In his latest research report, Grantham focused on the extremely bullish sentiment of the U.S. stock market, underscored by the Shiller P/E Ratio — a key valuation measure for the S&P 500 — standing at 34 as of Mar. 1. This places it in the top 1% historically, a position Grantham views with caution due to the historical rarity of sustained market rallies from such a valuation.