SRAS vs LRAS: The Difference Explained for AP Macro

Short-run aggregate supply (SRAS) slopes upward because some input prices — especially nominal wages — are sticky in the short run. Long-run aggregate supply (LRAS) is vertical at full-employment output because in the long run every price adjusts, so output depends only on the economy's resources and technology, not on the price level. Almost every AD-AS question on the AP Macro exam comes down to knowing which curve moves, and why.

Why SRAS Slopes Upward

Imagine the price level rises while workers' wages are locked in by contracts. Firms sell their output at higher prices but pay the same wages, so profit per unit rises — and they respond by producing more. Reverse it and production falls. That positive relationship between the price level and real output supplied is the upward slope of SRAS.

The slope exists only because some prices are sticky: wage contracts, menu costs, and input prices that adjust slowly. If every wage and price adjusted instantly, changing the price level would change nothing real — which is exactly the long-run story.

Why LRAS Is Vertical

In the long run, workers renegotiate wages, suppliers reprice inputs, and every nominal price catches up. Once that happens, the price level has no effect on how much the economy can produce. Output settles at full-employment output (also called potential output), determined by three real factors:

The quantity and quality of resources — the size and skill of the labor force, the capital stock, and available land and raw materials.

The level of technology — how productively those resources can be combined.

Institutions — property rights, rule of law, and incentives that determine whether resources get used efficiently.

Because none of those depend on the price level, LRAS is a vertical line at potential output. It is the AD-AS twin of the production possibilities curve: moving LRAS right and shifting the PPC outward are the same idea — economic growth.

What Shifts SRAS (But Not LRAS)

The classic SRAS-only shifters are changes in production costs:

Input prices. Higher nominal wages or a spike in oil prices raise costs and shift SRAS left. Falling input prices shift it right. This is the single most tested shifter.

Inflation expectations. If workers and firms expect higher inflation, wages and prices get set higher today, shifting SRAS left.

Per-unit taxes, subsidies, and regulation. Higher business taxes or costly regulation shift SRAS left; subsidies and deregulation shift it right.

A negative SRAS shock produces the exam's favorite scenario: stagflation — the price level rises while real GDP falls and unemployment rises. It is the one case where inflation and unemployment worsen together, and it is why the Phillips curve shifts.

What Shifts LRAS (and SRAS With It)

Anything that changes the economy's productive capacity shifts LRAS: more workers, more capital investment, better technology, improved human capital through education, or discovery of new resources. Growth shifts LRAS right; a disaster that destroys capital shifts it left.

Here is the nuance graders reward: a change in productive capacity shifts both curves, because an economy that can produce more at full employment can also produce more in the short run. But a change in input prices alone — say, higher wages — shifts only SRAS, because the economy's underlying capacity has not changed. Writing "higher wages shift LRAS left" is one of the most common ways students lose an FRQ point.

Self-Correction: How the Gaps Close

The interaction between SRAS and LRAS explains how the economy fixes itself without policy.

Recessionary gap — equilibrium output below potential. Unemployment is high, so wages eventually fall. Lower wages cut production costs, SRAS shifts right, and output returns to potential at a lower price level.

Inflationary gap — output above potential. Labor is scarce, wages get bid up, SRAS shifts left, and output falls back to potential at a higher price level.

The mechanism is always the same: wages adjust, SRAS moves, the gap closes. LRAS never moves during self-correction. The AP exam loves asking what happens "in the long run with no policy action" — the answer is always an SRAS shift driven by wage adjustment, ending back on LRAS.

Drawing It for Points

A full-credit AD-AS graph has: the price level on the vertical axis and real GDP on the horizontal, a downward-sloping AD, an upward-sloping SRAS, a vertical LRAS labeled at full-employment output, and the equilibrium marked where the curves cross. In long-run equilibrium, all three curves intersect at a single point on LRAS.

To show a recessionary gap, draw the AD-SRAS intersection to the left of LRAS; for an inflationary gap, to the right. Label the gap itself — the horizontal distance between current output and potential output.

Practice the mechanics with the interactive graphs in the aggregate supply module and the aggregate demand module, where you can shift each curve and watch the gaps open and close.

The Quick Reference

SRAS: upward-sloping, exists because wages and input prices are sticky, shifts with production costs and expectations.

LRAS: vertical at potential output, shifts only with resources, technology, and institutions — the same forces that shift the PPC.

Self-correction: wage adjustment moves SRAS until output returns to potential.

Stagflation: SRAS shifts left — higher price level, lower output, higher unemployment at the same time.

Get those four lines automatic and every AD-AS question — multiple choice or FRQ — becomes a matter of applying them in order.