S&P 500 Shiller CAPE Tops 40 as AI Valuation Risks Mount

The Shiller cyclically adjusted price-to-earnings ratio, or CAPE, compares the S&P 500 with a 10-year average of inflation-adjusted earnings, smoothing out the business cycle. Its rise reflects investor confidence that artificial intelligence will deliver exceptional productivity and profit growth. The gauge matters because extreme starting valuations have historically constrained long-term returns, leaving the index vulnerable if heavy data-center spending fails to generate earnings quickly enough.

YCharts data cited on July 20, 2026 showed the CAPE nearing 42 after holding above 40 since May, a level sustained only once before, from January 1999 through September 2000. The dot-com-era measure peaked near 44. Microsoft, Alphabet, Amazon and Meta have committed a combined $710 billion to AI infrastructure in 2026, raising concern that returns may lag spending. Analysts say a reset would favor companies with sound balance sheets, credible paths to profitability and durable competitive advantages.