Are Index Funds a Bubble? What Michael Burry Warned, and What Happened Next
In September 2019, the man who saw the 2008 crash coming said the money in your retirement fund was the next bubble.
His name was Michael Burry. Christian Bale played him in The Big Short. And in a long email interview with Bloomberg News, he compared index funds, the "safe, boring" investment millions of ordinary people trust with their retirement, to the toxic mortgage securities that nearly destroyed the world's financial system.
"Like most bubbles, the longer it goes on, the worse the crash will be," he said.
If you've already lost money in the stock market once, a warning like that hits differently. You wonder: is the one thing I was told was safe about to blow up too?
Here's the short answer, and then the whole story.
The short answer: No, index funds are not a bubble in the way Michael Burry warned. Seven years after his 2019 warning, the S&P 500 is up about 160%, the crash he feared never came from index funds, and even the sharp COVID crash of 2020 was fully recovered within six months. But Burry raised two fair worries that still matter: index funds now own more of the market than ever, and the S&P 500 is unusually top-heavy. Neither is a reason to sell. Both are reasons to understand what you own. |
In this article:
The Warning That Scared Millions of Index Investors
Michael Burry is not a man people ignore. Before 2008, while banks were piling into mortgage bonds, he studied the loans one by one, saw they were rotten, and bet against them. He made a fortune, and the story became a best-selling book and an Oscar-winning film.
So when Bloomberg published his views on September 4, 2019, the headline spread fast: The Big Short's Michael Burry Explains Why Index Funds Are Like Subprime CDOs.
That same day, the S&P 500 closed at 2,937.78.
Financial websites, forums and podcasts lit up. People who had finally found peace with a simple index fund, many of them after being burned by stock tips and "sure things", started asking the same question you may be asking now. Should I get out?
To answer that honestly, we need to look at exactly what he said, not the scary headline version.
What Michael Burry Actually Said About Index Funds
Burry made three main claims. Each one is serious, and each one deserves a fair hearing.
Claim No. 1: Index funds kill "price discovery"
When you buy an index fund, you don't study the companies. You simply buy all of them, in proportion to their size. Burry argued that if enough money does this, nobody is doing the hard work of deciding what each company is really worth.
In his words, "passive investing has removed price discovery from the equity markets." He compared it to the CDO market before 2008, where prices were set "not by fundamental security-level analysis, but by massive capital flows."
Claim No. 2: The exit door is too small
This was his most vivid point. Index funds hold every stock in the index, including small ones that barely trade. On the day of his interview, Burry counted 1,049 stocks in the Russell 2000 small-company index that traded less than $5 million in value that day. Yet hundreds of billions of dollars were tied to them through index funds.
"The theater keeps getting more crowded," he wrote, "but the exit door is the same as it always was."
The fear: when everyone tries to sell at once, there won't be enough buyers, and prices will collapse.
Claim No. 3: The flows will reverse, and "it will be ugly"
Burry said the money pouring into index funds would one day turn around and pour out. He also admitted, "I just don't know what the timeline will be."
That last sentence matters more than most people noticed. A warning without a timeline can be "right" one day, 10 years or 30 years later. Meanwhile, the person who sells today pays the price of being out of the market the whole time.
What Happened Next: Seven Years of Evidence
We now have something Burry didn't have in 2019: seven years of real results. Let's look at them.
The S&P 500 Since the Warning
Imagine two neighbours, both with $10,000 in an S&P 500 index fund on the day Burry's interview came out. One sells in fear. The other does nothing.
The neighbour who did nothing saw $10,000 grow to about $25,950 by October 23, 2026, and that's before counting dividends. At the record close of 7,818.93 on October 6, 2026, it was worth about $26,615.
Notice something else in that chart. Even at the very bottom of the 2020 COVID crash, the neighbour who stayed was only down about 24% from the day of the warning. The terrifying headlines measured the fall from the February 2020 peak, not from the day you might have panicked.
If you want to see how this fits the long-term record, I went through every 20-year period since 1926 in Has the S&P 500 Ever Lost Money Over 20 Years?
The Crash Came, and the Exit Door Held
Burry's "crowded theater" was tested sooner than anyone expected. Between February 19 and March 23, 2020, the S&P 500 fell about 34%, the fastest bear market on record. Trading was halted several times as prices fell.
If index funds were a trap, this was the moment for it to snap shut.
It didn't. The market kept functioning, buyers kept showing up, and on August 18, 2020, the S&P 500 closed at a new record of 3,389.78, less than six months after the bottom. To be fair, the Federal Reserve and governments stepped in with enormous support, so this wasn't a "pure" test. But the index fund machinery itself never broke.
And the index investors? They were calmer than the experts. Vanguard, the world's best-known index fund company, found that only about 8% of its retail and workplace-plan clients made any trade at all between February 19 and March 20, 2020. More than 90% simply held on.
That's the opposite of a stampede for the exit.
Burry's Own Bets Since 2019
Here's the part of the story most "Burry warns" headlines never follow up on.
