Passive investing through ETFs has grown from a niche strategy to a dominant force in global markets. Index funds and ETFs now hold roughly half of all US stock fund assets, prompting a contentious debate: do ETFs distort the market? The answer is more nuanced than either the critics or the defenders suggest.
The Rise of Passive: By the Numbers
The shift from active to passive has been dramatic. In 2005, index funds and ETFs held roughly 15% of US stock fund assets. By 2025, that figure crossed 50%. The three largest asset managers -- BlackRock (iShares), Vanguard, and State Street (SPDR) -- collectively manage trillions in index products and are among the largest shareholders of virtually every company in the S&P 500.
This concentration has raised questions about market structure, price discovery, and corporate governance that go well beyond the simple "active vs. passive" performance debate.
The Case That ETFs Distort Markets
Critics raise several interconnected concerns about passive investing's market impact.
Reduced Price Discovery
The core argument is that passive funds buy stocks based on their index membership, not their fundamentals. When money flows into an S&P 500 ETF, every stock in the index gets bought -- the overvalued and undervalued alike. If passive funds grow large enough, the argument goes, prices will no longer reflect fundamental value because not enough investors are doing the analysis to determine fair prices.
Michael Burry (of "Big Short" fame) compared passive investing to the subprime CDO bubble, arguing that money flows into index funds regardless of the price or quality of the underlying stocks. Economist Jeffrey Wurgler has published research showing that stocks added to the S&P 500 experience price increases unrelated to changes in their fundamentals.
Increased Stock Correlations
When passive funds buy and sell stocks as a basket, they can cause stocks within an index to move together more than fundamentals justify. Research by Antti Petajisto and others has found that stocks added to major indexes experience increased correlation with the index, while stocks removed see decreased correlation. This suggests that index inclusion itself -- separate from any fundamental change -- affects stock behavior.
The Index Inclusion Effect
When a stock is added to the S&P 500, billions of dollars of passive money must buy it. The stock typically rises on the announcement and experiences elevated volume around the inclusion date. The reverse happens for deletions. This creates mechanical price pressure unrelated to the company's business performance.
For large, liquid stocks, this effect is temporary and modest. But for smaller stocks being added to popular niche or smart beta indexes, the index inclusion effect can be more pronounced.
The Case That ETFs Do Not Distort Markets
Defenders of passive investing push back on each of these concerns with compelling counterarguments.
Passive Funds Are Quiet Holders, Not Active Traders
While passive funds hold roughly 50% of stock fund assets, they account for a much smaller share of trading volume -- estimated at 5-10%. The vast majority of daily price discovery is still performed by active traders: hedge funds, proprietary trading firms, active mutual funds, and individual investors. Passive funds mostly sit still. They trade only when index constituents change, when they receive inflows and outflows, or during rebalancing events.
The Self-Correcting Mechanism
If passive investing truly degraded price discovery, it would create more opportunities for active investors to profit from mispricing. This would attract more capital to active strategies, restoring the equilibrium. In a well-functioning market, the balance between active and passive adjusts naturally. As long as some minimum level of active management exists to correct mispricings, passive investing does not pose a systemic threat.
Historical Context
Markets had plenty of distortions, bubbles, and crashes long before passive investing existed. The dot-com bubble (1999-2000) and the housing crisis (2007-2008) occurred when passive funds were a fraction of their current size. Attributing market distortions to passive investing ignores the much larger forces of leverage, speculation, and human behavior that have always driven markets.
ETFs Provide Price Discovery During Stress
During the March 2020 bond market crisis, bond ETFs traded actively even when the underlying bond market was essentially frozen. The ETFs provided real-time price discovery when no other mechanism could. Far from distorting the market, ETFs served as a pressure release valve and an information source.
The Concentration of Ownership Problem
The most legitimate concern about passive investing may not be about price discovery at all. It is about corporate governance.
BlackRock, Vanguard, and State Street collectively own 20-25% of most S&P 500 companies through their index funds. These three firms have enormous voting power on shareholder proposals, board elections, and executive compensation. Unlike an active fund that buys a stock because it believes in the company's strategy, an index fund must own every stock in the index regardless of management quality.
This raises several questions. Are index fund providers good stewards of shareholder voting rights? Do they have the resources to engage meaningfully with thousands of portfolio companies? And does common ownership by the same three firms across an entire industry (say, all major airlines) reduce competitive incentives?
The evidence on common ownership is debated. Some studies have found that common ownership by index funds is associated with higher prices in concentrated industries (like airlines), while other studies have found no effect. Regulators are watching this issue closely.
What the Academic Research Says
Academic research on passive investing's market impact is genuinely mixed.
Evidence of distortion: Studies find increased stock correlations for index members, price distortions around index rebalancing events, and some evidence that passive ownership reduces the informativeness of stock prices.
Evidence against distortion: Other studies find that market efficiency has not deteriorated as passive investing has grown, that active managers' pricing power remains intact, and that the trading volume share of passive funds is too small to meaningfully impair price discovery.
The most balanced reading of the evidence is that passive investing has modest effects on market microstructure -- slightly increased correlations, some index inclusion effects -- but has not fundamentally broken price discovery. The market continues to incorporate new information into prices rapidly, which is the essential function that needs to work.
What This Means for Your Portfolio
For individual investors, the passive investing debate has limited practical implications.
If you invest in broad market index ETFs, you are getting diversified market exposure at rock-bottom costs. The theoretical concerns about market distortion do not change this value proposition. If anything, any distortions created by passive investing create opportunities for the active managers who set prices at the margin.
The most actionable takeaway is to be aware of index rebalancing events. When the S&P 500 adds or removes stocks, the price impact can create short-term trading opportunities or temporary distortions. If you are buying or selling a stock around an index change, be aware that passive flows may be temporarily affecting the price.
Beyond that, the choice between passive and active should be driven by costs, your investment goals, and the specific market you are investing in -- not by macro concerns about whether passive investing is bad for the market. Explore your options with ETF Beacon's fund screener and make the choice that fits your situation.
