Jeremy Grantham Calls U.S. Stock Market the Most Expensive in American History

Jeremy Grantham, the veteran investor and co-founder of asset manager GMO, has declared that the United States stock market has reached its most expensive valuation in American history, driven by what he describes as soaring valuations tied to artificial intelligence. The warning marks one of the starkest assessments yet from one of the investment world's most closely watched long-term bears.

The AI Premium at the Center of Grantham's Warning

Grantham's argument centers on the AI-driven enthusiasm that has propelled U.S. equity valuations to levels he views as historically unprecedented. In his assessment, the excitement surrounding artificial intelligence has pushed prices to a point that has no precedent in the long arc of American market history — a claim that, if accurate, places current conditions beyond even the peaks of the dot-com bubble and the period preceding the 2008 financial crisis.

The significance of the AI angle is that it gives this cycle a narrative justification that valuations alone do not supply. Markets tend to sustain elevated prices longest when investors believe a genuine technological transformation is underway, making the eventual correction harder to time and harder to trade against.

What It Means for Positioning

For portfolio managers, Grantham's framing carries a direct implication: the standard tools for gauging when a market is stretched — historical price-to-earnings comparisons, cyclically adjusted ratios, market-cap-to-GDP — are all, by his reading, now flashing red simultaneously. That is a different situation from a market that looks expensive on one measure but fair on another.

Grantham has a long record of identifying major market dislocations ahead of significant drawdowns, which lends his current assessment institutional weight even among managers who do not share his conviction. Ignoring a signal from a source with that track record requires a deliberate, documented thesis.

The Macro Context

The backdrop amplifies the warning. U.S. equities have been on a sustained run underpinned by AI-related capital expenditure expectations and a resilient earnings backdrop, even as interest rates have remained elevated relative to the prior decade. High valuations in a high-rate environment reduce the margin for error: the discount rate working against future cash flows is larger, meaning any disappointment in AI's commercial timeline would hit prices harder than it would in a low-rate world.

Grantham's assessment does not come with a timing call, and market history is littered with expensive markets that became more expensive before reversing. But the combination of a named, historically grounded valuation warning and a specific macro driver — AI enthusiasm — gives investors a concrete framework to stress-test their exposure.