Why Some Investors Think Index Funds Are a Bubble | Risks, Valuation, and Concentration

Your retirement account is probably in one. Your coworker brags about it at lunch. Every personal finance influencer on the internet swears by it. The index fund has become the universal answer to the question: “How do I invest without blowing everything up?”

So when someone says that an index fund is the thing blowing up, people get uncomfortable fast.

In 2019, Michael Burry, the man who famously bet against the U.S. housing market before the 2008 collapse, called index funds a bubble. The financial world mostly laughed it off. But the argument has not gone away. It has gotten louder, and the data behind it has gotten harder to dismiss.

Here is the thing, though: the people on the other side are not idiots either. Some of the sharpest financial minds alive disagree with Burry completely. They think the “index fund bubble” thesis is a myth, a distraction, or flat-out wrong.

So who is right? That is exactly what we are going to work through. Not to sell you a product. Not to push a particular ideology. Just to lay out the strongest version of both arguments and give you the tools to think for yourself.

Buckle up. This one gets genuinely interesting.

First, What Is an Index Fund Actually Doing?

Before we get into the bubble debate, we need a clear picture of the mechanism. Because a lot of the argument hinges on exactly how these funds operate.

An index fund is a type of investment fund designed to replicate the performance of a specific market index, like the S&P 500, the Nasdaq Composite, or the MSCI World. It is not trying to beat the market. It is trying to be the market.

Most major indices are weighted by market capitalisation. That means the bigger a company’s total market value, the bigger its slice of the index. Apple, Microsoft, and Nvidia are not just in the S&P 500. They dominate it. A single dollar going into an S&P 500 index fund today sends a disproportionate chunk of that dollar straight into those three companies.

This is the first crack in the foundation. And it matters more than most people realise.

The Scale of Passive Investing Today

Passive investing was a niche idea when John Bogle launched the first retail index fund at Vanguard in 1976. Today, it is the dominant force in global markets. By 2024, passive funds overtook active funds in total U.S. equity assets for the first time in history.

That is a seismic shift. The original theory behind index investing assumed that active traders would do the heavy lifting of price discovery. Passive funds could simply free-ride on their collective intelligence. But what happens when passive investors become the majority?

That is precisely the question at the heart of this debate.

The Case FOR a Bubble: Michael Burry’s Warning

Michael Burry does not make predictions casually. His track record demands attention, even when his conclusions are uncomfortable.

In a now-famous 2019 interview with Bloomberg, Burry drew a direct parallel between today’s index fund market and the collateralised debt obligations (CDOs) that blew up the housing market in 2008. The analogy is jarring but worth examining seriously.

His core argument: just as CDOs packaged toxic mortgages into vehicles that got bought blindly because of their ratings, index funds package stocks into vehicles that get bought blindly because of their inclusion in an index. The underlying asset quality becomes irrelevant. What matters is whether something is in the index.

“The flows into index funds are like rivers with no end. Like a glacier that doesn’t stop moving. They are relentless. And the stocks in the index get bid up regardless of their fundamentals.” — Michael Burry (paraphrased from Bloomberg interview, 2019)

The Price Discovery Problem

Healthy markets rely on a process called price discovery. Buyers and sellers exchange information through trades, pushing prices toward something approximating fair value. It is messy, imperfect, and sometimes brutal. But it works.

Index funds do not do this. They are price-takers, not price-makers. When a billion dollars flows into a broad market ETF, that fund does not ask whether Apple is fairly valued at its current price. It just buys Apple in proportion to its market cap weight.

Investor and fund manager Mike Green has been one of the most persistent voices on this issue. His argument, laid out in detail on the Odd Lots podcast, is that we are approaching a tipping point where passive flows are actually setting prices rather than reflecting them. At that point, the self-correcting mechanism of markets starts to break down.

Think of it this way. If the dog (active traders) used to wag the tail (passive funds), the tail is increasingly wagging the dog. And a tail-wagging-dog market is a market that has lost its anchor to reality.

Valuations: The Shiller PE Alarm Bell

Numbers do not lie, even when they are inconvenient. One of the most respected valuation tools in finance is the Shiller PE ratio, also known as the Cyclically Adjusted Price-to-Earnings ratio, or CAPE. It was developed by Nobel Prize-winning economist Robert Shiller.