In January 2023, Burry posted a single word on social media: "Sell." By late March, he took it back: "I was wrong to say sell." Then, by mid-2023, regulatory filings showed his fund held put options with a notional value of about $1.6 billion betting against funds tracking the S&P 500 and the Nasdaq 100. The market kept rising, and he exited those positions within a quarter.
In October 2025, he wrote to his investors that he would close his fund and return their money, explaining that his "estimation of value in securities is not now, and has not been for some time, in sync with the markets." His firm, Scion Asset Management, withdrew its registration with the SEC in November 2025. He now writes a paid newsletter, Cassandra Unchained, named after the prophet in Greek myth who was always right and never believed. It passed 300,000 subscribers in July 2026.
None of this makes Burry a bad investor. He is brilliant, and he was spectacularly right once. But it shows something every burned investor needs to hear: even a genius cannot time the market reliably. If he can't, you and I shouldn't bet our retirement on trying.
Scorecard: Burry's 2019 claims vs. what happened "Index fund flows will reverse and it will be ugly." Index funds kept growing. ICI data show their share of the US stock market rose from 16% in 2019 to 19% in 2025. "The exit door is too small." Tested in March 2020's 34% crash. The market kept working and recovered to a record within six months. "Passive investing has removed price discovery." Still debated, but about 80% of US stock market value is still held by investors other than index funds. "The longer it goes on, the worse the crash." No index-fund crash yet. He admitted he didn't know the timeline. |
Where Burry Had a Point
It would be easy, and lazy, to say "Burry was wrong, the end." He wasn't wrong about everything. A fair investor looks at both sides.
Index funds really do own more of the market
According to the Investment Company Institute's 2026 Fact Book, index mutual funds and ETFs that buy US stocks held 11% of the US stock market in 2015, 16% in 2019, and 19% by the end of 2025. Actively managed funds shrank from 18% to 11% over the same years.
Some researchers think the true passive share is even bigger. A 2024 study by Alex Chinco and Marco Sammon in the Journal of Financial Economics looked at the huge trading spikes on the days stocks join or leave an index. They estimated that, counting pension funds and other big investors who quietly copy indexes, about 33.5% of the US market was effectively passive in 2021, roughly double the index-fund figure.
So the trend Burry pointed to is real and still growing.
The S&P 500 has become top-heavy
The bigger risk today isn't the index fund itself. It's what's inside the S&P 500. According to S&P Global, the 10 largest companies made up almost 40% of the index by mid-2025, a level of concentration not seen since the mid-1960s, and it has stayed close to that level through 2026.
That means when you buy "500 companies", a large slice of your money rides on about 10 of them. If those giants stumble together, an S&P 500 fund will feel it.
Interestingly, this doesn't seem to be caused by index funds. Research from Dimensional Fund Advisors in 2026 found that the 10 biggest US stocks actually have lower index-fund ownership than other large companies. Most of their value is held by active funds, institutions and households who chose to buy them.
Why an Index Fund Is Not a CDO
Burry's comparison was memorable. But when you put the two side by side, the differences are huge.
2008 subprime CDO | S&P 500 index fund | |
|---|---|---|
What you own | Slices of pooled mortgage loans most buyers never inspected | Real shares in about 500 public companies |
Can you see inside? | Hidden, complex, rated by agencies paid by the sellers | Every holding published, usually daily |
Borrowed money (leverage) | Often heavy, layered on top | A plain fund uses none |
What backs it | Home loans, many made to people who couldn't repay | Company profits, sales and assets |
Worst case | Many tranches became nearly worthless | Price falls, as in 2008, 2020 and 2022, then historically recovered |
A CDO's value depended on loans that were secretly going bad. An index fund's value depends on the profits of the biggest companies in America. Those can fall, sometimes hard, but they don't vanish because a credit rating turned out to be a lie.
There's another reason the comparison breaks down. Remember, about 80% of US stock market value is still held by investors other than index funds. Some of them quietly track indexes too, but hedge funds, active managers and millions of individuals still study companies and trade every day. They keep setting prices. A Bloomberg Intelligence analysis even found that stocks with lower passive ownership performed better over the long run than those most favoured by index strategies, the opposite of what you'd expect if index money were blindly inflating prices.
And if you're worried about the fund company itself failing, that's a separate question with a reassuring answer. I explain it in Is Your Money Safe in an Index Fund? What Happens If Vanguard Goes Bankrupt.
The Real Risk Isn't the Index Fund. It's What You Do Next.
Here is the uncomfortable truth for anyone who has lost money in the market. The danger is rarely the investment. It's the decision made in fear.
The professionals who are paid to beat the index mostly don't. S&P Dow Jones Indices tracks this in its SPIVA scorecards.
In 2025, 79% of actively managed US large-company funds did worse than the S&P 500. Over the 20 years to the end of 2025, about 93% fell behind. If the full-time experts can't reliably beat the index, a scary headline is not a good enough reason to abandon it. I go deeper into this in Stock Picking vs. the S&P 500: What 20 Years of Results Say, and Warren Buffett proved the same point with real money in his famous $1 million bet against hedge funds.
