Every quarter, index funds and ETFs shuffle their holdings to match changes in their benchmark. The date is printed in prospectuses and posted on fund websites. Yet a small group of insiders gets to see exactly which stocks will be bought and sold before the rest of the market does. The result is a quiet transfer of wealth from long-term shareholders to the fastest hands in the market.

The Clock That Rewards the Fastest Hands

Index providers like S&P Dow Jones Indices, MSCI, and FTSE Russell publish their constituent changes weeks before they take effect. A stock added to the S&P 500, for example, is announced in advance, giving everyone time to prepare. But the full list of additions and deletions, along with weight changes, is distributed to a select group of clients under embargo. Those clients include the very market makers and arbitrage desks that will trade against the funds on rebalance day.

The gap between the embargoed list and the public announcement is measured in hours, sometimes days. During that window, the insiders know precisely what the fund will be forced to do. They can accumulate the stocks being added and short the ones being removed, positioning themselves to profit when the fund executes its trades. The fund, meanwhile, has no choice but to buy and sell at whatever prices prevail when it must act.

This is not a secret. It is a feature of the ecosystem, discussed in academic papers and whispered about on trading floors. But it is rarely explained to the retail investor who owns the ETF in a retirement account. The cost is real, but it is also small enough that most people never notice it. Over a decade, however, it can shave a meaningful slice off a portfolio's return.

The mechanics are straightforward. When an index changes, the fund must adjust its holdings to stay in line. The larger the change, the bigger the trade. An S&P 500 addition might require the fund to buy millions of shares, moving the price upward. The arbitrageur who bought before that move sells into the fund's demand, capturing a profit that comes directly out of the fund's performance.

How the Mechanism Lets Some Trade Ahead

Authorized participants (APs) are the gatekeepers of the ETF creation and redemption process. They are the only entities that can create new shares of an ETF by delivering a basket of underlying securities to the fund sponsor. Because of this role, they receive the full portfolio composition of the fund on a daily basis, including the changes that will occur at the next rebalance.

That daily file is a map of the fund's future trades. An AP that sees a large addition to the portfolio can buy the underlying stock before the fund does, then deliver it to the fund in exchange for ETF shares. The profit is the difference between the price the AP paid and the price the fund pays when it eventually buys. The fund's shareholders eat the difference.

The same logic applies to deletions. An AP that sees a stock being removed can short it ahead of the fund's sale, then cover the short when the fund dumps the shares. The short sale pushes the price down, and the fund sells into that weakness. The AP profits from the decline; the fund loses the same amount.

This is not illegal. It is a form of front-running that is explicitly allowed because APs are seen as providing liquidity. But the liquidity they provide comes with a price, and that price is paid by the fund's long-term holders. The cost is tiny per dollar traded, but it happens every time an index changes, and indices change constantly.

The Rule That Was Supposed to Fix It

In 2019, the Securities and Exchange Commission (SEC) adopted a rule that allowed ETFs to use custom baskets and to create and redeem shares more quickly. The stated goal was to lower costs and improve liquidity for investors. The rule was a response to a series of exemptive orders that had given individual ETFs permission to deviate from the traditional in-kind process.

Custom baskets let an ETF deliver a portfolio that is not exactly proportional to the index, which can reduce the trading costs associated with rebalancing. The SEC hoped this would make ETFs more efficient and reduce the drag on returns. In many ways, it worked. Trading costs for ETFs have fallen, and spreads have narrowed.

But the rule did nothing about the information gap. The APs still see the full portfolio daily, and the index providers still share their changes under embargo. The rule changed the mechanics of how funds trade, but it did not change who gets to see the cards before the game starts.

Some observers argue that the rule actually made the problem worse. By allowing faster and more flexible creations, it gave APs more opportunities to trade ahead of the fund. The intent was to lower costs, but the effect may have been to increase the advantage of the fastest players.

What the Data Shows About the Drag

Quantifying the cost is difficult, but several academic studies have tried. One well-known paper, by Antti Petajisto, estimated that front-running in index funds costs investors a few basis points per year. The exact number depends on the fund and the index, but a range of 1 to 5 basis points is a reasonable estimate for large-cap funds. Small-cap and less liquid indexes can see costs several times higher.

To put that in perspective, a 2 basis point drag on a $10,000 investment is $2 per year. That sounds trivial. But over 30 years, with compounding, it can reduce the final portfolio value by more than 1%. For a fund with $100 billion in assets, a 2 basis point drag is $20 million per year transferred from shareholders to intermediaries.

