Jeremy Grantham Warns U.S. Stock Market Is at Record Valuation Amid AI Boom

Veteran investor Jeremy Grantham, co-founder of GMO, told CNBC’s “Squawk Box” that the U.S. equity market’s total market-capitalization-to-GDP ratio now stands at 235 %, a level he described as “the most expensive market in American history.” The ratio, tracked by Longtermtrends.com, indicates that the combined value of all U.S. listed equities exceeds twice the size of the nation’s gross domestic product.

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The metric Grantham cites is commonly known as the Buffett Indicator, popularized by Warren Buffett. Buffett warned that when the ratio “approaches 200 % — as it did in 1999 and a part of 2000 — you are playing with fire.” The 2000 dot-com bubble peaked near that threshold, and Grantham notes that the current 235 % reading surpasses that historic high.

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Grantham linked the valuation surge to the artificial-intelligence boom, arguing that speculative expectations are inflating prices beyond fundamentals. He highlighted SpaceX, estimating that roughly 90 % of its valuation rests on AI assumptions, which he labeled an “AI bubble within a bubble.” Grantham also suggested that the market could experience a crash “like Amazon,” calling the recent IPO “one of the defining peaks of all time.” While he stressed that the timing of any correction is “terribly uncertain,” he warned that the long-run outlook for the broad U.S. market “looks as poor as almost any other time in history.”

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• Market-cap-to-GDP ratio: 235 % (Longtermtrends.com)

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• Buffett’s fire-threshold: 200 % (1999-2000)

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• SpaceX AI exposure: ~90 % of valuation (Grantham)

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• Previous warning: March 2024, Grantham warned the market’s long-run prospects were “as poor as almost any other time in history,” yet equities continued to rise afterward.

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The combination of a record-high valuation metric and AI-driven hype raises concerns of a systemic correction. Grantham’s analogy to the dot-com era suggests that investors could be “playing with fire” if price growth outpaces economic fundamentals. A sharp decline could affect not only technology stocks but also broader market indices that are now heavily weighted toward AI-related firms.

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Grantham’s public remarks emphasize caution: he frames the current valuation as a warning signal rather than a definitive prediction, acknowledging uncertainty about when a downturn might occur. Buffett’s earlier commentary on the indicator provides a historical benchmark, reinforcing the view that sustained ratios above 200 % have preceded market stress. Grantham’s March 2024 warning, which preceded continued market gains, illustrates the difficulty of timing corrections even when valuation metrics appear extreme.

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Grantham admits he is “not sure there was a comparable period,” indicating a gap in historical analogues for the present AI-driven surge. No alternative valuation models or dissenting expert opinions appear in the supplied sources, leaving the assessment of risk largely dependent on the Buffett Indicator and Grantham’s qualitative judgment.

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• “Based on the value of the stock market compared to GDP, with modifications, this is the most expensive market in American history,” — Jeremy Grantham, GMO

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• “The long-run prospects for the broad U.S. stock market here look as poor as almost any other time in history,” — Jeremy Grantham, GMO

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• “approaches 200% — as it did in 1999 and a part of 2000 — you are playing with fire.” — Warren Buffett

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• “Is it going to have a crash like Amazon? Yes, very likely,” — Jeremy Grantham, GMO

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Grantham suggests that market participants should monitor the Buffett Indicator closely and remain prepared for a correction, though he offers no specific timeline. Analysts and investors are likely to watch upcoming earnings reports and AI-related IPO activity for signs that valuation pressures are intensifying or easing.

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