The investors who got hurt after Burry's warning weren't the ones who held their index funds. They were the ones who sold, waited for the crash to "prove" him right, and watched the market double without them. If that has happened to you before, you're not alone, and it isn't a character flaw. I explain the psychology in Why Smart, Hard-Working People Sell at the Very Bottom.
Will the market fall again? Almost certainly. Prices are high, interest rates are high, and a few giant companies carry a lot of weight. A fall of 20% or 30% would be normal history, not a broken system. That's very different from a "bubble" in index funds bursting and never coming back. If you're worried about the worst case, read Can the S&P 500 Go to Zero?
What to Do Tonight If the "Index Fund Bubble" Headlines Worry You
You don't need to predict the next crash. You need a plan that survives it. Use this simple flow the next time a famous name warns of doom.
Check your time horizon. Money you need within about five years shouldn't be in stocks at all, index fund or not. Move it to cash or short-term bonds on a calm day, not a scary one.
Look at your top 10. Open your fund's holdings page and see how much sits in the 10 largest companies. If it makes you uncomfortable, you could add a global or equal-weight index fund to spread the risk. Keep it simple, though. Most people need fewer funds than they think, as I show in How Many Index Funds Should You Own?
Write your "crash rule" now. For example: "If the market falls 20%, I will keep my monthly investment going and not sell." Decisions written down in calm times beat decisions made at 2 a.m. Here's what calm investors do after a 20% drop.
Keep investing a fixed amount every month. Regular investing turns falling prices into a discount instead of a disaster.
Stop treating famous predictions as instructions. Ask one question: "Has this person given a timeline, and would I have made money following their last five calls?" Usually the answer settles it.
Frequently Asked Questions
Are index funds a bubble?
No. A bubble means prices far above real value, driven by speculation, that eventually collapse. Index funds simply own shares in real companies. Their prices can fall when the market falls, but since Michael Burry's 2019 warning the S&P 500 has risen about 160%, and no index-fund collapse has happened.
What did Michael Burry say about index funds?
In a September 2019 Bloomberg interview, Burry said money pouring into index funds was distorting stock and bond prices, much like CDO buying did before 2008. He warned that many stocks in indexes trade too little to handle mass selling, and that when flows reverse, "it will be ugly." He gave no timeline.
Was Michael Burry right about index funds?
Not so far. The market recovered from the 2020 crash within six months and later reached record highs. He was right that index funds keep growing and that the market has become concentrated in a few giant stocks. Those are real risks, but they have not caused the collapse he predicted.
Could index funds cause a stock market crash?
Index funds could deepen a sell-off if many investors redeemed at once, but evidence so far is reassuring. In March 2020, more than 90% of Vanguard's investors made no trades. About 80% of US stock value is held by investors other than index funds, many of whom keep setting prices, so index funds alone are unlikely to trigger a crash.
Is it safe to invest in an S&P 500 index fund right now?
No investment is risk-free, and the S&P 500 can fall 20% to 30% or more. For long-term money, a low-cost index fund has historically been one of the most reliable ways to build wealth. Only invest money you won't need for at least five years, and keep investing steadily instead of guessing the timing.
Does Michael Burry still run a hedge fund?
No. Burry told investors in October 2025 that he would close his fund and return their money, and Scion Asset Management withdrew its SEC registration in November 2025. He now publishes his views through a paid newsletter called Cassandra Unchained.
The Bottom Line
Michael Burry asked a smart question in 2019, and it's healthy to keep asking it. But seven years of evidence say index funds are not the next subprime CDO. They are a low-cost way to own real businesses, and they have quietly beaten most of the professionals who warned against them.
The real "bubble" to watch is the one in our own heads: the urge to act on every frightening headline. Protect yourself from that, and an index fund can do what it was designed to do. Help you build wealth slowly, steadily, while you get on with your life.
If you've been burned before and want to understand exactly why it happened, and how to make sure it never happens again, start with my free report.
Why You Lost Money in the Stock Market (And Why It Wasn't Your Fault) is a free 38-page guide for investors who want to start again on solid ground.
About the Author
Stephen Loke has been a student of the market for more than 15 years. He is also an agritourism entrepreneur whose farm has been featured in Bloomberg News and Asahi Shimbun.
This article is general education, not personal financial advice. I'm not a licensed financial adviser, and nothing here is a recommendation to buy or sell any investment. Past performance does not guarantee future results.
Sources
CNBC: Asia markets and US close, September 5, 2019 (S&P 500 at 2,937.78)
NPR: S&P 500 Closes at Record High 6 Months After Coronavirus Plunge (Aug 18, 2020)
BenefitsPRO: Vanguard trading activity up, but most staying the course (March 2020)
Investment Company Institute: 2026 Investment Company Fact Book, Chapter 2
Bloomberg Intelligence: Passive's no bubble as active retains market control
ETF Stream: Big Short's Michael Burry bets against S&P 500 and Nasdaq 100 (2023)
Barchart: Michael Burry names successor as he deregisters hedge fund (Nov 2025)
Stocktwits: Burry's Cassandra Unchained passes 300,000 subscribers (July 2026)
Reuters via Virginia Business: S&P 500 notches record-high close (Aug 13, 2026)
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