Aggregate Demand, Aggregate Supply, and Economic Fluctuations

Production, capacity, and potential output

An economy produces real output by combining resources and using them efficiently. The aggregate production model represents this relationship as

Here, $Y$ is real output, usually measured by real GDP; $L$ is labor; $K$ is physical capital such as factories and equipment; $H$ is human capital such as education and skills; $R$ is natural resources; and $A$ is technology and efficiency.

Real GDP measures the value of final goods and services produced within an economy, adjusted for changes in prices. This adjustment makes it more useful for comparing production across time.

Potential output is the sustainable level of real GDP that can be produced when existing resources are used at normal rates. It is associated with the natural rate of unemployment, not with zero unemployment. Potential output rises when the labor force, capital stock, human capital, natural resources, technology, or institutional quality improves.

A key distinction is between actual output and productive capacity. A fall in consumer spending can reduce actual GDP because factories operate below capacity, while potential output remains unchanged. A permanent loss of factories or a lasting decline in productivity reduces potential output itself.

Takeaway: Short-run changes in spending mainly affect actual output, while long-run changes in resources and productivity affect potential output.

Business cycles and short-run fluctuations

The business cycle describes recurring fluctuations in real GDP, employment, income, and spending around the economy's long-run growth trend.

Its four common stages are:

1. Expansion: Real GDP, employment, income, and production generally rise.

2. Peak: Economic activity reaches a temporary high point.

3. Contraction: Economic activity declines. A broad and sufficiently persistent contraction is called a recession.

4. Trough: Economic activity reaches a temporary low point before recovery begins.

Short-run fluctuations can result from changes in consumer confidence, business expectations, government spending, taxes, interest rates, credit conditions, foreign demand, exchange rates, energy prices, productivity, or major disruptions such as natural disasters and wars.

A recession is not defined officially by a simple rule requiring two consecutive quarters of declining real GDP. The National Bureau of Economic Research evaluates the breadth, depth, and duration of a decline across several economic indicators.

Takeaway: Business-cycle fluctuations can originate in either total spending or economy-wide production conditions.

Aggregate demand and total spending

Aggregate demand is total planned spending on domestically produced final goods and services at different overall price levels. It is represented by

The components are:

• $C$: household consumption spending.

• $I$: business and household investment spending.

• $G$: government purchases of goods and services.

• $X-M$: net exports, equal to exports minus imports.

On an AD–AS graph, the vertical axis measures the price level and the horizontal axis measures real GDP. The AD curve slopes downward for three main reasons:

• Real wealth effect: A higher price level reduces the purchasing power of money balances and other nominal assets, tending to reduce consumption.

• Interest-rate effect: A higher price level increases the amount of money needed for transactions. With an unchanged money supply, interest rates may rise, reducing interest-sensitive consumption and investment.

• International-trade effect: If domestic prices rise relative to foreign prices, domestic goods become less competitive. Exports tend to fall, imports tend to rise, and net exports decrease.

These effects describe movement along the AD curve. A shift of the entire curve occurs when a non-price determinant of spending changes.

Takeaway: The price level changes the quantity of real GDP demanded, while income, expectations, policy, wealth, interest rates, and international conditions shift aggregate demand.

Short-run and long-run aggregate supply

Aggregate supply describes the total quantity of goods and services firms are willing and able to produce at different price levels.

Short-run aggregate supply is generally upward-sloping because some input prices, especially wages, adjust slowly. If firms receive higher prices for their products while many costs remain fixed, profit margins increase and firms have an incentive to produce more. As the economy approaches capacity, additional production becomes increasingly difficult and costly, so the curve may become steeper.

Long-run aggregate supply is vertical at potential output. Once wages and other input prices have time to adjust, the overall price level does not determine the economy's sustainable productive capacity. LRAS shifts right when the labor force, capital stock, human capital, natural resources, technology, or institutional efficiency increases. It shifts left when productive capacity is permanently lost.

Some versions of the model identify three regions of SRAS:

• Keynesian zone: The economy has substantial unused labor and capital, so output can rise considerably with little increase in the price level.

