Aggregate Demand vs Long-Run Aggregate Supply
Aggregate Demand and Long-Run Aggregate Supply are two Aggregate Demand & Supply concepts in AP Economics that students often mix up. Aggregate demand is the total demand for final goods and services in an economy at a given time. Long-run aggregate supply is the total supply of goods and services when all factors of production are fully employed. Here is how they compare side by side.
Aggregate demand is the sum of consumption, investment, government spending, and net exports. It represents the total amount of goods and services that households, businesses, the government, and foreigners plan to buy at a given level of income.
In the long run, an economy's potential output is determined by its factors of production, such as labor, capital, and technology. Changes in the price level do not affect the quantity of goods and services supplied in the long run. The LRAS curve is vertical.
Aggregate Demand vs LRAS: Spending Power Against Productive Capacity
| Aggregate Demand | Long-Run Aggregate Supply | |
|---|---|---|
| What it fixes once input prices have adjusted | The price level | Real output |
| Reaction to a stimulus package | Shifts right within months | Does not move on the diagram at all |
| Effect of its own rightward shift on the price level | Raises it | Lowers it, since the same spending meets more goods |
| Speed at which it moves | Fast, since confidence and policy can change it in a quarter | Slow, since capital, skills and technology change over years |
| Whether the gain in output lasts | No, it fades as wages and input prices catch up | Yes, because the sustainable level itself has risen |
| Policy aimed at it | Stabilization: fiscal and monetary tools | Growth: investment, education, technology, institutions |
Both shifts raise output, and only one of the gains is still there afterward
Set potential output at 820 billion dollars. Push aggregate demand right and the short-run intersection moves to 860 billion dollars with the price index up from 100 to 104. Output sits above potential, unemployment is below its natural rate, and for a while the economy looks 40 billion dollars better off. Then the labor market does what tight labor markets do. Wages and other input prices get bid up, short-run aggregate supply drifts left, and output slides back to 820 billion dollars while the price index finishes at 109. Count what survived: no extra goods, and a higher price level than before. Now run the other shift. New capital and better production methods raise potential output from 820 to 860 billion dollars while aggregate demand stays exactly where it was. Output rises to 860 and stays there, and the price index falls to 96, because the same spending is now chasing more goods. Two rightward shifts, both of which raise output on the day they happen, ending in opposite places: one leaves the country with a higher price level and nothing else, the other leaves it with more goods and cheaper ones. That contrast is the reason economists keep stabilization and growth in separate chapters.
The one channel through which demand policy reaches the capacity line
On a diagram the rule has no exceptions: shift aggregate demand and long-run aggregate supply stays put. Answer every shift question that way. Over years rather than quarters, though, one component of aggregate demand is also a source of the capacity line, and that component is investment. Spending on machines, buildings and research becomes part of the capital stock, and the capital stock is one of the things the long-run curve counts. The demand side therefore reaches capacity through investment, and it reaches it in both directions. Government borrowing that pushes interest rates up can crowd out private investment, leaving a smaller capital stock and a long-run curve further left than it would otherwise be, which is set out at /glossary/crowding-in. Public spending aimed at infrastructure, research or schooling adds to capital and skills, moving the line right after a lag too long for any exam diagram to show. Economists point to a labor-market version as well: a shortfall in demand deep enough and long enough can let skills decay and push discouraged workers out of the labor force, lowering what the economy can produce even after spending recovers. Use these arguments in an essay about growth, and keep them off a question asking what a stimulus does this year.
Frequently asked questions
Can an increase in aggregate demand shift long-run aggregate supply?
An increase in aggregate demand does not shift long-run aggregate supply on the diagram, and drawing it that way is a standard error. One indirect route exists, through investment: spending on machines, buildings and research adds to the capital stock over years, and the capital stock is a determinant of the long-run curve. That argument belongs in an essay about growth rather than on a graph about this year's stimulus.
Why does a rightward shift in LRAS lower the price level when a rightward shift in AD raises it?
Long-run aggregate supply shifting right adds goods without adding spending, so the same demand spreads over more output and the price level falls. Aggregate demand shifting right adds spending without adding capacity, so more spending chases the same goods and the price level rises. Both moves lift real output at first, and only the supply-side one keeps that output after input prices have fully adjusted.
Does fiscal stimulus create long-run economic growth?
Fiscal stimulus operates on aggregate demand, so its lasting effect in this model falls on the price level rather than on output, and the early gain in production fades as input prices adjust. Stimulus can still touch growth in one specific way, by financing capital the private sector would not have built, or by damaging it, if the borrowing crowds private investment out. Which of the two happens depends on what the money buys.
Live AD/AS Model graph. Drag the curves, or open the full version.
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