Index funds are not replacing price discovery, but they are changing where it happens, who supplies liquidity and how benchmark-driven flows move through markets.
The rise of passive investing is often described as a competition between cheap index funds and expensive active managers. That framing misses the more consequential market-structure question. As more assets follow benchmarks, the mechanism by which prices are formed is changing: a growing share of capital is allocated according to index rules, while a smaller but still crucial set of participants decides whether the prices produced by those flows are economically justified.
The scale is now large enough that this distinction matters. In its latest monthly survey, the Investment Company Institute reported that U.S. long-term index mutual funds and exchange-traded funds held $21.88 trillion in June 2026, compared with $18.83 trillion in active funds. Index products represented 53.7% of the combined long-term mutual fund and ETF assets covered by the survey, and 63.8% of domestic-equity assets. In June alone, index funds recorded $119.32 billion of net inflows while active funds recorded a $7.78 billion net outflow. These figures describe fund ownership and flows, not the share of daily trading that is “passive,” but they show why benchmark mechanics have become a first-order issue for market structure.
Price discovery happens at the margin, not by counting assets
The first mistake in the passive-investing debate is to assume that the largest owner must also be the price setter. Securities prices are determined by marginal transactions: the buyers and sellers willing to trade at the next available price. A long-term index fund can own a large position while doing little on a normal day. A much smaller active hedge fund, market maker, fundamental investor or derivatives desk can be the participant whose order changes the price.
That is why the question “What percentage of the market is passive?” does not have a direct answer to the question “Who discovers the price?” Index ownership can rise substantially while active traders continue to set relative prices. But this does not make passive ownership irrelevant. Index funds bring flows that must be executed, and their rules determine which securities receive those flows. The market still needs someone to take the other side.
The Federal Reserve has reached a similarly nuanced conclusion. Its study of the shift from active to passive investing found that some passive strategies can amplify volatility and that the shift has increased concentration in asset management, while also reducing some liquidity and redemption risks. On return and liquidity co-movement, the evidence was mixed. The policy lesson is important: passive investing changes several market channels at once, so a simple claim that it either “destroys” or “improves” price discovery is too coarse.
Rule-based demand changes the market even when it expresses no valuation view
A fundamental investor buys a stock because it is believed to be worth more than the market price. A broad market-capitalisation index fund typically buys because the stock is in the benchmark and receives a prescribed weight. Those are different motives, yet both create real orders. The passive fund is price-insensitive in a narrow sense: its mandate is to track an index, not to decide whether a company is cheap. The market mechanism must still find a price at which another holder is willing to sell.
Recent academic work shows that the counterparty to passive demand is not always another investor. A 2026 Review of Financial Studies paper by Marco Sammon and John J. Shim finds that firms themselves are the primary sellers that clear index-fund buying in their sample, supplying shares at nearly a one-for-one rate. The authors find that companies respond to exogenous passive demand largely through non-primary-market issuance, including employee stock-based compensation, with prices acting as the coordinating mechanism. The finding broadens the debate: passive flows can affect not only secondary-market trading but also how corporations supply equity.
That does not mean every dollar entering an index fund causes new issuance, nor that passive funds determine corporate capital structures. It does show that the market-clearing response can extend beyond active managers selling existing shares. The economy adapts to persistent benchmark demand through prices, issuance, arbitrage and portfolio rebalancing.
Index rebalancing turns a portfolio rule into a market event
The effect becomes most visible when an index changes. Additions, deletions and weight changes translate a published methodology into a known future trading requirement. Funds that closely track the benchmark need to hold the revised portfolio at the implementation point; arbitrageurs and liquidity providers can estimate those needs in advance. Price discovery around the event therefore mixes information about corporate fundamentals with information about benchmark demand.
The numbers have become enormous. FTSE Russell said in June 2026 that approximately $12.2 trillion in assets were benchmarked to Russell US Indexes. During the June 2026 Russell reconstitution, Nasdaq reported that its Closing Cross executed 4.59 billion shares worth $334.027 billion in 1.630 seconds across Nasdaq-listed securities, the exchange’s largest liquidity event for the Russell reconstitution. Not all of that money was passive, and benchmarked assets include active strategies. The event nevertheless illustrates how index rules can synchronize extraordinary volumes around one reference price.
This predictability has two opposite effects. It can create temporary price pressure because many funds want the same execution window. It can also attract liquidity because market makers and active traders know that demand will arrive. Rebalancing therefore makes passive investing easier to observe, but it does not eliminate the role of active intermediation; it creates a concentrated reason for active intermediation to exist.
Liquidity is increasingly clustered in time
The closing auction is one of the clearest places where this change appears. A passive manager seeking to minimise tracking error often wants the same official closing price used by the benchmark. The result is a natural migration of flow toward the end of the session, particularly on rebalance days.
