The question of whether the Federal Reserve should adjust interest rates to deflate risky financial market bubbles split some of its top policymakers on Friday, suggesting the controversial idea is re-emerging as the U.S. central bank approaches an historic policy tightening. Giving the central bank an effective third mandate – beyond its formal objectives for inflation and employment – has won more adherents since the 2007-2009 financial crisis, which some blame in part on too-easy monetary policy in the preceding years that allowed risks to take root.