"While the degree of policy inertia in central banks' reaction functions is a central ingredient in theoretical and empirical monetary economics, the source of the observed policy inertia in the U.S. is controversial, with tests of competing hypotheses such as interest-smoothing and persistent-shocks theories being inconclusive. This paper employs real time data; nested specifications with flexible time series structures; narratives; interest rate forecasts of the Fed, financial markets, and professional forecasters; and instrumental variables to discriminate competing explanations of policy inertia. The presented evidence strongly favors the interest-smoothing explanation and thus can help resolve a key puzzle in monetary economics"--National Bureau of Economic Research web site.