菲利普斯曲线与自然失业率 — AP 宏观经济学

1. 核心概念：菲利普斯曲线与自然失业率 ★★☆☆☆ ⏱ 3 min

菲利普斯曲线是描述通货膨胀与失业之间关系的核心宏观经济学模型，从早期实证研究发展而来，修正后纳入了长期通货膨胀预期调整。本主题占AP宏观经济学第5单元分值的10-15%，几乎每次考试都会出现在MCQ和FRQ部分。

现代菲利普斯曲线模型的核心结论是：通货膨胀与失业之间的权衡取舍仅存在于短期；长期中不存在永久权衡取舍，因为通货膨胀预期会调整至实际通货膨胀水平，推动短期曲线回移，与自然失业率处的长期垂直曲线对齐。

2. 短期菲利普斯曲线（SRPC） ★★★☆☆ ⏱ 4 min

短期菲利普斯曲线（SRPC）是一条向下倾斜的曲线，显示当预期通货膨胀和自然失业率保持不变时，通货膨胀与失业之间的反向关系。反向关系来自短期工资和价格粘性：总需求的意外增加会促使企业提高产出、雇佣更多工人（失业率降低），推高价格（通货膨胀升高）。

pi = pi^e - alpha(u - u_n)

Where: $\pi$ = 实际通货膨胀, $\pi^e$ = 预期通货膨胀, $\alpha$ = a positive constant measuring how responsive inflation is to cyclical unemployment, $u$ = 实际失业率, and $u_n$ = 自然失业率. When actual unemployment falls below $u_n$, actual inflation rises above expected inflation, giving the SRPC its downward slope.

Exam tip: 在AP FRQ题目中，务必给SRPC标注负斜率、坐标轴（Y轴：通货膨胀率，X轴：失业率），记住总需求的变化只会导致沿SRPC移动，不会移动曲线本身。

3. Long-Run Phillips Curve (LRPC) and Long-Run Equilibrium ★★★☆☆ ⏱ 4 min

In the long run, wages and prices are fully flexible, and workers adjust their inflation expectations to match actual inflation. Any attempt to keep unemployment below the natural rate via expansionary policy will only lead to permanently higher inflation, not sustained lower unemployment. This means the Long-Run Phillips Curve (LRPC) is a vertical line drawn exactly at the natural rate of unemployment $u_n$, with no trade-off between inflation and unemployment in the long run.

There is a direct 1:1 mapping between the LRPC and the long-run aggregate supply (LRAS) curve in the AD-AS model: LRAS is vertical at potential output, which corresponds to an unemployment rate exactly equal to the natural rate. If expansionary policy shifts AD right, output rises above potential, unemployment falls below $u_n$, and inflation rises in the short run. In the long run, workers adjust wage expectations, SRAS shifts left, output returns to potential, unemployment returns to $u_n$, and inflation is permanently higher, which corresponds to the SRPC shifting upward to intersect LRPC at the new higher inflation rate.

Exam tip: If a question asks how a change in frictional unemployment affects the Phillips curve, remember it shifts both LRPC and SRPC, since SRPC always intersects LRPC at the new NRU.

4. Shifts of the SRPC and LRPC ★★★★☆ ⏱ 3 min

It is critical to distinguish between movements along a curve and shifts of the entire curve, a common source of error on AP exams. Movements along the SRPC are only caused by changes in aggregate demand that change actual inflation and unemployment. Shifts of the SRPC are caused by two factors: (1) Changes in expected inflation: higher expected inflation shifts SRPC upward/rightward (higher inflation at every unemployment rate), lower expected inflation shifts it downward/leftward. (2) Aggregate supply shocks: a negative supply shock (e.g., rising oil prices) shifts SRPC up/right, a positive supply shock shifts it down/left.

Shifts of the LRPC are only caused by changes in the natural rate of unemployment, which stem from changes to labor market structure, frictional unemployment, or structural unemployment. For example, increased job training reduces structural unemployment, so NRU falls and LRPC shifts left. A permanent increase in the minimum wage raises structural unemployment, so NRU rises and LRPC shifts right.

Exam tip: On AP MCQs, if you are asked to identify the effect of a supply shock, always eliminate options that show a shift of LRPC unless the question explicitly states the shock changed the natural rate of unemployment.