The short run long run Phillips curve describes how inflation and unemployment relate in the immediate period versus after expectations have fully adjusted, offering a cornerstone framework for understanding monetary policy trade‑offs and the limits of demand‑side stimulus.
Introduction
Economists use the Phillips curve to trace the inverse relationship between inflation and unemployment. In the long run, however, the curve becomes vertical at the natural rate of unemployment, indicating that any attempt to push unemployment below its structural level only generates accelerating inflation without lasting gains in employment. In practice, in the short run, policymakers can exploit this trade‑off by expanding aggregate demand to lower unemployment at the cost of higher inflation. Grasping the distinction between the short run and long run Phillips curve is essential for interpreting historical episodes such as the 1970s stagflation and for designing credible monetary strategies today Less friction, more output..
The Phillips Curve: Basic Idea
The original Phillips curve, observed by A. Consider this: w. Day to day, phillips in 1958, plotted wage inflation against unemployment in the United Kingdom and suggested a stable, downward‑sloping line. Later adaptations replaced wage inflation with price inflation, yielding the familiar inflation‑unemployment trade‑off.
- Demand‑pull pressure – when aggregate demand rises, firms hire more workers, reducing unemployment and pushing up wages and prices.
- Cost‑push pressure – when unemployment falls below a certain threshold, bargaining power shifts to workers, leading to higher wage demands that feed into prices.
These forces generate a negative slope in the short run, but they do not explain why the relationship might disappear over time.
Short‑Run Phillips Curve
Derivation and Shape
The short‑run Phillips curve (SRPC) can be expressed as
[ \pi_t = \pi_t^e - \beta (u_t - u_n) + \nu_t, ]
where (\pi_t) is actual inflation, (\pi_t^e) is expected inflation, (u_t) is the observed unemployment rate, (u_n) is the natural rate of unemployment, (\beta > 0) measures the sensitivity of inflation to unemployment gaps, and (\nu_t) captures supply shocks. The curve slopes downward because a lower unemployment gap ((u_t < u_n)) raises actual inflation when expectations are held constant.
Role of Expectations
Expectations are the linchpin that separates short‑run from long‑run dynamics. If workers and firms form adaptive expectations, they base (\pi_t^e) on past inflation ((\pi_{t-1})), creating a lagged response. A surprise expansionary policy can thus lower unemployment temporarily, as actual inflation exceeds expected inflation Practical, not theoretical..
If expectations are rational, agents incorporate all available information, including the likely policy response. In that case, only unanticipated shocks move the economy off the SRPC; anticipated policy changes are immediately reflected in (\pi_t^e), leaving unemployment unchanged.
Policy Implications
In the short run, a central bank can achieve a temporary reduction in unemployment by tolerating higher inflation. Worth adding: the magnitude of the gain depends on the slope (\beta) and the credibility of the policy announcement. On the flip side, the SRPC is not a permanent menu of options; repeated use of demand stimulus shifts expected inflation upward, flattening the curve’s effectiveness over time Small thing, real impact..
Long‑Run Phillips Curve
Concept of NAIRU
The non‑accelerating inflation rate of unemployment (NAIRU), denoted (u_n), is the unemployment rate at which inflation neither rises nor falls. When the economy operates at (u_n), actual inflation equals expected inflation ((\pi_t = \pi_t^e)), and the SRPC collapses to a point.
Vertical Long‑Run Curve
In the long run, expectations fully adjust so that (\pi_t^e = \pi_t). Substituting this equality into the SRPC equation yields
[ \pi_t = \pi_t - \beta (u_t - u_n) + \nu_t ;\Rightarrow; u_t = u_n - \frac{\nu_t}{\beta}. ]
Ignoring supply shocks ((\nu_t = 0)), the long‑run Phillips curve (LRPC) is a vertical line at (u = u_n). This implies that monetary policy cannot sustainably lower unemployment below its natural rate; any attempt merely generates ever‑higher inflation.
Expectations Adjustment
The transition from SRPC to LRPC occurs as agents revise (\pi_t^e) to match actual inflation. So under adaptive expectations, this adjustment is gradual, producing a period of inflation inertia. Under rational expectations, the adjustment is instantaneous, making the LRPC the relevant benchmark for policy analysis almost immediately after a shock.