The European Central Bank’s November 2024 Financial Stability Review documented this mechanism in euro-area equities. It found that passive ownership was associated with stronger return co-movement and highlighted a “lumping” of liquidity around closing auctions. The ECB noted that closing-auction volume is significantly larger on index-rebalancing days and warned that a structural preference for the close could weaken the market’s ability to absorb shocks during continuous trading hours.
The important qualifier is that concentrated liquidity is not necessarily bad liquidity. Research from the Federal Reserve Bank of New York on benchmark U.S. Treasuries found that since 2020 trading activity has been roughly 46% higher on the last trading day of the month, while price-impact coefficients for benchmark notes have been about 28% lower on average. The authors conjecture that increased month-end rebalancing by funds managed relative to fixed-income indexes helps explain the pattern. In that market, predictable benchmark activity appears to deepen liquidity at the moment when trading concentrates.
These findings are not contradictory. A market can be highly liquid during a closing or month-end window and less liquid outside it. Passive growth may therefore change the liquidity clock: investors gain deeper pools at predictable times while the continuous session potentially carries less of the day’s risk transfer. For trading desks, execution quality increasingly depends on knowing not only what security to trade, but when benchmark-sensitive liquidity will appear.
ETFs connect basket trading to individual securities
Exchange-traded funds add another layer because the fund itself trades throughout the day while authorised participants and arbitrageurs keep its market price aligned with the value of the underlying basket. A shock can therefore move through the ETF, the futures or options used to hedge it, and the securities it owns.
A widely cited NBER review by Itzhak Ben-David, Francesco Franzoni and Rabih Moussawi describes the trade-off clearly: ETF arbitrage can enhance price discovery by transmitting information across the fund and its underlying securities, but it can also transmit non-fundamental volatility and alter return correlations and liquidity, especially in stressed conditions. That literature is older than today’s market, but the mechanism remains relevant because the ETF ecosystem has only grown larger and faster.
It is also important not to equate “ETF” with “passive.” ETFs can be actively managed, while index strategies can be held in mutual funds, institutional accounts or derivatives. The market-structure issue is the rule-based allocation and arbitrage architecture, not the legal wrapper alone.
Market-cap weighting can reinforce concentration - but the feedback is subtle
The most contested part of the debate concerns market-capitalisation-weighted indexes. Because larger companies receive larger weights, every new dollar entering a cap-weighted fund directs more money to the largest companies. This can make the ownership structure of mega-cap stocks increasingly connected to broad-market flows.
A 2025 Review of Financial Studies paper by Hao Jiang, Dimitri Vayanos and Lu Zheng develops and tests a mechanism in which flows into passive funds disproportionately raise the prices of the economy’s largest firms, with the strongest effects among large firms already in high demand. The paper reports that the prices and idiosyncratic volatility of the largest S&P 500 firms rise the most following flows into that index. This is evidence for a potential feedback channel, not proof that passive investing is the primary cause of large-company valuations.
There is a technical nuance that is often lost in popular discussion. A cap-weighted index fund does not normally need to buy more shares simply because an existing constituent’s price rises; its portfolio weight rises automatically with the market value. The stronger mechanical channel comes from fresh inflows, index additions and deletions, changes in free float or shares outstanding, and benchmark reconstitution. Passive investing can reinforce the importance of large firms without creating a perpetual “price rises, therefore index buys” loop.
The counterargument: passive and active price discovery can be complements
The strongest counterargument to fears about passive dominance is economic rather than ideological. If index flows create mispricing, they also create potential profit for investors willing to research securities, take the other side and wait for prices to converge with fundamentals. In that sense, a smaller active sector can still do a large amount of price discovery if it is sufficiently informed, liquid and willing to trade.
A recent 2026 working paper by Youngmin Choi adds an interesting qualification. Using Russell 1000/2000 reconstitutions as an instrument for passive ownership, the paper finds that increases in passive holdings are associated with better price-efficiency measures when active mutual-fund ownership is already substantial, but little effect where active ownership is limited. Because the study is a working paper rather than a peer-reviewed conclusion, it should be treated as emerging evidence. Its implication is nevertheless useful: passive and active capital may be complements, with passive liquidity and active information working best when both are present.
This also explains why passive market share can rise for a long time without prices becoming obviously detached from information. Price discovery does not require every investor to analyse every security. It requires enough informed risk-bearing capacity at the margin. The harder question is whether that capacity remains available during stress, when leverage, funding, short-selling constraints and risk limits can force the very traders who normally correct dislocations to step back.
What changes for asset managers, exchanges, issuers and regulators
For asset managers, the shift changes the economics of active management rather than making active management obsolete. Research-heavy strategies face relentless fee pressure when they deliver benchmark-like portfolios, but genuinely differentiated investors can become more important to price formation as benchmark ownership grows. The scarce resource is not “active” branding; it is capital willing and able to trade against mechanical flows when the expected return justifies the risk.
