Smart indexing: added value for index funds around corporate actions and index changes

Advanced11 min read · updated 9 September 2026 · 18 sources

Smart indexing means that an index fund stays close to its index but uses room for maneuver that the index itself does not have. In corporate actions, it can choose an option that is economically more favorable than the one the index rule assumes, and for index changes it does not have to trade at the closing price on the effective date, at which everyone else is trading too. Here you will learn where this added value comes from, how large the index effect was and still is today, and where the limits lie.

Why an index costs money

An index is a paper portfolio: it rebalances at closing prices without costs and, in every corporate action, makes the choice its rules prescribe. A fund that tracks it trades in real markets. Harding (2016) of State Street names, among the levers of good index managers, effective strategies for trading index changes and for elections in corporate actions, as well as skipping small index changes, all with controlled risk. At the same time, all market participants know what index funds have to buy and when. This gives rise to the index effect, which Petajisto (2011) describes as a hidden cost of index funds and of the indices themselves.

Elections in corporate actions

Many corporate actions give shareholders a choice. The index has to commit to one; MSCI, for example, assumes the company's default option for a scrip dividend and, if there is none, the cash payment (MSCI Corporate Events Methodology 2022). A fund may choose differently if it benefits its investors:

• Scrip dividend: cash or new shares, sometimes at a subscription price with a discount. If the new shares are worth more than the cash dividend, the share option is better, provided taxes and costs do not eat up the discount.
• Subscription rights: in a capital increase, the fund can exercise or sell the rights. What the index assumes is set out in the provider's rules; if the rights fetch more on the exchange than their theoretical value, selling them can pay off.
• Takeover and exchange offers: for a friendly offer that is expected to succeed, MSCI deletes the takeover target at the end of the first acceptance period and announces this at least two business days in advance; for hostile offers, MSCI waits for the outcome (MSCI Corporate Events Methodology 2022). A fund can sell earlier on the exchange or accept the offer, depending on whether the gap to the offer price outweighs the risk of failure.
• Spin-offs: if the spun-off company meets the criteria, MSCI adds it at the close of its first trading day (MSCI Corporate Events Methodology 2022). If it does not, index funds hold shares that do not belong in the index and have to sell them; anyone who anticipates this can spread the sale over several days.

Anticipating index changes

Index changes do not come out of the blue. Rules and data are public; anyone who applies them can estimate many additions and deletions before the official list. The schedules set the framework:

• MSCI: the results of the August 2026 review were published on August 12 and implemented at the close of trading on August 31 (MSCI Index Review 2026).
• Russell: for the US indices, 30 April was the rank day for market capitalization in 2026; preliminary lists appeared from May 22, and the reconstitution was implemented after the close on June 26. From 2026, FTSE Russell reconstitutes the US indices semi-annually, in June and December (FTSE Russell Reconstitution 2026).
• S&P 500: since October 1989, S&P has announced changes about a week in advance where possible, as studies from the 1990s describe (Lynch and Mendenhall 1997; Beneish and Whaley 1996).
• Solactive: for the GBS indices, the composition is fixed on the selection day, 20 trading days before the rebalancing (Solactive GBS Guideline 2026).
• FTSE global: FTSE Russell publishes a provisional list of upcoming changes on the first business day of each month (FTSE Russell GEIS Dates 2023).

Before the official list, uncertainty remains: borderline cases depend on prices up to the cut-off date, on buffer zones and on free-float data that the provider may estimate differently. A fund that buys an expected addition in advance is betting on its forecast. If it is wrong, it holds a stock that does not enter the index. And because arbitrageurs apply the same rules, it is precisely the predictability of the changes that allows wealth to be transferred from fund investors to them (Chen et al. 2006).

The index effect

Between the announcement and the effective date, arbitrageurs buy stocks that are being added and sell those that are being dropped (front-running in the broader sense). Index funds, by contrast, usually trade at the closing price on the effective date, because that is the only way to keep their tracking error small; many agree with liquidity providers to trade at exactly that price, and the providers get paid for the risk through the price patterns around the change (Blume and Edelen 2004). The result: additions rise before the effective date, deletions fall, and the index buys at higher prices and sells at lower prices than at the announcement. Petajisto (2011) calls the consequence index turnover costs:

These costs do not show up in any tracking difference, because the index and the fund bear them together. They reduce the return of the index itself, measured against an index that would have rebalanced at the prices on announcement. If a fund spreads its purchases over several days or buys earlier, it can avoid part of them; it then temporarily deviates from the index, however.

