For most of the inflation-targeting era, macroeconomic models treated fiscal policy as a background condition. The central bank set the short-term rate according to a Taylor-type rule, and the government was assumed to adjust taxes or spending, eventually and quietly, so that public debt remained sustainable. That assumption, usually relegated to a footnote about Ricardian fiscal behavior, did an enormous amount of work. It is what allowed monetary policy alone to pin down the price level.

Remove the assumption and the analysis changes character. The intertemporal government budget constraint links the real value of outstanding nominal debt to the expected present value of future primary surpluses. If surpluses do not respond to debt, something else must adjust: the price level, the path of real interest rates, or the monetary authority's willingness to accommodate. Inflation becomes a joint product of two institutions, only one of which is formally charged with controlling it. Neither can fully disclaim responsibility for the result.

This article examines that joint production through three lenses. First, the strategic structure of interaction between independent authorities with distinct objectives and commitment technologies. Second, Eric Leeper's active/passive taxonomy, which classifies policy combinations by which authority anchors expectations and which adjusts. Third, the coordination failures that emerge when implicit commitments are mutually inconsistent: nonexistence of equilibrium, indeterminacy, and regime uncertainty. The common thread is that policy outcomes are properties of regimes, not of individual instruments, and must be analyzed accordingly.

The modern framing traces to Sargent and Wallace's Some Unpleasant Monetarist Arithmetic (1981), which posed the question as one of sequencing. If the fiscal authority commits first to a path of deficits, and the monetary authority must ultimately ensure that debt is financed, then tight money today merely postpones inflation. Rising debt accumulates interest obligations that future seigniorage must cover, so restraint now can imply higher inflation later. The identity of the leader determines the outcome, not the stance of either authority taken in isolation.

Subsequent work formalized this as a strategic game. Each authority minimizes its own loss function: the central bank weights inflation and output-gap stabilization, while the fiscal authority weights output, the provision of public goods, tax-smoothing distortions, and often electoral considerations. Under simultaneous Nash play, outcomes are typically inefficient. Dixit and Lambertini showed that divergent targets between the authorities can produce worse inflation and output outcomes than either would choose under cooperation, and that fiscal leadership can undermine the benefits of monetary commitment.

The interaction is frequently characterized as a game of chicken over debt sustainability. Each authority prefers that the other bear the adjustment: the fiscal authority would rather the central bank tolerate higher inflation, while the central bank would rather the legislature raise surpluses. Resolution depends on relative credibility. Central bank independence functions as a commitment device, but only on one margin. It constrains the monetary authority's reaction function; it does not compel fiscal adjustment, and it cannot by itself determine who ultimately yields.

In dynamic stochastic general equilibrium settings, the strategic dimension operates primarily through expectations. Forward-looking households and investors form beliefs about the end-game, about which authority will adjust when debt dynamics become binding. Those beliefs feed back into current pricing decisions, long-term yields, and inflation expectations. The equilibrium is therefore selected less by current policy actions than by perceived commitments regarding future behavior. Time-consistency problems compound this: a discretionary government holding nominal debt faces persistent incentives to erode its real value through surprise inflation.

The policy implication is that institutional design must address both commitment problems simultaneously. A credible central bank paired with a fiscal authority lacking credible rules does not deliver monetary dominance by default; it delivers an unresolved game whose resolution markets must forecast. Fiscal councils, expenditure rules, and debt-management frameworks matter for price stability precisely because they shape the fiscal side of that game. Independence, in this light, is a statement about decision rights, not a guarantee that monetary policy is insulated from fiscal choices.

Takeaway

Central bank independence settles who controls the policy rate, not who adjusts when debt becomes binding. The price level ultimately reflects the market's forecast of how that unresolved question gets answered.

Leeper's 1991 contribution provided a parsimonious language for these interactions. An active authority pursues its objectives unconstrained by the current state of government debt. A passive authority is constrained by private-sector optimization and by the actions of the active authority, adjusting its instrument to satisfy equilibrium conditions. The taxonomy shifts attention from the level of instruments to the structure of feedback rules, specifically how interest rates respond to inflation and how primary surpluses respond to outstanding debt.

Active monetary policy satisfies the Taylor principle: the nominal rate responds more than one-for-one to inflation, so real rates rise when inflation rises. Passive fiscal policy means surpluses respond to debt strongly enough to stabilize it, specifically by more than the real interest rate in the linearized debt dynamics. This combination, often labeled Regime M, is the configuration implicit in standard New Keynesian analysis following Woodford. Monetary policy determines inflation, and fiscal policy accommodates whatever debt service the chosen interest-rate path implies.

