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Jeremy Grantham says, 'This is the most expensive market in American history'

Jeremy Grantham says soaring valuations around AI makes the U.S. stock market historically expensive.

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Jeremy Grantham says, 'This is the most expensive market in American history'
Source: Investing

AI Frenzy Pushes U.S. Stock Market to 'Most Expensive' Levels in History, Warns Jeremy Grantham

Jeremy Grantham, the co-founder of Boston-based asset management firm GMO and a legendary investor renowned for identifying major market tops, has issued a stark warning regarding the current state of Wall Street. According to Grantham, the U.S. stock market has reached unprecedented valuation levels, making it "the most expensive market in American history." He attributes this record-breaking premium primarily to the speculative frenzy surrounding artificial intelligence (AI), which has artificially extended a broader market cycle that he believes was already primed for a correction.

Grantham’s analysis centers on the rapid escalation of equity prices, particularly among mega-cap technology firms. While indicators like the Shiller cyclically adjusted price-to-earnings (CAPE) ratio have soared to levels rivaled only by the 1929 pre-crash era and the height of the late-1990s dot-com bubble, the AI narrative has acted as a powerful accelerant. Grantham suggests that while AI represents a genuine, long-term technological shift, the immediate market pricing has far outpaced actual corporate utility and revenue generation, creating a fragile foundation for everyday investors.

Key Drivers of the Market's Historic Valuations:

  • The AI Multiplier: Speculative capital has concentrated heavily in a small group of tech giants, driving up index-weighted valuations.
  • Interrupted Correction: The enthusiasm for generative AI in early 2023 effectively halted a necessary market downturn that began in 2022.
  • Macroeconomic Divergence: Equity prices continue to rise despite persistent economic headwinds, including elevated interest rates and global geopolitical instability.

Historically, Grantham has been a staunch advocate of "mean reversion"—the financial theory that asset prices eventually return to their long-term historical averages. In his view, the sudden burst of enthusiasm for AI bypassed traditional market discipline. By shifting investor attention and capital toward a handful of dominant tech stocks, the market has temporarily ignored macroeconomic warning signs. Grantham warns that this delay will not prevent an eventual market correction, but may instead exacerbate the ultimate fallout when speculative expectations inevitably fail to align with real-world corporate earnings.

While optimistic Wall Street analysts argue that the AI boom represents a structural shift in corporate productivity that justifies these historic multiples, Grantham remains highly skeptical of a painless "soft landing." For long-term portfolio managers, the current environment demands extreme caution. Historically, periods of extreme overvaluation have been followed by prolonged periods of low or negative real returns. As the debate over market sustainability continues, Grantham’s cautionary outlook serves as a sobering reminder of the cyclical nature of global finance.