Robust Monetary Policy Rules with Unknown Natural Rates

The natural rate is an abstraction; like faith, it is seen by its works. One can only say that if the bank policy succeeds in stabilizing prices, the bank rate must have been brought in line with the natural rate, but if it does not, it must not have been.1 THE CONVENTIONAL PARADIGM for the conduct of monetary policy calls for the monetary authority to attain its objectives of a low and stable rate of inflation and full employment by adjusting its short-term interest rate instrument—in the United States, the federal funds rate—in response to economic developments. In principle, when aggregate demand and employment fall short of the economy’s natural levels of output and employment, or when other deflationary concerns appear on the horizon, the central bank should ease monetary policy by bringing real interest rates below the economy’s natural rate of interest for some time. Con-versely, the central bank should respond to inflationary concerns by adjusting interest rates upward so as to bring real interest rates above th