Economist Paul Krugman gives credit for the compression not only to progressive income taxation but to other New Deal and World War II policies of President Franklin D. Roosevelt. From about 1937 to 1947 highly progressive taxation, the strengthening of unions of the New Deal, and the wage and price controls of the National War Labor Board during World War II, raised the income of the poor and working class and lowered that of top earners. Krugman argues these explanations are more convincing than the conventional Kuznets curve cycle of inequality driven by market forces because a natural change would have been gradual and not sudden as the compression was.[6]