The mirror image, Regime F, pairs passive monetary policy with active fiscal policy. Surpluses evolve independently of debt, and the central bank responds weakly to inflation or pegs the nominal rate. Here the price level adjusts to equate the real value of nominal debt with the present value of expected surpluses. This is the logic of the fiscal theory of the price level developed by Sims, Woodford, and Cochrane. A deficit-financed tax cut becomes inflationary without any change in money, because households rationally perceive no future fiscal backing.

Regime F inverts several standard intuitions. Higher policy rates raise nominal interest payments on debt that is not offset by future surpluses, which can increase rather than reduce inflation over the medium run. With long-maturity debt, Cochrane has shown that a rate increase may lower inflation temporarily while raising it later, as bond prices absorb part of the fiscal adjustment. The sign of the monetary transmission mechanism, in other words, is regime-dependent. Identical rate decisions carry different implications depending on the fiscal rule in force.

A central result is that determinacy in this class of models generally requires exactly one active authority. Empirical work suggests that the prevailing regime has not been constant. Davig and Leeper estimated Markov-switching policy rules for the United States and found recurrent shifts between configurations. Bianchi and Ilut attributed much of the 1970s inflation to a fiscally led regime and argued that the subsequent disinflation required not only a monetary shift but fiscal adjustment that rendered the monetary-led regime credible. Regimes, in practice, are estimated rather than declared.

Takeaway

Ask not whether monetary policy is tight or loose, but which authority is active. The same interest-rate decision can stabilize inflation in one regime and fuel it in another.

The two remaining configurations in Leeper's taxonomy illustrate distinct coordination failures. When both authorities are active, each insists on its own objective and neither stabilizes debt. A central bank raising rates aggressively against inflation increases debt service; a fiscal authority refusing to adjust surpluses allows debt to grow. No stationary equilibrium exists. Debt or inflation follows an explosive path until one authority abandons its rule. The configuration is unsustainable by construction, and its resolution typically arrives abruptly rather than through gradual convergence.

When both authorities are passive, the opposite problem arises: too many equilibria rather than none. The central bank accommodates inflation, the fiscal authority accommodates debt, and nothing anchors the nominal side of the economy. The model admits a continuum of price-level paths consistent with equilibrium, including those driven by sunspots, extrinsic shifts in beliefs unrelated to fundamentals. Volatility in this configuration is not caused by shocks to preferences or technology; it emerges from the absence of any institution willing to pin down expectations.

Regime uncertainty introduces a subtler failure. Even when the current configuration is active monetary and passive fiscal, agents may assign positive probability to a future switch. Davig and Leeper showed that expectations of possible fiscal dominance can generate fiscal effects on inflation within an ostensibly monetary-led regime. Bianchi and Melosi extended this logic, modeling inflation outcomes as dependent on whether the fiscal authority is expected to back new debt. Policy credibility thus depends on beliefs about regimes not yet in place.

Fiscal limits sharpen these dynamics. As debt rises, the political and economic capacity to raise surpluses may approach a ceiling. Work by Bi and others treats this limit as stochastic and state-dependent. Near the limit, fiscal policy is effectively forced into an active stance, and the monetary authority must choose between accommodation and permitting default risk premia to emerge. Similar logic applies at the effective lower bound, where monetary policy is constrained to passivity and determinacy may require fiscal policy to take on the anchoring role.

These failures share a common origin in incomplete or inconsistent commitments. Each authority communicates its reaction function as though the other's behavior were fixed and favorable. Central bank forward guidance often assumes, without stating, fiscal backing for the implied interest-rate path; fiscal projections often assume, without stating, monetary accommodation of the implied debt path. When those implicit assumptions are incompatible, markets eventually force reconciliation. The task of policy design is to make that reconciliation explicit and orderly before it is imposed under stress.

Takeaway

Too much activism produces no equilibrium; too much accommodation produces too many. Stability depends on a clear and credible division of labor between the authorities, not on the virtue of either one alone.

The monetary-fiscal literature reframes a question often posed narrowly. Inflation is not determined by the central bank's reaction function alone, but by the joint configuration of monetary and fiscal rules and by the private sector's beliefs about how that configuration will evolve when debt dynamics bind.

For policy design, three implications follow. Central bank communication should acknowledge the fiscal assumptions embedded in its projections, rather than treating them as exogenous. Fiscal frameworks should be evaluated for their contribution to nominal anchoring, not only for debt sustainability. And models used for policy analysis should incorporate regime uncertainty, since credibility depends on expected future regimes, not merely the present one.

Woodford's framework demonstrated how much monetary policy can achieve under passive fiscal backing. Leeper's taxonomy demonstrated how conditional that achievement is. The research frontier lies in treating that condition as an object of analysis, institutional design, and explicit communication rather than an assumption.