Instead of using a single year’s earnings, CAPE smooths earnings over 10 years, adjusted for inflation. It removes the noise of a single boom or bust year and gives a longer view of whether stocks are cheap or expensive relative to their actual earning power.

Here is the data as of early 2026:

Time Period	Shiller PE Ratio (CAPE)	Context
Long-term historical average	~17x	Baseline for “fair value”
Peak before the 1929 crash	~30x	Black Tuesday was preceded by extreme overvaluation
Peak of the dot-com bubble (2000)	~44x	The highest CAPE in history at that point
Pre-2008 financial crisis	~27x	Still well above average before the housing collapse
January 2025	~38x	More than double the long-term average
Early 2026	>40x	Approaching dot-com era extremes

Let that sink in. Today’s market valuations are sitting at levels only previously seen at the peak of the dot-com bubble. The stock market’s current earnings yield sits around 2.6%. Meanwhile, U.S. Treasury bonds are yielding around 4.5%. You are accepting significantly more risk for significantly less return. That math deserves serious scrutiny.

The ETF Mechanics Problem: Forced Selling in a Crisis

Here is a risk that rarely gets discussed in the mainstream conversation about index funds. It is technical, but it is important.

Exchange-traded funds (ETFs) are the vehicle most people use to hold index funds today. By design, when the market rises, ETFs buy. When the market falls and investors panic and redeem their shares, ETFs must sell. There is no discretion. No manager is making a judgment call that this selloff is overdone and we should hold.

This creates a dangerous procyclical dynamic. Falling prices trigger outflows. Outflows trigger more selling. More selling pushes prices lower. Lower prices trigger more outflows. You can see where this spiral leads.

In a severe enough market dislocation, the sheer scale of passive ETF assets could amplify the decline dramatically. We got a small preview of this dynamic in the March 2020 COVID crash, when markets fell faster and further in 10 days than they did at almost any comparable point in history.

The Cap-Weight Concentration Trap

There is another quiet problem that is getting worse every year. When you buy a standard S&P 500 index fund today, you are not really buying 500 equally weighted companies. You are buying a fund increasingly concentrated in a handful of mega-cap technology stocks.

As of early 2026, Apple, Microsoft, Nvidia, Amazon, and Alphabet together account for roughly 25-30% of the S&P 500’s total weight. That means one dollar in four of your “diversified” index fund investment is concentrated in five companies.

Diversification was supposed to be the whole point. At these concentration levels, that promise is becoming harder to keep.

The Case AGAINST a Bubble: The Debunkers Fight Back

Now for the other side. And this side is not flimsy. Some very smart people think the index fund bubble thesis is, to put it plainly, wrong.

Ben Carlson of Ritholtz Wealth Management has written one of the most cited rebuttals to the passive bubble narrative. His central point is elegant: index funds are not new, and neither is overvaluation. Markets got into bubbles long before passive investing existed.

The Great Depression. The 1973-74 bear market. The dot-com crash. The Japanese asset bubble of the late 1980s. None of these had anything to do with index funds. They had everything to do with human nature: greed, fear, extrapolation, and leverage.

Passive Still Owns a Minority of the Market

Here is a crucial data point that the bubble advocates tend to underweight. Even though passive funds have grown enormously, they still represent a minority of total market ownership when you include direct stock ownership, private equity, hedge funds, sovereign wealth funds, and foreign investors.

Institutional investors who actively trade make up a massive portion of daily market volume. The tail-wagging-the-dog metaphor is vivid, but the math of who actually sets prices at the margin is more complicated than the metaphor suggests.

Ben Carlson’s rough framework: wake him up when passive funds control 90% of the market. Until then, the active trading that still dominates volume is doing enough price discovery to keep the system functional, if imperfect.

Index Funds Are Not the Cause; Markets Are Just Expensive

Canadian portfolio manager Ben Felix makes a related and important point. High valuations are real. Nobody disputes the Shiller PE data. But attributing those high valuations to passive investing specifically requires proving a causal link that simply does not exist in the data yet.

Markets were getting expensive before passive funds dominated flows. Low interest rates, corporate buybacks, and genuine technological innovation in the largest companies all explain elevated valuations without needing a passive-investing villain.

