Aggregate Demand and Aggregate Supply Model
The AD/AS model is the central framework in macroeconomics for understanding how price levels, real GDP, and employment are determined. It brings together the demand side and the supply side of an entire economy into one diagram, making it possible to analyze everything from recessions to inflation to the effects of government policy.
The model has both short-run and long-run components. In the short run, prices and output can fluctuate together. In the long run, the economy gravitates toward its potential output. Understanding how these two timeframes interact is what makes the AD/AS model so useful for analyzing economic shocks, policy changes, and business cycles.
Aggregate Supply and GDP Relationships
The aggregate supply (AS) curve shows the relationship between the overall price level and the total quantity of goods and services firms are willing to produce.
A few GDP concepts are essential here:
- Real GDP measures the total value of all final goods and services produced in an economy, adjusted for inflation. It excludes intermediate goods to avoid double-counting.
- Potential GDP (also called full-employment output or the natural level of output) represents the maximum sustainable output when all resources are fully employed. The economy isn't necessarily at potential GDP at any given moment; it can be above or below.
There are two versions of the AS curve, and they behave very differently:
- The short-run aggregate supply (SRAS) curve slopes upward. When the overall price level rises, firms can sell their output at higher prices while some of their costs (especially wages) stay fixed in the short run. That gap between rising revenues and sticky costs makes higher production profitable, so firms produce more.
- The long-run aggregate supply (LRAS) curve is vertical at the level of potential GDP. In the long run, all input prices (including wages) fully adjust to match changes in the price level. That means price level changes don't affect real output. Output is determined instead by the economy's productive capacity: its technology, capital stock, and labor force.
Price Levels and Aggregate Demand
The aggregate demand (AD) curve shows the relationship between the overall price level and the total quantity of goods and services demanded across the economy. It slopes downward: higher price levels correspond to lower quantity demanded.
Three effects explain why the AD curve has this negative slope:
- Wealth effect: When the price level rises, the real purchasing power of people's existing wealth (savings, cash holdings) falls. Consumers feel poorer and spend less, reducing aggregate demand.
- Interest rate effect: A higher price level means people and businesses need more money to carry out transactions. This increased demand for money pushes interest rates up, which discourages borrowing and investment, pulling aggregate demand down.
- Exchange rate effect: Higher domestic price levels can cause the domestic currency to appreciate relative to foreign currencies. That makes exports more expensive for foreign buyers and imports cheaper for domestic buyers, so net exports fall and aggregate demand decreases.
Each of these effects works independently, and together they give the AD curve its downward slope.
Equilibrium in the AD/AS Model
Equilibrium occurs where the AD curve intersects the AS curve. At that point, the quantity of goods and services demanded equals the quantity supplied.
- Short-run equilibrium is where AD intersects SRAS. This determines the economy's actual price level and real GDP at a given point in time. The economy can be in short-run equilibrium above, below, or exactly at potential GDP.
- Long-run equilibrium is where AD intersects LRAS (and SRAS). This occurs at potential GDP. In long-run equilibrium, there's no pressure on wages or prices to change because the economy is producing at its full-employment level.
The economy transitions from short-run to long-run equilibrium through a self-correcting mechanism. If the economy is producing above potential GDP, wages and input prices eventually rise (shifting SRAS left) until output returns to potential. If the economy is below potential GDP, wages and input prices eventually fall (shifting SRAS right), pushing output back toward potential.
Short-Run vs. Long-Run Aggregate Supply
Understanding the difference between SRAS and LRAS comes down to one thing: whether input prices (especially wages) have had time to adjust.
| Feature | SRAS | LRAS |
|---|---|---|
| Shape | Upward sloping | Vertical at potential GDP |
| Key assumption | Some input prices are sticky (fixed in the short run) | All input prices are fully flexible |
| What determines output | Both price level and productive capacity | Only productive capacity (technology, capital, labor) |
| Price level effect on real GDP | Yes, temporarily | None |
In the short run, sticky wages mean that a rise in the price level increases firms' profit margins, encouraging them to hire more workers and expand production. In the long run, workers renegotiate wages to keep up with price changes, profit margins return to normal, and output settles back at potential GDP.
Applications of the AD/AS Model
The real power of the AD/AS model is in analyzing how specific events shift the curves and change economic outcomes.
Demand-side shifts (AD curve):
- Increase in government spending shifts AD to the right. In the short run, both the price level and real GDP rise. This is a standard fiscal expansion.
- Decrease in taxes also shifts AD right, because consumers have more disposable income to spend. Short-run result: higher real GDP and a higher price level.
Supply-side shifts:
- Technological advancements shift LRAS to the right, increasing potential GDP. The economy can now produce more at any price level. This represents genuine long-term growth.
- Supply shocks like a spike in oil prices shift SRAS to the left. Firms face higher input costs, so they produce less at every price level. The short-run result is a higher price level combined with lower real GDP.
The model also explains two important types of inflation:
- Demand-pull inflation happens when AD increases faster than AS. Too much spending chases too few goods, pulling prices up.
- Cost-push inflation happens when rising production costs shift SRAS to the left, pushing prices up even as output falls.
Economic Fluctuations and Policy Implications
The AD/AS model provides a visual framework for the business cycle, the recurring pattern of expansions and contractions in economic activity. Recessions show up as leftward shifts in AD or SRAS that push real GDP below potential. Expansions can be driven by rightward shifts in AD or improvements in aggregate supply.
One particularly difficult scenario the model helps explain is stagflation: the combination of high inflation and stagnant (or falling) output. Stagflation typically results from a leftward shift in SRAS, such as a major oil price shock. The price level rises while real GDP falls, creating a policy dilemma. Stimulating demand (shifting AD right) would fight the recession but worsen inflation. Reducing demand (shifting AD left) would fight inflation but deepen the recession.
This trade-off is exactly why policymakers rely on the AD/AS framework. It clarifies the likely consequences of fiscal and monetary policy decisions and reveals why some economic problems don't have easy solutions.