Transition from Short‑Run to Long‑Run
Adaptive vs Rational Expectations
- Adaptive expectations: (\pi_t^e = \lambda \pi_{t-1} + (1-\lambda) \pi_{t-1}^e). Inflation expectations lag behind actual inflation, allowing policymakers to exploit the SRPC for several periods before the curve shifts upward.
- Rational expectations: (\pi_t^e = E[\pi_t | \mathcal{I}_t]), where (\mathcal{I}_t) is the information set. Anticipated policy changes are instantly incorporated, nullifying the short‑run trade‑off.
The 1970s provided a natural experiment. Oil price shocks raised (\nu_t), shifting the SRPC upward. Simultaneously, inflation expectations adapted slowly, causing unemployment to rise alongside inflation—a phenomenon termed stagflation. The episode highlighted the limitations of relying solely on demand‑side policies when supply shocks dominate.
Stagflation and the 1970s
During the early 1970s, expansionary monetary policy attempted to keep unemployment low. So as oil prices surged, cost‑push inflation ((\nu_t > 0)) moved the SRPC upward. Eventually, expectations adjusted, the SRPC shifted further upward, and the economy settled at a higher inflation rate with unemployment returning to its natural rate. Because expectations had not yet caught up, actual inflation exceeded expected inflation, worsening the unemployment‑inflation trade‑off. This sequence illustrated why the LRPC is vertical: long‑run unemployment is immune to demand manipulation.
Policy Making
Policy Making
Given the structural verticality of the long‑run Phillips curve, any credible commitment by the central bank to anchor inflation expectations forces the economy back onto the LRPC, rendering attempts at persistent discretionary stimulus futile in lowering real employment. So empirically, the post‑1970 experience shows that only policies which simultaneously raise the natural rate of unemployment—through structural reforms, labor‑market regulation, or investment in human capital—can restore stability. These measures shift the entire SRPC outward, thereby moving the LRPC inward toward a lower unemployment equilibrium while leaving inflation on an acceptable trajectory.
A prudent strategy therefore involves two interlocking components. Second, fiscal authorities must avoid financing deficits that rely on accommodative monetary policy during periods of rising oil prices or other exogenous supply shocks. First, the central bank should adopt a forward‑guided stance that publicly signals a clear path for tightening once inflation departs from the target. Which means by anchoring expectations through transparent communication, it reduces the lag inherent in adaptive dynamics and helps the SRPC pivot earlier than would otherwise be possible under slow learning. Instead, they should employ counter‑cyclical spending that supports productivity gains rather than temporary demand boosts, ensuring that the aggregate supply side remains resilient.
On top of that, institutional design matters. Independent monetary institutions that are insulated from political pressure can reinforce credibility, because their mandate to maintain price stability depends on maintaining public confidence in the inflation forecast. When such an institution enforces a stable growth rule—often expressed as “price stability first, full employment second”—the feedback loop between expectations and actual outcomes becomes less volatile, reducing the risk of abrupt policy reversals that could reignite inflationary spirals The details matter here..
Finally, the lesson from the 1970s is that “demand‑side” interventions alone are insufficient when supply constraints dominate. And policymakers must recognize that the LRPC sets a hard ceiling on sustainable unemployment, and any deviation from that boundary inevitably translates into higher inflation unless accompanied by genuine structural improvements. As a result, a balanced approach that couples macro‑economic stabilization with long‑term investment in education, technology adoption, and labor market flexibility offers the most solid pathway to durable prosperity.
And yeah — that's actually more nuanced than it sounds.
Conclusion
The long‑run structure of the Phillips curve reveals a fundamental incompatibility between cyclical fluctuations in unemployment and sustained reductions in real joblessness. Only expectations anchored to a credible, low‑inflation regime allow the economy to return to the natural rate of unemployment. Also, central banks, in partnership with governments, must therefore prioritize expectation management, structural reform, and disciplined monetary frameworks over short‑run attempts to exploit the short‑run trade‑off. By doing so, societies can break the cycle of stagflation and achieve a stable equilibrium where inflation stays near target and unemployment hovers around its potential, delivering lasting economic well‑being That alone is useful..