The Federal Reserve’s decade of near-zero interest rates after 2008 compressed yields on every alternative asset class. When bonds pay nothing, stocks become relatively more attractive even at high absolute valuations. That is basic TINA economics: There Is No Alternative.

Liquidity and Arbitrage Keep the System Honest (For Now)

One underappreciated mechanism is arbitrage. For ETFs specifically, a class of sophisticated traders called authorised participants can create and redeem ETF shares at the value of the underlying assets. This keeps ETF prices anchored closely to the net asset value of what they hold.

If passive flows were wildly distorting individual stock prices, active traders would pounce on that mispricing. They would short the overpriced stocks and buy the underpriced ones. In doing so, they would actually correct the distortion. This arbitrage mechanism is a real and significant counterweight to passive distortion.

It is not perfect. It is not instantaneous. But it exists, and ignoring it understates the self-correcting resilience of the market.

Historical Parallels: What Bubbles Actually Look Like

Context matters enormously when you are evaluating bubble claims. Let’s look at what real bubbles looked like structurally and see how well today’s index fund market fits the pattern.

The Dot-Com Bubble (1995-2000)

The dot-com bubble had clear hallmarks. Companies with no revenue, no earnings, and sometimes no customers were trading at astronomical valuations. The narrative was intoxicating: the internet changes everything, profits can come later, eyeballs are the new currency.

When the bubble burst, the Nasdaq fell roughly 78% from peak to trough. Many individual companies went to zero. The destruction was concentrated and severe.

Today’s mega-cap tech companies are genuinely profitable. Apple generates more than $100 billion in annual net income. Microsoft and Alphabet are similarly robust cash machines. So the direct analogy to dot-com profitless speculation has real limits.

However, the valuation premium being paid for those earnings is the concern. At 40x cyclically adjusted earnings for the whole market, you are paying a dot-com-level multiple for companies that are, in many cases, better businesses than 1999’s darlings. That is nuanced, and it matters for how you interpret the risk.

The Housing Bubble (2003-2008)

The housing bubble Burry shorted had a specific structural problem: leverage. Ordinary homebuyers were taking on mortgages they could not sustain. Banks were packaging those mortgages into securities and selling them to investors as safe assets. Rating agencies blessed the whole thing with AAA ratings.

The index fund market has its own leverage-adjacent concern. Retail investors increasingly use margin accounts to amplify their index fund exposure. Options markets around index ETFs like SPY and QQQ have exploded in scale. Leveraged ETFs have become accessible to retail investors who may not fully understand the risks of daily rebalancing compounding against them.

The system is not as leveraged as the 2006 housing market. But it is not unleveraged either.

The Japanese Asset Bubble (1986-1991)

The Japanese asset bubble is the most chilling historical parallel for a different reason. The Nikkei 225 peaked in December 1989 at around 38,900. It would not return to that level for over 34 years.

Japan was not a basket-case economy. It was the world’s second-largest economy, running massive trade surpluses, with globally competitive companies in electronics and automobiles. Yet it took three and a half decades to recover from the bubble’s aftermath.

This matters for index fund investors because international index funds are heavily weighted toward Japan. Japan represents between one-fifth and one-quarter of many international indices. A country with a shrinking population, the world’s highest public debt-to-GDP ratio, and still-suppressed equity valuations is the anchor for global diversification. That is not a comfortable position.

The Framework: How to Steelman Both Sides Simultaneously

Rather than picking a team, let’s build a structured framework. Both sides of this debate are making partially valid points. The truth is in the synthesis.

Argument	Team Bubble (Burry/Green)	Team Debunk (Carlson/Felix)	Our Assessment
Price discovery degradation	Passive flows are becoming the market	Active volume still dominates price-setting	Partially valid concern; not yet at the crisis threshold
Valuations (Shiller PE)	40x CAPE is historically extreme	Low rates justify higher multiples structurally	Valuations are genuinely stretched; return expectations should be lowered
ETF forced selling risk	Creates procyclical crash amplification	Arbitrage mechanisms dampen distortions	Real tail risk in severe dislocations, but not a daily threat
Concentration in mega-caps	Top 5 = ~25-30% of S&P 500	Those companies are genuinely dominant and profitable	Concentration risk is real; “diversified” is less true than marketed
Historical bubble comparison	Structural parallels to CDOs and dot-com	Bubbles predate passive investing; correlation is not causation	The vehicle is not the bubble; market overvaluation is the bubble

The most honest framing might be this: index funds themselves are not a bubble. But the market they are relentlessly buying into is potentially in one. And that distinction, while important, does not actually protect your portfolio.

