson runs the other way. Index membership is better thought of as a source of liquidity and flows than as a source of alpha, which fits the broader view in factor investing: durable returns tend to come from compensated exposures, not from being early to a known forced buyer.

Limitations

The evidence here has real gaps. The magnitudes in the 1986 studies are the abstract-level headlines; this article did not verify further detail from the full texts, and the numeric magnitudes in Chen, Noronha and Singal (2004) were not verified either. The decline documented by Greenwood and Sammon is measured over event windows defined in their paper, and different window definitions can give different numbers. Finally, the 2020 Tesla episode rests on press releases and news reports, not on a systematic event study.

The eligibility rules also move. The US$22.7 billion threshold is current as of S&P’s July 1, 2025 announcement, the latest we found; a later revision could exist. Several criteria above come from a 2025 SEC filing reproducing the methodology rather than from the methodology document itself, which was not accessible. Check S&P Dow Jones Indices directly before relying on any threshold. Nothing here is investment advice.

FAQ

What is the S&P 500 index inclusion effect?

It is the abnormal price change a stock experiences when it is announced as an addition to the S&P 500 (or a deletion). Early studies such as Harris and Gurel (1986) found a rise of more than 3% after an announcement. The Journal of Finance version of Greenwood and Sammon (2025) finds the average abnormal return on additions has fallen from 7.4% in the 1990s to under 1% over the past decade.

Does buying a stock when it is added to the S&P 500 still work?

The recent evidence says the average announcement premium is now small, under 1% in the most recent decade. Harris and Gurel also found that the early jump was nearly fully reversed after two weeks. After costs, the pattern is hard to rely on.

What does a company need to join the S&P 500?

As of S&P’s July 1, 2025 update, a market cap of US$22.7 billion or more, positive GAAP earnings over the latest four quarters and the latest quarter, adequate liquidity, and at least 12 months since IPO. A committee also weighs sector balance, so meeting the screens does not guarantee inclusion.

Why did the index effect shrink?

Greenwood and Sammon document the decline, and note that it happened despite a significant rise in the share of market assets linked to the index. This article does not claim a proven cause. One plausible reading is that the market now absorbs index demand more efficiently, but that is our interpretation.

References

1. Chen, H., Noronha, G. & Singal, V. (2004). “The Price Response to S&P 500 Index Additions and Deletions: Evidence of Asymmetry and a New Explanation.” The Journal of Finance, 59(4), 1901–1930. doi:10.1111/j.1540-6261.2004.00683.x
2. Greenwood, R. & Sammon, M. (2025). “The Disappearing Index Effect.” The Journal of Finance, 80(2), 657–698. doi:10.1111/jofi.13410 (NBER Working Paper 30748: doi:10.3386/w30748)
3. Harris, L. & Gurel, E. (1986). “Price and Volume Effects Associated with Changes in the S&P 500 List: New Evidence for the Existence of Price Pressures.” The Journal of Finance, 41(4), 815–829. doi:10.1111/j.1540-6261.1986.tb04550.x
4. Lynch, A. W. & Mendenhall, R. R. (1997). “New Evidence on Stock Price Effects Associated with Changes in the S&P 500 Index.” The Journal of Business, 70(3), 351–383. doi:10.1086/209722
5. Shleifer, A. (1986). “Do Demand Curves for Stocks Slope Down?” The Journal of Finance, 41(3), 579–590. doi:10.1111/j.1540-6261.1986.tb04518.x
6. S&P Dow Jones Indices. (2025, July 1). “S&P Dow Jones Indices Announces Update to S&P Composite 1500 Market Cap Guidelines.” press.spglobal.com
7. S&P Dow Jones Indices. (2026, June 4). “Consultation on Treatment of MegaCap Companies – Results.” press.spglobal.com
8. S&P Dow Jones Indices. (2014, February 24). “S&P U.S. Indices Methodology Update.” press.spglobal.com
9. S&P Dow Jones Indices. (2020, November 16). “Tesla Set to Join S&P 500.” press.spglobal.com
10. S&P Dow Jones Indices. (2020, November 30). “S&P Dow Jones Indices Announces Implementation of Tesla’s Addition to S&P 500.” press.spglobal.com
11. Citigroup Global Markets Holdings Inc. (2025). Form 424B2, reproducing the S&P 500 index methodology. SEC EDGAR
12. Associated Press. (2020, November 16). Tesla to be added to benchmark S&P 500 index. TechXplore
13. Al Jazeera. (2020, November 16). “Tesla shares up 12% after announcement it will zoom onto S&P 500.” aljazeera.com