Example: what the run-up costs

What the research shows

The early studies found the effect in the S&P 500. According to Shleifer (1986), stocks added to the index from September 1976 onwards earned a clearly positive excess return on the announcement that did not disappear for at least ten days after inclusion. This is consistent with downward-sloping demand curves. Harris and Gurel (1986) found a rise of more than 3% immediately after the announcement, which almost completely reversed after two weeks, and interpreted this as price pressure. Since S&P began announcing changes in advance, prices have risen between the announcement and the effective date (Beneish and Whaley 1996; Lynch and Mendenhall 1997). Chen et al. (2004) showed an asymmetry: added stocks rise permanently, while deleted stocks do not fall permanently.

For the costs to index funds, Petajisto (2011) worked with data from 1990 to 2005: from the announcement to the effective date, additions to the S&P 500 rose by 8.8% on average and those to the Russell 2000 by 4.7%; deletions fell by 15.1% and 4.6% respectively. He estimates the hidden costs of index funds as follows:

For the Russell 2000, he gives 38 to 77 basis points. Chen et al. (2006) put the losses of investors in funds tracking the S&P 500 and Russell 2000 at US$1.0 to 2.1 billion per year and see the cause in the fact that fund managers are measured on tracking error. Madhavan (2003) found significant return effects around the annual Russell reconstitution from 1996 to 2002. They were costly for funds that rebalance on the effective date, and profitable but risky for liquidity providers. Blume and Edelen (2004) calculated that S&P 500 funds could gain 19.2 basis points per year if they traded at the open right after an announcement, albeit at the price of a significantly higher tracking error. Gastineau (2002) concluded that the concentrated trading around index changes makes index funds on popular indices more expensive than they need to be.

More recent work shows that the effect has shrunk. According to Patel and Welch (2017), there has been clear evidence of a reversal since the 2000s; inclusion or deletion no longer shifts demand permanently. Greenwood and Sammon (2025) measure a decline in the excess return on additions to the S&P 500 from an average of 7.4% in the 1990s to less than 1% in the last decade; from 2010 to 2020, deletions brought only 0.1% on average. They find a similar decline in other index families, even though the share of assets tied to indices has risen sharply.

Advanced: added value versus tracking error risk

Every deviation from the index is a small active bet. If the fund buys an expected addition at the announcement rather than on the effective date, it gains the run-up on average, but bears the stock's specific risk for those days:

• Correlated bets: additions in the same review are not independent. If newcomers fall together, all bets lose at the same time, and the risk grows faster than with the square root of the number of cases.
• Forecast errors: anyone who wrongly predicts an addition bears the full risk without the effect.
• Competition: the more market participants run the same strategy, the earlier the price rise sets in and the less is left for latecomers.

Limits and risks

• Prospectus and mandate: index funds may only deviate from the index within the limits of their prospectus.
• Tracking error versus return: if a fund is measured on tracking error, every attempt to add value costs it deviation. Chen et al. (2006) see precisely this measurement as the cause of the losses.
• Costs and taxes: scrip dividends, subscription rights and exchange offers can be treated differently for tax purposes; the theoretical advantage can disappear after tax. This is not tax or investment advice.
• Reversal after the effective date: part of the run-up is lost again after the effective date (Harris and Gurel 1986; Patel and Welch 2017); this affects the index and the fund equally.
• Shrinking effect: what amounted to several percent in the 1990s is now below 1% for S&P 500 additions (Greenwood and Sammon 2025). Historical figures are not a forecast.
• Liquidity: on the day of the Russell reconstitution, intraday prices fluctuated strongly (Madhavan 2003). Trading beforehand or spreading trades out swaps this risk for spreads and market risk on other days.

How Portfolio Lab calculates it

Portfolio Lab has no dedicated tool for corporate actions or index changes and does not predict additions. The price series are adjusted for distributions and splits; whatever choice a fund made in a corporate action is reflected in its price. Whether an index fund does better or worse than its index thanks to such room for maneuver can be seen in the Backtest with a benchmark, ideally from the difference over many years and from the tracking error based on monthly returns. In Look-through, you can see which securities a fund actually holds according to its holdings list. More on the metrics in the metrics methodology.

This article explains a method; it is not investment or tax advice. Worked examples are simplified and rounded. All terms in the glossary.