Key Points
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The evolution of artificial intelligence (AI) and better-than-expected earnings have lifted the Dow Jones Industrial Average, S&P 500, and Nasdaq Composite to new heights.
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Stock valuations are officially in bubble territory, based on what history tells us.
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Furthermore, outsize risk-taking by investors points to potential disaster for the stock market.
For the better part of the last four years, Wall Street can do no wrong. Since early June, the Dow Jones Industrial Average(DJINDICES: ^DJI), S&P 500(SNPINDEX: ^GSPC), and Nasdaq Composite(NASDAQINDEX: ^IXIC) have rocketed to fresh all-time highs.
Although catalysts have been bountiful, with the evolution of artificial intelligence (AI) and better-than-expected corporate earnings each playing a key role, historical headwinds are mounting for the high-flying stock market. While history conclusively shows that optimism and long-term investors are handsomely rewarded — the S&P 500 has never failed to rise over any rolling 20-year period, including dividends — the argument can be made that the stock market is less attractive now than it’s ever been.
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Image source: Getty Images.
Stock valuations have reached dot-com bubble territory
Even though history can’t guarantee what’s to come, past events have an uncanny ability to predict the future on Wall Street. Based solely on historical precedent, stock valuations are sending all the wrong signals to investors.
The S&P 500’s Shiller Price-to-Earnings (P/E) Ratio, also known as the Cyclically Adjusted P/E Ratio (CAPE Ratio), is based on average inflation-adjusted earnings over the last decade. When backtested to January 1871, the Shiller P/E Ratio has averaged 17.4. As of Aug. 21, the CAPE Ratio was almost 42, which is a stone’s throw from the dot-com bubble peak of 44.19 in December 1999.
Stock Market Shiller PE Ratio on the verge of taking out its Dot Com Bubble all-time high 🚨 🤯 👀 pic.twitter.com/CtCmSgWnLt
— Barchart (@Barchart) July 11, 2026
The S&P 500’s Shiller P/E has surpassed 30 only six times over nearly 156 years, including the present. The previous five occurrences all ended in disaster for the stock market, with the Dow, S&P 500, and/or Nasdaq Composite losing between 20% and 89% of their respective values.
Outstanding margin debt has gone parabolic
Rapidly rising margin debt is another blaring historical red flag for the stock market.
Margin represents money that investors borrow from their broker, with interest, to short-sell (wager against) or purchase securities. When used to buy stocks or exchange-traded funds, margin acts as leverage. In essence, higher margin use equates to greater risk-taking by investors.
Total Margin Debt hits $1.5 Trillion, a new all-time high 🤯 👀 pic.twitter.com/1IXqZGgrqs
— Barchart (@Barchart) July 20, 2026
Although outstanding margin debt is expected to steadily climb over the long term, parabolic increases in margin debt are never a good sign for Wall Street. Between April 2025 and June 2026, outstanding margin debt surged 77% to a record $1.502 trillion, according to FINRA.
Over the last three decades, margin debt has skyrocketed by at least 65% over a short period on four occasions, including the present. The prior three instances were either immediately or shortly followed by the bursting of the dot-com bubble, the financial crisis, and the 2022 bear market.
Image source: Getty Images.
Game-changing technologies and bubbles go hand in hand
Lastly, history tells us that stock market rallies driven by game-changing technologies invite trouble.
Every next-big-thing innovation since (and including) the advent of the internet has undergone a bubble-bursting event. These bubbles form because investors persistently overestimate the pace of adoption and optimization of new technologies.
For example, businesses didn’t optimize their internet solutions until well after the dot-com bubble had burst. While demand for AI infrastructure is off the charts, proving that adoption isn’t a concern, we’re likely several years away from businesses optimizing AI solutions to bolster sales and profits. When otherworldly investor expectations aren’t met, bubbles burst.
Historically speaking, this may be the least attractive the stock market has ever been.
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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.
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