What “Smart Money” Is Actually Doing Right Now

Here is where it gets instructive. How are sophisticated allocators responding to these concerns? Not with panic. But also not with blind complacency.

Moving Toward Factor-Based Investing

Factor investing, sometimes called “smart beta,” tries to capture the benefits of passive investing while tilting toward characteristics that have historically generated better risk-adjusted returns. Common factors include:

• Value: Companies trading at low multiples relative to earnings, book value, or cash flow, as popularised by Benjamin Graham and Warren Buffett
• Quality: Companies with strong balance sheets, high return on equity, and stable earnings
• Low Volatility: Stocks that historically exhibit lower price swings, which paradoxically tend to outperform in the long run
• Small Cap: Smaller companies that have historically delivered higher long-run returns (with higher volatility) than large caps
• Momentum: Securities that have recently outperformed, which tend to continue doing so over intermediate timeframes

Funds like Dimensional Fund Advisors (DFA) have built their entire business model around factor-based passive strategies. They are passive in the sense that they do not try to pick individual stocks. But they are active in tilting away from pure market cap weighting.

Equal-Weight Indexing as an Alternative

One direct response to the concentration problem is equal-weight indexing. Instead of weighting companies by market cap, every company in the index gets the same allocation. The Invesco S&P 500 Equal Weight ETF (RSP) does exactly this.

Over long periods, equal-weight strategies have historically outperformed their cap-weighted counterparts. The reason: they systematically buy more of smaller companies that are cheaper and sell proportionally more of the largest companies that become more expensive. It is a built-in value discipline.

The trade-off: equal-weight funds have higher turnover, slightly higher fees, and in momentum-driven markets like 2023-2024, they will underperform the cap-weighted version as the giants keep growing.

International and Emerging Market Diversification

The valuation case for emerging markets is genuinely compelling right now. Countries like India, Brazil, South Korea, and Taiwan are growing faster, have younger populations, and trade at dramatically lower valuation multiples than the U.S. market.

As Lyn Alden of Lyn Alden Investment Strategy has noted, a meaningful allocation to emerging markets could provide a significant return tailwind over the next decade relative to an all-U.S. or all-developed-market portfolio. Funds like Vanguard’s VWO offer broad emerging market exposure at very low cost.

The caveats are real: emerging markets carry currency risk, political risk, and governance risk that developed markets do not. Sizing matters. A 10-20% allocation is very different from going all-in.

The Case for Some Cash and Short-Term Bonds

When the earnings yield on stocks is 2.6% and Treasury bills are yielding 4.5%, holding some portion of your portfolio in short-term, high-quality bonds is not being a coward. It is being rational.

The SPDR Bloomberg 1-3 Month T-Bill ETF (BIL) and iShares 1-3 Year Treasury Bond ETF (SHY) offer liquid, low-risk yield while you wait for better equity entry points. This is not a prediction that the market will crash. It is an acknowledgement that the risk-reward equation at current valuations is less favourable than it has been historically.

What You Should Actually Do With This Information

Here is where we stop being abstract and get practical. You are probably not a hedge fund manager. You likely cannot short the market or run complex factor-tilt portfolios with institutional-grade precision. But you are not powerless either.

Step 1: Adjust Your Return Expectations

This is the most actionable and underappreciated response to the current environment. At a Shiller PE of 40+, historical data suggest that long-run forward returns for U.S. equities are likely to be lower than their historical average of roughly 10% per year nominal.

Vanguard’s own research and GMO’s 7-year asset class forecasts have consistently projected lower-than-historical U.S. equity returns over forward windows from high starting valuations. Plan accordingly. If you are relying on 10% annual returns to hit your retirement number, model out what 6% or 7% does to your timeline.

Step 2: Stress-Test Your Diversification

Look inside your index funds. If 90% of your equity exposure is in U.S. large-cap stocks through an S&P 500 fund, you are more concentrated than you might think. A genuine diversification audit might lead you toward:

• Adding international developed market exposure (VEA)
• Tilting toward small-cap value (VBR) for factor diversification
• Including some real estate investment trusts (REITs via VNQ) for inflation hedging
• Considering a small allocation to gold or commodity ETFs as a tail-risk hedge

Step 3: Do Not Try to Time the Market

This is where we have to be brutally honest. Even if everything Burry and Green are saying is correct, timing the exact peak of a bubble is essentially impossible. Burry himself was early on the housing trade by years and nearly got wiped out before being proven right.

