Historical Context & Motivation
In the decades following the Great Depression and World War II, policymakers around the world grappled with a central question: could governments use fiscal and monetary policy to simultaneously achieve low unemployment and stable prices? Classical economists had generally assumed that markets would self-correct, but the Keynesian revolution of the 1930s and 1940s shifted attention toward active demand management. It was in this intellectual climate that A.W. Phillips, a New Zealand-born economist working at the London School of Economics, published a landmark empirical paper in 1958. Phillips documented an apparent inverse relationship between the rate of wage inflation and the unemployment rate in the United Kingdom over nearly a century of data, suggesting that low unemployment came at the cost of rising wages—and, by extension, rising prices.
The story of the Phillips Curve is ultimately a story about expectations. The central question it forces us to address is: Can policymakers permanently reduce unemployment by tolerating higher inflation, or does the tradeoff erode once people adjust their expectations? Understanding why the answer differs in the short run versus the long run is essential for mastering AP Macroeconomics.
Core Principles & Definitions
Before diving into the graphs and equations, it is important to establish the foundational concepts that underpin the Phillips Curve framework. The model rests on several interconnected ideas about labor markets, inflation, and expectations formation—each of which plays a distinct role in explaining why the inflation–unemployment tradeoff behaves differently across time horizons.
Short-Run Phillips Curve (SRPC)
Long-Run Phillips Curve (LRPC)
Natural Rate of Unemployment (NRU)
Inflation Expectations
Supply Shocks
Visual Explanation — The Short-Run Phillips Curve
The diagram below illustrates the fundamental short-run Phillips Curve. The horizontal axis measures the unemployment rate and the vertical axis measures the inflation rate. The downward slope captures the inverse relationship: as aggregate demand increases, firms hire more workers (lowering unemployment) while bidding up wages and prices (raising inflation). Notice that the LRPC is drawn as a vertical line at the natural rate of unemployment, emphasizing that no permanent tradeoff exists once expectations adjust.
The key insight from this diagram is that movements along the SRPC represent demand-side fluctuations (corresponding to movements along the aggregate demand curve), whereas shifts of the entire SRPC occur when inflation expectations change or when supply shocks hit the economy. When policymakers pursue expansionary policy—say, increasing the money supply—the economy moves from Point A toward Point B in the short run, but as workers and firms revise their inflation expectations upward, the SRPC itself shifts upward, pulling the economy back to the NRU at a higher inflation rate.
Mathematical Framework
The Phillips Curve can be expressed algebraically using the expectations-augmented Phillips Curve equation, which formalizes the relationship between actual inflation, expected inflation, the unemployment gap, and supply shocks. This equation is the workhorse model tested on the AP exam and connects directly to the AD-AS framework you have already studied.
This equation tells us several things simultaneously. First, when actual unemployment equals the natural rate (U = Uₙ), the unemployment gap term vanishes and actual inflation equals expected inflation plus any supply shock. Second, when unemployment falls below the natural rate, the term −α(U − Uₙ) becomes positive, pushing actual inflation above expected inflation—this corresponds to an economy operating beyond full employment with an inflationary gap. Third, the supply shock term ε captures events like oil price spikes that shift the SRPC.
Shifts of the SRPC & Supply Shocks
Understanding what shifts the SRPC is one of the most frequently tested concepts on the AP Macroeconomics exam. Two primary forces shift the curve: changes in inflation expectations and supply shocks. The diagram below shows how these shifts work, contrasting a demand-driven shift (via expectations) with a supply-shock-driven shift.
| Cause of Shift | Direction of SRPC Shift | Example |
|---|---|---|
| ↑ Inflation expectations | Upward (rightward) | Workers expect 5% inflation and negotiate higher wages, shifting cost structures upward. |
| ↓ Inflation expectations | Downward (leftward) | Central bank credibly commits to low inflation; expectations anchor at 2%, reducing wage pressures. |
| Adverse supply shock | Upward (rightward) | Oil embargo raises production costs across the economy (e.g., OPEC 1973). |
| Favorable supply shock | Downward (leftward) | Major technological breakthrough reduces production costs (e.g., the IT revolution of the 1990s). |
Worked Example — From AD-AS to the Phillips Curve
This worked example walks through a complete policy scenario, tracing the effects of expansionary monetary policy through both the AD-AS model and the Phillips Curve framework. This type of dual-diagram analysis is commonly required on AP free-response questions.