Bond ETFs face a similar issue, though the mechanism is slightly different. When an index changes in the bond market, the fund must trade in a market that is less liquid than equities. The price impact is often larger, and the arbitrageurs are just as quick to exploit it.

The drag is invisible in any single year. A fund that returns 9.9% instead of 10% looks fine. But over a career of investing, the difference between 10% and 9.9% compounded is substantial. It is the kind of cost that does not show up on your statement, so it is easy to ignore.

Who Profits From the Status Quo

The beneficiaries of this system are the market makers, high-frequency trading desks, and the index providers themselves. Market makers profit from the spread and from the price moves they anticipate. High-frequency traders use algorithms to trade ahead of the fund in milliseconds. Index providers sell their data to a select group for a fee, creating a revenue stream that depends on the information gap.

Brokerage firms also benefit indirectly. The extra trading that happens around rebalance dates generates commissions and order flow that they can monetize. Even the fund sponsors themselves are not entirely unhappy, because the system keeps trading costs low in the aggregate, which makes their funds more attractive to investors.

The costs are spread across all fund holders, so no single victim loses enough to complain. A retail investor with a $50,000 ETF position might lose $10 per year to front-running. That is not enough to notice, let alone to file a complaint. But when you multiply that by millions of investors, the total is enormous.

This is a classic collective action problem. The harm is diffuse, the beneficiaries are concentrated, and the fix is technical and unglamorous. As a result, the status quo persists.

Why Regulators Haven't Closed the Gap

The SEC has considered proposals to require index providers to disclose changes to the public at the same time they share them with APs. In 2024, the agency floated a concept release asking for comment on the issue. But the rulemaking process has stalled, in part because of industry lobbying and in part because the harm is difficult to measure.

Opponents argue that the current system benefits investors by providing liquidity and keeping trading costs low. They point out that APs take on risk when they create and redeem shares, and that the ability to see the portfolio in advance is part of the compensation for that risk. Remove the advantage, they say, and ETFs would become more expensive to trade.

There is some truth to this. The creation and redemption process is what keeps ETF prices close to their net asset value. Without the information edge, APs might demand higher fees for their services, which could offset any gains from reduced front-running. The trade-off is not simple.

But the argument that the system is necessary does not mean it is fair. It is possible to design a system where APs are compensated for their risk without giving them an information advantage. The SEC has been slow to explore those designs, in part because the issue is technical and the public pressure is minimal.

What an Investor Can Actually Do

For the individual investor, the practical response is to be aware of the drag and to choose funds that minimize it. One approach is to prefer ETFs that use custom baskets, which can reduce the need to trade exactly at the rebalance date. Another is to avoid trading ETFs on rebalance days, when the price impact is likely to be highest.

Investors should also look at the total cost of ownership, not just the expense ratio. A fund with a lower expense ratio but higher trading costs may be more expensive in the long run. Some funds are more transparent about their rebalancing practices than others, and that transparency is worth something.

There is also a case for using index funds that are less prone to front-running, such as those that track broad, liquid indexes. The drag is smaller for these funds because the price impact of a trade is smaller. Small-cap and niche funds are more vulnerable.

None of these measures will eliminate the cost entirely. The information gap is structural, and as long as index providers sell data to a select group, someone will trade on it. But understanding the cost is the first step to reducing it. A careful investor can at least make an informed choice about which funds to own and when to trade them.

The Case for Transparency

Some argue that the solution is not to eliminate the information advantage but to make it available to everyone. If the full list of index changes were published simultaneously to all market participants, the arbitrage opportunity would vanish. The fund would still trade, but no one would have a head start.

This approach has precedent. In some markets, index providers have experimented with publishing changes in real time. The result has been a reduction in price impact and a more level playing field. The concern is that without the embargo, market makers might be less willing to provide liquidity, leading to wider spreads and higher costs for all investors.

But the evidence from markets that have moved to greater transparency suggests that the fears are overblown. The liquidity that APs provide is valuable, but it does not depend on an information edge. The creation and redemption process itself provides incentives for APs to participate, and the risk they take on can be compensated through fees and spreads rather than through early access to information.

In the end, the question is one of fairness. The current system quietly transfers wealth from the many to the few. It is a tax on long-term investors that is invisible in any single year but significant over a lifetime. The fix is not complicated, but it requires the will to challenge an entrenched industry.

For a related look at how fees and execution costs hide in plain sight, see this piece on brokerage fee schedules. And if you are thinking about refinancing, be aware that closing costs can hide a second price, as this article on refinance closing explains.

This article is for informational purposes only and does not constitute personalized investment advice. Consult a qualified professional before making any financial decisions.