As investor enthusiasm continues to surge—and related concerns about overenthusiasm in financial markets grow—an indicator devised by the renowned investor Warren Buffett is now flashing red and signaling potential trouble ahead for the U.S. economy. The “Buffett Indicator,” proposed by and named after the recently retired Berkshire Hathaway CEO, divides the total value of publicly traded U.S. equities by the country’s gross domestic product (GDP) to assess whether the stock market is