Wall Street's Biggest Bubble May Be Popping (No, Not AI), and It Has Dire Consequences for the Stock Market

Despite short-lived volatility tied to the Iran war in March, 2026 is shaping up to be another banner year for the stock market. Since early June, the iconic Dow Jones Industrial Average (DJINDICES:^DJI), benchmark S&P 500 (SNPINDEX:^GSPC), and technology-powered Nasdaq Composite (NASDAQINDEX:^IXIC) have blasted to respective all-time highs.

There's little question that the rise of artificial intelligence (AI) has been Wall Street's No. 1 catalyst. Otherworldly spending on the AI infrastructure build-out has increased corporate growth rates and expanded stock valuations to levels last seen in the months leading up to the bursting of the dot-com bubble.

While evidence is mounting that an AI bubble may be brewing, this, arguably, isn't Wall Street's biggest bubble. Something far more sinister lurks in the shadows and, based on historical precedent, the bursting of this bubble may already have begun. If history were to repeat, the consequences for the stock market would be dire.

Image source: Getty Images.

The stock market's risk-taking bubble may be popping

Headwinds are always present for the stock market. Whether it's above-average inflation, weak job growth, or historically pricey valuations, something is always threatening to pull the rug out from beneath investors.

However, no warning over the last three decades has been more prescient or worrisome than outstanding margin debt.

Margin is the money an investor borrows from their broker, with interest, to short-sell (wager against) or purchase securities. When used to buy stocks or exchange-traded funds (ETFs), margin acts as a form of leverage and can be used as a crude gauge of investors' willingness to take risks.

Over several decades, it's perfectly normal for outstanding margin debt to steadily rise in lockstep with the overall value of public companies. But things tend to go awry when margin debt goes parabolic over a relatively short time frame (i.e., when investors' willingness to take risks increases dramatically).

In June 2026, outstanding margin debt, published monthly by FINRA, jumped to an all-time high of $1.502 trillion. What's worth noting is that margin debt spiked 77%, from approximately $850.6 billion in April 2025 to $1.502 trillion in June 2026, over 14 months.

Total Margin Debt hits $1.5 Trillion, a new all-time high 🤯 👀 pic.twitter.com/1IXqZGgrqs

— Barchart (@Barchart) July 20, 2026

There have only been four relatively short periods over the last three decades in which outstanding margin debt has spiked by at least 65%:

• March 1999 to March 2000: In the 12 months leading up to the official bursting of the dot-com bubble, outstanding margin debt soared 80% to just shy of $300 billion. In the wake of this bubble-bursting event, the S&P 500 and Nasdaq Composite lost 49% and 78% of their values, respectively.
• June 2006 to July 2007: Mere months before the financial crisis really took hold, margin debt surged by 66% to approximately $416 billion. The Great Recession took an even greater toll on the S&P 500 than the dot-com bubble did, with this iconic index shedding 57% of its value.
• March 2020 to October 2021: Following the height of the short-lived COVID-19 crash, and amid several rounds of fiscal stimulus, outstanding margin debt exploded by 95%. It peaked just three months before the 2022 bear market took shape, which lopped 25% and 33% off the S&P 500 and Nasdaq Composite, respectively.
• April 2025 to June 2026: Outstanding margin debt peaked at a 77% increase over 14 months.

In July, FINRA reported that outstanding margin debt fell to $1.417 trillion, meaning it's risen by 67% over the last 15 months. When outsize risk-taking begins to wane on Wall Street, history tells us it never happens quietly. Every instance when outsize risk-taking reversed (vis-à-vis margin debt) was almost immediately followed by a significant reversal in equities.

While a one-month retracement in July doesn't make a trend -- outstanding margin debt briefly shrank for two months in February-March 2026 -- parabolic moves in margin debt that eventually reverse have consistently foreshadowed bear markets on Wall Street.

We may very well be witnessing Wall Street's biggest bubble, outstanding margin debt, popping.

Image source: Getty Images.

Bubble-bursting events offer a silver lining for optimistic, long-term investors

Although history can't guarantee short-term directional moves on Wall Street, it has an even better track record of forecasting long-term trends.

On the one hand, we've just seen that outstanding margin debt has, for three decades, accurately foreshadowed significant downside in equities. Something similar has been observed with the S&P 500's Shiller Price-to-Earnings Ratio, which has a perfect track record of forecasting significant stock market declines when backtested over nearly 156 years.

But history is a two-way street, and bull and bear markets aren't mirror images of one another.

In late May, the analysts at Bespoke Investment Group published a data set on X (formerly Twitter) that compared the calendar-day length of every S&P 500 bull and bear market since the start of the Great Depression (September 1929). It demonstrated what a night-and-day difference optimism and pessimism yield for investors.

The current bull market that began on 10/12/22 is now the 9th longest in S&P 500 history, surpassing the 1,324-day bull that ended on 2/9/1966: pic.twitter.com/4mGsS2t2ft

— Bespoke (@bespokeinvest) May 30, 2026

On the one hand, the average of 27 bear markets found its trough after 286 calendar days, or roughly 9.5 months. Furthermore, no 20% or greater decline in Wall Street's benchmark index has lasted longer than 630 calendar days.

In comparison, the typical S&P 500 bull market has lasted 1,023 calendar days as of late May 2026, which is roughly 3.6 times longer than the average bear market. Additionally, more than half (14) of all S&P 500 bull markets have persisted longer than the lengthiest bear market.

The point is that downturns create opportunities for long-term-minded optimists to pounce. Even though it's impossible to know ahead of time precisely when a downturn will begin, how long it'll last, and how much the Dow Jones Industrial Average, S&P 500, and Nasdaq Composite will drop, more than a century of historical data shows that Wall Street's major indexes (and top companies) increase in value over time.

Sean Williams has no position in any of the stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. The Motley Fool has a disclosure policy.