Recent studies argue that the spread-adjusted Taylor rule (STR), which includes a response to the credit spread, replicates monetary policy in the United State. We show (1) STR is a theoretically optimal monetary policy under heterogeneous loan interest rate contracts in both discretionay and commitment monetary policies, (2) however, the optimal response to the credit spread is ambiguous given
... [Show full abstract] the financial market structure in theoretically derived STR, and (3) there, a commitment policy is effective in narrowing the credit spread when the central bank hits the zero lower bound constraint of the policy rate.