This paper overviews recent studies on optimal monetary policy in a liquidity trap, starting from Krugman (1998). First, we point out that these studies focus on a phenomenon in which short-term, rather than long-term, nominal interest rates face the zero bound constraint, which is quite different from the original definition provided by Keynes. In this sense, recent studies focus on a temporary, rather than permanent, liquidity trap. Second, somewhat surprisingly, policy prescriptions to the temporary trap proposed by the recent studies are very close to the standard one in light of the modern theory of optimal monetary policy. Specifically, as shown by many studies, the optimal monetary policy rule in a liquidity trap could be expressed as versions of inflation or price-level targeting.
