Key takeaways
- Index domestic equity mutual funds and ETFs held 19% of the value of US stocks at year-end 2025 on the Investment Company Institute's count. Active funds held 11%.
- Chinco and Sammon put the true passive ownership share at 33.5% of the US market in 2021, against 16% for index funds alone, once internal and closet indexing are counted.
- The abnormal return on joining the S&P 500 fell from 7.3% in the 1990s to 0.8% in the 2010s, even as indexation kept climbing. Deletions went from -16.1% to -0.6%.
- Greenwood and Sammon's price-impact multiplier for index additions fell from 6.76 in the late 1990s to 0.36 in the last decade, a drop of more than 20 times.
- What survives is narrower: passive investing has made demand for individual stocks 11 percent more inelastic over 20 years, on Haddad, Huebner and Loualiche's estimate.
The index fund bubble claim, and what the ownership data says
Someone has told you index funds are a bubble. The argument usually runs like this. Passive money buys whatever the index holds, at any price, without an opinion. It now owns so much of the market that prices have stopped reflecting anything. One day the flows reverse, and the whole structure unwinds.
On the evidence available in August 2026, the strong version of that claim fails, and it fails on the evidence its own proponents choose. Index funds own less of the US market than the argument assumes. Where their forced buying is easiest to measure, at index reconstitution, the price impact has collapsed rather than grown. What's left standing is a smaller, better documented finding about how elastic the market is and how much information gets into prices. That finding is real. It isn't a bubble.
Index funds own about a fifth of the US market, not most of it
Two very different numbers get used interchangeably here, and the confusion does most of the work in the index fund bubble argument.
The first is a share of the fund industry. The Investment Company Institute's 2026 Fact Book reports that index mutual funds and index ETFs held $19.1 trillion at year-end 2025, and "accounted for the majority (52%) of assets in long-term funds, up from 19% at year-end 2010". That is the number people have in mind when they say passive has won.
The second is a share of the market. On the same page, the ICI reports that index domestic equity mutual funds and ETFs held "19% of the value of US stocks at year-end 2025". Actively managed domestic equity funds held another 11%. Everyone else, which the ICI lists as hedge funds, pension funds, life insurance companies and individuals, held 69%.
So index funds are the majority of one industry and a minority owner of the market that industry invests in. The UK picture is smaller again. The Investment Association counted £383.7 billion in tracker funds at the end of 2024, which was 26% of industry funds under management excluding ETFs.
One caveat about who is counting. The ICI is the fund industry's trade body, and it titles that chart "Index Domestic Equity Fund Share of US Stock Market Is Small". It has an interest in the answer. Which is why the next number matters more.
The real passive ownership share is roughly double the headline
Index funds have to disclose their holdings, so their share is easy to count. Other passive investors don't. Large pension funds and sovereign wealth funds often run index-tracking portfolios in house. Individuals can hold index-like separate accounts, which is often done for tax reasons. And some nominally active managers hug their benchmark closely enough that the difference is cosmetic, which is what closet indexing means.
Alex Chinco and Marco Sammon found a way to measure all of it at once. Passive rebalancing happens in a spike right at the market close on the day an index changes, so they ask how much money would have to be tracking an index to explain the volume they see. Their April 2024 paper reports that "while index funds held 16% of the US stock market in 2021, we put the overall passive ownership share at 33.5%". Their reading of that gap is blunt: "for every $1 held by an index fund, there is another $1 held by another kind of passive investor".
The direction of travel is not in dispute either. The same paper notes that index funds held "just $0.4t in 2000, roughly 3% of the value of the US stock market". Federal Reserve staff put US stocks held in passive funds and ETFs at "about 14 percent of the domestic equity market" as of March 2020, up from less than four percent in 2005. Sammon's separate work dates the passive share of US equities at "less than 1% to almost 15%" between 1990 and 2019.
Take the honest number, then. Roughly a third of the US market is held passively, not a fifth. That's the strongest fact the bubble case has, and it deserves to be stated at full strength before anything is said against it.
Owning a share and setting a price are different jobs
Here's where the argument has to do work it usually skips. Ownership is a stock. Price setting is a flow. The price of a share today is set by whoever is willing to transact today, not by everyone who happens to hold it.
William Sharpe's 1991 note defines the terms precisely. "A passive investor always holds every security from the market, with each represented in the same manner as in the market." An active investor "is one who is not passive". From that alone, Sharpe derives that "before costs, the return on the average actively managed dollar will equal the return on the average passively managed dollar". No forecasting, no data, just addition and subtraction.
The relevant consequence is that an index fund has almost no discretionary trading. It buys when money arrives, sells when money leaves, and rebalances when the index changes. It expresses no view on price on any other day. A market where a third of the shares sit still is not a market where a third of the price discovery has stopped. It's a market where the remaining traders set the price against a smaller free float. Whether that makes prices worse is an empirical question, and it has been measured.
The index inclusion effect collapsed while indexation grew
The cleanest test available is what happens when a stock joins an index. Every tracker has to buy it, on a known date, for reasons that have nothing to do with the company. If mechanical index demand moves prices, this is where you'd see it, and you'd expect the effect to grow as indexation grows.
