A metaphor attributed to John Maynard Keynes maintains that using monetary policy to fight a severe recession is like “pushing on a piece of string.” When the problem is inflation, pushing up interest rates (pulling on a string) is a pretty effective policy tool — ask anyone who lived through the Volcker recession of the early 1980s. But when rates are pushed down to stimulate economic activity the ‘push’ becomes less and less effective the closer to zero rates get.