Jeremy Grantham, a well-known fund manager, believes that the stock market could decline by 50% and advises investors to avoid U.S. stocks due to overvaluation. He recommends focusing on high-quality stocks, which have consistently outperformed the overall market for years. This strategy has been successful during market downturns, as high-quality stocks tend to perform well while weaker stocks struggle. While some experts question using company quality to predict stock performance, others believe quality strongly predicts returns. MSCI data shows quality stocks have consistently outperformed the market overall, despite periods of underperformance.