For passive managers, the competitive edge increasingly sits in implementation. Tracking error, market impact, tax efficiency, securities lending, corporate-action processing and auction execution can matter more than security selection. Two funds following the same index can deliver different realised outcomes because of how efficiently they navigate the market mechanics created by the benchmark.
For exchanges and brokers, predictable concentration of volume raises the value of robust closing auctions, transparent imbalance information and resilient capacity. The June 2026 Nasdaq event shows the engineering challenge: hundreds of billions of dollars can need a common clearing price in seconds. Market design therefore becomes part of the investment-management value chain.
For listed companies, index membership is increasingly relevant to the shareholder base, trading calendar and potential demand for shares. The 2026 evidence that firms can help clear passive demand through issuance suggests that benchmark flows may also interact with compensation, capital raising and treasury decisions. Corporate executives should not confuse index-driven demand with a permanent valuation guarantee, but neither can they treat benchmark ownership as a purely passive background condition.
For regulators, the challenge is to focus on market function rather than on the label attached to a fund. The relevant questions are whether auctions remain resilient under extreme volume, whether liquidity is becoming too concentrated in particular windows, whether index methodologies create avoidable cliff effects, and whether active liquidity providers retain sufficient capacity to correct dislocations. The Federal Reserve and ECB evidence both point toward mixed effects, which argues for surveillance and market-design improvements rather than a presumption that passive investing is inherently destabilising.
Evidence versus inference: Evidence shows that index funds now exceed active funds in the U.S. long-term mutual fund and ETF asset pool measured by ICI; benchmark events can concentrate very large trading volumes; ECB research links passive ownership with higher co-movement and closing-auction concentration; and recent academic work finds measurable effects on market clearing and large-firm prices. The inference is that price discovery will become more concentrated in specialised active investors, market makers, arbitrageurs and corporate issuers. That direction is plausible, but the evidence does not establish that passive investing will systematically produce mispricing, market crashes or the disappearance of fundamental price discovery.
Price discovery is moving, not disappearing
The debate over passive investing is often framed as though markets must choose between two states: active investors who discover prices and passive investors who merely accept them. Modern markets are more interconnected than that. Passive funds create benchmark-linked demand. Active investors decide whether to accommodate it. Market makers and arbitrageurs translate basket flows into security-level prices. Exchanges concentrate execution into auctions. Companies can respond by supplying shares. Derivatives transmit information and risk across instruments.
The result is a different price-discovery system. More of the market’s ownership is rule-based, but the marginal work of valuation becomes more important, not less. Liquidity becomes deeper at some benchmark-sensitive moments and potentially thinner at others. Common flows can increase co-movement, while arbitrage can improve information transmission. Index events create predictable pressure, but that predictability also attracts capital willing to trade against it.
For investors, the practical conclusion is not that passive investing has become unsafe or that active management is destined for a revival. It is that the low-cost simplicity of an index fund rests on an increasingly sophisticated market infrastructure underneath. The more capital follows rules, the more important the institutions become that set marginal prices, provide liquidity and decide when a mechanical order has pushed a security too far from fundamental value.
References
1. Investment Company Institute - Active and Index Investing, June 2026 (30 July 2026).
2. Board of Governors of the Federal Reserve System - The Shift from Active to Passive Investing: Potential Risks to Financial Stability? (August 2018; revised June 2020).
3. European Central Bank - Financial Stability Review, November 2024 - Box 3: Passive investing and its impact on return co-movement, market concentration and liquidity (November 2024).
4. Federal Reserve Bank of New York, Liberty Street Economics - End-of-Month Liquidity in the Treasury Market (24 September 2024; charts revised 8 October 2025).
5. Nasdaq - Record Trading Volume on the Nasdaq Closing Cross During the June 2026 Russell US Indexes Reconstitution (26 June 2026).
6. LSEG / FTSE Russell - Growth, value or both? Key style shifts in the June 2026 Russell reconstitution (18 June 2026).
7. Marco Sammon and John J. Shim, The Review of Financial Studies - Who Clears the Market When Passive Investors Trade? (3 April 2026; corrected/typeset 21 April 2026).
8. Hao Jiang, Dimitri Vayanos and Lu Zheng, The Review of Financial Studies - Passive Investing and the Rise of Mega-Firms (10 October 2025).
9. Youngmin Choi, SSRN working paper - The Conditional Price Efficiency of Passive Investing (Last revised 12 April 2026).
10. Itzhak Ben-David, Francesco Franzoni and Rabih Moussawi, NBER - Exchange Traded Funds (ETFs) (Working Paper 22829; revised September 2017).