Fiscal Sustainability in a New Keynesian Model

Most recent work deriving optimal monetary policy utilising New Neo-Classical Synthesis (NNCS) models abstract from the impact of monetary policy on the government`s finances, by assuming that any change in the government`s budget can be financed through lump sum taxes. In this paper, we assume that the government does not have access to such taxes to satisfy its intertemporal budget constraint in the face of shocks. We then consider optimal monetary and fiscal policies under discretion and commitment in the face of technology, preference and cost-push shocks. We confirm that the optimal precommitment policy implies a random walk in the steady-state level of debt. We also find that the time-inconsistency in the optimal precommitment policy is such that governments are tempted, given inflationary expectations, to utilise their monetary and fiscal instruments in the initial period to change the ultimate debt burden they need to service. We show that this temptation is only eliminated if following shocks, the new steady-state debt is equal to the original (efficient) debt level. This implies that under a discretionary policy the random walk result is overturned: debt will always be returned to this initial steady-state even although there is no explicit debt target in the government`s objective function. Analytically and in a series of numerical simulations we show which instrument is used to stabilise the debt depends crucially on the degree of nominal inertia and the size of the debt-stock. We also show that the welfare consequences of introducing debt are negligible for precommitment policy, but can be significant for discretionary policy.