Janet Yellen, chair of the U.S. Federal Reserve, center, stands in an elevator after a Senate Banking Committee hearing in Washington, D.C., U.S., on Thursday, Feb. 11, 2016. Yellen said the Fed was taking another look at negative interest rates as a potential policy tool if the U.S. economy faltered, a scenario some investors view as a mounting possibility amid a darkening outlook for world growth. Photographer: Andrew Harrer/Bloomberg via Getty Images
