S&P 500 CAPE hits historic high as AI fuels 23% rally

The S&P 500’s Shiller CAPE ratio has risen above its 17‑year average to a level seen only twice before in 155 years of data, coinciding with a 23% year‑to‑date gain driven by AI‑related stocks【2】.

AI boom lifts the market but valuation warnings remain

The index’s 13.4% annualized total‑return over the past decade, amounting to roughly a 250% gain, has been turbo‑charged this year by AI enthusiasm, pushing the S&P 500 up close to 23%【2】. That surge has lifted the CAPE ratio well above its long‑run mean of 17, a reading that historically precedes only two other periods of extreme overvaluation: the late 1920s before the Great Depression and the late 1990s before the dot‑com crash【2】.

Analysts note that despite the lofty valuation, market breadth is at its narrowest since 2000, a metric that tracks the number of advancing versus declining stocks【1】. Goldman Sachs warns the breadth is low but still “doesn’t yet rival the depths of the pre‑crisis era,” suggesting that while the rally is broad, it may lack the depth that preceded past crashes【1】.

Divergence between “new” and “old” era stocks tempers risk

Former Leuthold Group strategist Jim Paulsen argues that the split between “new” (technology) and “old” (the other nine S&P sectors) stocks is more pronounced than in the 1990s, reducing overall market risk. He points out that “old era” stocks now provide a larger stabilizing force, encouraging broader diversification than during the dot‑com era【1】. This lower correlation between sectors, he says, could blunt the impact of a potential downturn in AI‑heavy names, though he acknowledges the risk remains “substantially mitigated” only if the “music finally stops for new era stocks”【1】.

What to watch

• CAPE trajectory – A move back toward the 17 average would signal easing valuation pressure; a further rise could heighten crash risk.
• Market breadth data – Upcoming weekly breadth reports (usually released on Fridays) will show whether the narrowness deepens.
• AI earnings season – Quarterly results from leading AI‑focused firms (e.g., Nvidia, Microsoft) will test whether earnings growth can justify the elevated multiples.

The current combination of an AI‑driven rally and a historically high CAPE ratio mirrors the conditions that preceded past market collapses, yet the stronger role of “old era” stocks adds a layer of uncertainty. Whether the market can sustain its lofty valuation without a sharp correction remains an open question.