It shrank. Robin Greenwood and Marco Sammon measured the abnormal return around addition to the S&P 500 by decade. It was "3.4% in the 1980s to 7.3% in the 1990s", then "decreased somewhat in the first decade of the 2000s to 5.1%, and then fell further to 0.8% in the most recent decade, statistically indistinguishable from zero". Deletions traced the same path: "-4.6% in the 1980s, -16.1% in the 1990s, -12.4% from 2000-2009, and -0.6% from 2010-2020". The chart above plots both series.
This happened while the demand shock got bigger, not smaller. The same paper finds that in the early 1990s net buying by trackers around an addition "was close to 0% of shares outstanding, while now it is over 6%", and almost 8% of shares outstanding for deletions. Funds tracking the S&P 500 have grown "from essentially zero in the 1980s to approximately 7 percent of market capitalization in recent years".
Converting that into a price-impact multiplier makes the size of the change plain. The multiplier "has fallen by a factor of more than 20, from 6.76 in the late 1990s to 0.36 in the last decade", with the deletion multiplier falling from 10.76 to 0.43. Bigger forced trades, smaller price effect.
The authors' explanation is that active investors show up to take the other side. "Although index trackers now buy about 7-8% upon index addition, total institutional ownership barely moves around index changes." Someone sells trackers the shares they need. Federal Reserve staff reached the same reading of the equity evidence in their staff paper on the shift from active to passive: for equities, "valuation effects have declined significantly since 2000", and comovement effects "have declined significantly since 2001".
The strongest objection, stated at full strength
None of that means index money is weightless. The best version of the sceptical case rests on three findings, and they're serious.
Xavier Gabaix and Ralph Koijen estimate that "investing $1 in the stock market increases the market's aggregate value by about $5". Aggregate demand for equities is far less elastic than textbook finance assumes, so flows move the index level a lot.
Valentin Haddad, Paul Huebner and Erik Loualiche then attribute part of that to passive itself. Their 2025 American Economic Review paper finds that "the rise in passive investing over the last 20 years has made the demand for individual stocks 11 percent more inelastic". They also find that competing investors only offset about two-thirds of a change in another group's behaviour, so the market is not the frictionless ideal.
And Sammon shows a cost to price discovery. Passive ownership "negatively affects the degree to which stock prices anticipate earnings announcements", with the information reaching prices ahead of results down by roughly a quarter of the sample average over 30 years.
Read those together and you get a market that is measurably more inelastic and measurably slower to price news. That is a real change, and it's contested only in magnitude. What it isn't is a bubble. An inelastic market amplifies flows in both directions. It says nothing about the level of prices, and none of these papers claims that stocks are overvalued.
Claim against evidence, one line each
- "Index funds own most of the market." Not supported. 19% for index domestic equity funds at end-2025, 33.5% for all passive investors in 2021.
- "Index buying mechanically inflates the stocks it buys." Contradicted. The addition effect fell to 0.8% and the multiplier to 0.36 while tracker buying rose above 6% of shares outstanding.
- "Passive has made markets less elastic and prices less informative." Supported, and modest. 11 percent more inelastic, price informativeness down about a quarter.
- "A passive unwind would be disorderly." Untested. Federal Reserve staff found passive fund flows less responsive to performance than active flows, which cuts the other way, but the era has had no sustained passive outflow to observe.
The limitations of all of this
Almost every figure here is American. The Investment Association's UK tracker number is the only domestic data point, and it counts funds sold in the UK, not ownership of UK-listed shares. Nothing above tells you the passive share of the FTSE All-Share.
The 33.5% estimate is an inference, not a holdings count. Chinco and Sammon infer it from reconstitution-day volume across five indexes, so it depends on their model of how trackers trade. The Greenwood and Sammon sample ends in 2020, and the authors flag that the late-2010s uptick was driven by Tesla joining the index in November 2020. The elasticity work by Gabaix and Koijen and by Haddad, Huebner and Loualiche comes from estimated demand systems, which are model-dependent by construction.
The data cannot tell you whether the market is expensive. Every study here measures ownership, price impact or elasticity. None of them values anything. If you think US equities are overpriced in August 2026, these papers neither support nor refute you, and the index fund bubble framing adds nothing to that argument either way. It's also worth separating the vehicle from the exposure: a fund that tracks a concentrated index inherits that concentration, which is a question about mega-cap concentration rather than about indexing.
What would change the conclusion
If the inclusion effect started rising again. The whole case above turns on one measured fact: index demand has become easier to absorb, not harder. That is a claim about liquidity provision, and liquidity provision is a behaviour, not a law. If additions started printing 1990s-style returns again, the mechanical-price-pressure story would be back on the table.
If active capital stopped showing up. Greenwood and Sammon's explanation depends on professional active investors taking the other side of tracker trades. That pool is not fixed. The ICI puts actively managed domestic equity funds at 11% of US stocks at the end of 2025, and their long-run record against index benchmarks, which the SPIVA scorecard tracks, is a subject of its own. Nobody has identified the share at which providing that liquidity stops being worth doing.
If flows turned persistently negative. Every estimate above was measured over a period in which the passive share only rose. Gabaix and Koijen describe a market where "flows in and out of the stock market have large impacts on prices", so the multiplier runs in both directions in their framework. It has never been tested at the current passive ownership share, because it has never had to be.
The number to watch isn't the passive share. It's the addition effect. It's published, it's measured the same way each decade, and it's the one series that would move first if the mechanism people are worried about were real.