Famed Investor Jeremy Grantham Warns S&P 500 Could Drop Another 26 Percent by 2023

Jeremy Grantham, founder of GMO, explores different perspectives on resource volatility, security, and planning at the ReSource 2012 conference in Oxford, England, on July 12, 2012. Matthew Lloyd/Getty Images for ReSource 2012

Veteran investor co-founder of Boston asset manager GMO, Jeremy Grantham, predicted that the S&P 500 could plummet by another 26 percent by 2023, while speaking at the Reuters Global Markets Forum on Sept. 8.

His comments following months of turbulence on Wall Street this year amid a series of challenges and signs that the United States is already in a silent recession.