Robust monetary rules under unstructured model uncertainty

This paper revisits a widely adopted approach to robust decision making developed by Hansen and Sargent (2003, 2008)—henceforth HS—and applies it to monetary policy design in the face of model uncertainty. We pay particular attention to two issues: first, we distinguish three possible forms of the implied game between malign nature and the policymaker in the HS procedure each leading to a different robust and approximating equilibria. Second, we impose the zero lower bound (ZLB) constraint on the nominal interest rate. We show that the ZLB constraint has serious consequences for a policymaker pursuing HS-type robustness, especially when accompanied by an inability to commit.