The market can stay “overvalued” by conventional measures for far longer than any individual investor can remain solvent betting against it. Keynes understood this more than a century ago.

What this means practically: do not liquidate your index funds in a panic. Do not try to pick the exact moment to rotate out of equities. Instead, rebalance gradually. Adjust your asset allocation at the margin. Add to undervalued asset classes over time. Let your regular contributions maintain discipline.

Step 4: Revisit Your Risk Tolerance Honestly

Most investors discover their actual risk tolerance during a crisis, not before one. If the S&P 500 dropped 40% tomorrow, what would you actually do? Honest answer: most retail investors would panic and sell, locking in losses at exactly the wrong moment.

At current valuations, a 40% drawdown is not a wild scenario. It would bring the S&P 500 back to roughly its historical average valuation. Building a portfolio you can psychologically hold through that scenario is more important than optimising for maximum theoretical return.

The Role of Fees, Taxes, and Time Horizons

Even in a bubble-adjacent environment, the core advantages of index funds do not disappear. They get recontextualised.

The fee advantage remains massive. The average Vanguard Total Market ETF (VTI) charges 0.03% per year. The average actively managed fund charges around 0.60-1.00% or more. That difference compounds dramatically over decades. A 1% annual fee difference on a $500,000 portfolio over 20 years at 7% nominal returns costs roughly $200,000 in lost compounding. That is real money.

The tax efficiency of index funds also remains a structural advantage. Low turnover means fewer realised capital gains are distributed to shareholders annually. For investors in taxable accounts, this is significant.

Time horizon matters enormously in this discussion. A 25-year-old investing in index funds for the next 40 years faces a very different risk profile than a 62-year-old who is 3 years from retirement and 80% in S&P 500 index funds. The bubble concerns are far more pressing for the latter than the former. Most of the scary scenarios involve a prolonged period of low returns from today’s starting valuations. For a long-horizon investor, a decade of low returns is recoverable. For someone about to retire, it is potentially catastrophic.

A Note on Prediction Versus Preparation

Here is something the financial media consistently conflates: predicting a crash and preparing for elevated risk are two completely different activities.

You do not need to believe index funds will cause the next market crisis to believe that portfolios built entirely on today’s U.S. equity valuations carry meaningful downside risk. You do not need to think Michael Burry is right about everything to acknowledge that Shiller PE ratios at 40+ have historically preceded periods of poor equity returns.

Preparation is not prediction. It does not require certainty. It requires clear-eyed probability assessment and a willingness to accept slightly lower returns in normal scenarios in exchange for better protection in tail scenarios.

The investors who got through the dot-com crash and the 2008 crisis most intact were generally not the ones who predicted the crash. They were the ones who had built diversified, appropriately risk-sized portfolios before the crisis hit, and who had the psychological fortitude not to blow up their strategy when everyone around them was panicking.

Spend some time for your future. 

To deepen your understanding of today’s evolving financial landscape, we recommend exploring the following articles:

The $10 Billion Bet Gone Wrong: Inside the Most Spectacular Hedge Fund Collapses of Our Time 

Doom Spending Is Real — And It’s Why Your Savings Account Is Empty Despite a Good Salary 

The Quiet Panic: Why High Earners Are the Most Financially Anxious People in America 

Position Sizing Strategies Every Global Trader Needs to Master 

Explore these articles to get a grasp on the new changes in the financial world.

Disclaimer

The content of this article is for informational and educational purposes only. Nothing in this article constitutes financial, investment, legal, or tax advice. The author is not a licensed financial advisor, broker-dealer, or registered investment advisor. All investment decisions involve risk, including the possible loss of principal. Past performance is not indicative of future results. Readers should conduct their own due diligence and consult a qualified financial professional before making any investment decisions. References to specific securities, funds, or financial products are for illustrative purposes only and do not constitute endorsements or recommendations. Market conditions, valuations, and regulatory environments change continuously, and information in this article may become outdated.

References

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