Policy Tradeoffs & Limitations
The Phillips Curve framework has profound implications for stabilization policy. The distinction between the short-run tradeoff and the long-run vertical curve creates a fundamental tension for policymakers: demand-side tools can smooth cyclical fluctuations but cannot permanently alter the unemployment rate below its natural level. The table below compares key aspects of the short-run and long-run perspectives.
| Feature | Short-Run Phillips Curve | Long-Run Phillips Curve |
|---|---|---|
| Shape | Downward-sloping | Vertical at the NRU |
| Inflation–Unemployment Tradeoff | Yes — lower U is achievable at the cost of higher π | No — any π is consistent with Uₙ |
| Expectations | Held constant (not yet adjusted) | Fully adjusted (π = πᵉ) |
| Policy Implication | Demand management can temporarily reduce unemployment | Only structural reforms (education, labor market) can lower Uₙ |
| AD-AS Parallel | Upward-sloping SRAS (output responds to price changes) | Vertical LRAS at Yf (output is supply-determined) |
Connection to Advanced Theory & Modern Debates
The expectations-augmented Phillips Curve studied on the AP exam is itself a simplification of a richer body of macroeconomic theory. Understanding where the AP model sits relative to more advanced frameworks will deepen your intuition and prepare you for college-level economics. The key development beyond the AP curriculum is the distinction between adaptive expectations (the Friedman-Phelps approach, which is what the AP exam tests) and rational expectations (the approach developed by Robert Lucas and Thomas Sargent in the 1970s).
| Feature | AP Model (Adaptive Expectations) | Rational Expectations (Advanced) |
|---|---|---|
| How expectations form | People look backward: πᵉ is based on recent past inflation | People look forward: πᵉ incorporates all available information, including policy announcements |
| Short-run tradeoff | Yes — expectations lag behind actual inflation | Only if policy is unexpected; anticipated policy has no real effect even in the short run |
| Speed of adjustment | Gradual — takes time for workers and firms to update expectations | Immediate — expectations jump to the new equilibrium if the policy is credible |
| Policy implication | Stabilization policy works in the short run | Only surprise policy works; credibility and commitment are paramount |
Modern central banking practice incorporates insights from both frameworks. The Federal Reserve's emphasis on forward guidance—clearly communicating its inflation targets and policy intentions—reflects the rational expectations insight that credible commitments can anchor inflation expectations and reduce the short-run costs of disinflation. Meanwhile, the observation that inflation expectations sometimes adjust sluggishly, especially among consumers and workers who do not closely follow Fed announcements, validates the adaptive expectations framework tested on the AP exam. The Phillips Curve remains a living area of macroeconomic research, with ongoing debates about whether the curve has "flattened" in recent decades as inflation expectations have become more firmly anchored around the Fed's 2% target.
Practice Problems
Summary — The Phillips Curve
The Phillips Curve captures the relationship between inflation and unemployment. The short-run Phillips Curve (SRPC) is downward-sloping, reflecting a temporary tradeoff: expansionary policy can lower unemployment at the cost of higher inflation, and contractionary policy can reduce inflation at the cost of higher unemployment. This tradeoff exists because inflation expectations are slow to adjust in the short run. The SRPC shifts when expectations change or when supply shocks hit the economy. The key equation is π = πᵉ − α(U − Uₙ) + ε, which links actual inflation to expected inflation, the unemployment gap, and supply shocks.
The long-run Phillips Curve (LRPC) is vertical at the natural rate of unemployment (NRU), demonstrating that there is no permanent tradeoff between inflation and unemployment. Once inflation expectations fully adjust so that π = πᵉ, the economy returns to the NRU regardless of the inflation rate. This parallels the vertical long-run aggregate supply (LRAS) curve in the AD-AS model. Permanently reducing the NRU requires supply-side policies—such as investments in education, job training, and labor market reforms—rather than demand-side stimulus alone.