• Intermediate zone: Resources become scarcer, so both real GDP and the price level rise as demand expands.

• Classical or neoclassical zone: The economy is near potential output, so additional demand mainly raises the price level.

Takeaway: The short run allows production to respond to prices, but the long run is determined by productive capacity.

Equilibrium, gaps, and adjustment

The AD–AS model determines macroeconomic equilibrium where aggregate demand intersects the relevant aggregate supply curve. This intersection identifies the equilibrium price level and equilibrium real GDP.

When equilibrium real GDP is below potential output, the economy has a recessionary gap. Labor and capital are underused, and cyclical unemployment is elevated. For example, if households become pessimistic and reduce consumption, AD shifts left. In the short run, real GDP falls, unemployment rises, and the price level tends to fall or rise more slowly.

When equilibrium real GDP is above potential output, the economy has an inflationary gap. The economy is operating beyond sustainable normal capacity, so wages and prices face upward pressure. For example, rising consumer confidence and business investment can shift AD right, increasing real GDP, employment, and the price level in the short run.

The size of the output and price effects depends on the economy's initial position. A demand increase produces a larger output response when substantial resources are unemployed. Near capacity, the same demand increase produces a larger price response and a smaller output response.

Over time, wages and other input prices may adjust. This can shift SRAS and move the economy toward potential output, but the adjustment may be slow and is not guaranteed to occur quickly.

Takeaway: Equilibrium output can be below, near, or above potential output, and the position of the economy determines whether demand changes mainly affect production or prices.

How to analyze AD–AS shifts

A reliable shift analysis separates movements along curves from shifts of curves.

1. Identify the affected curve. Ask whether the event changes total spending, short-run production costs, or long-run productive capacity.

2. Determine the direction. Decide whether AD, SRAS, or LRAS shifts right or left.

3. Find the new equilibrium. Compare the new intersection with the original intersection.

4. State both effects. Identify the likely change in real GDP and the price level.

5. Separate time horizons. Explain the immediate short-run result and any later adjustment toward potential output.

Typical cases include:

• Rising consumer confidence: AD shifts right; real GDP and the price level generally increase.

• Falling government purchases: AD shifts left; real GDP decreases and the price level decreases or rises more slowly.

• Currency depreciation: AD may shift right because net exports increase; real GDP and the price level generally increase.

• A sharp rise in oil prices: SRAS shifts left; real GDP decreases and the price level increases.

• Higher productivity: SRAS and LRAS shift right; real GDP increases and the price level may decrease or rise more slowly.

• Growth in the labor force: LRAS shifts right, raising potential output; the price effect depends on aggregate demand.

• Destruction of factories by a major disaster: SRAS and LRAS may shift left; real GDP decreases and the price level initially increases.

A leftward SRAS shift that raises the price level while reducing real GDP produces stagflation. This result combines inflationary pressure with weak or falling output.

Takeaway: Always identify the cause before predicting the curve shift; the same event can affect output and prices differently depending on the curve involved.

Stabilization policy and long-run growth

Stabilization policy uses the AD–AS framework to address short-run fluctuations.

During a recessionary gap, expansionary fiscal policy, such as higher government purchases or lower taxes, can increase AD. Expansionary monetary policy can also support spending by improving monetary and credit conditions. These policies are more likely to raise real GDP when the economy has substantial unused resources.

During an inflationary gap, contractionary fiscal or monetary policy can reduce AD and limit upward pressure on prices. However, reducing demand may also lower real GDP and employment in the short run.

Supply-side policies affect productive capacity rather than only total spending. Policies that encourage capital formation, improve education and skills, increase labor-force participation, support technological progress, or strengthen economic institutions can shift LRAS right and promote long-run economic growth. Productivity improvements can also shift SRAS right by reducing per-unit production costs.

The effects of demand-management policy depend on the economy's position. Far below potential output, an increase in AD may produce a substantial increase in real GDP with a modest price effect. Near potential output, it may produce mostly inflation.

Takeaway: Demand-management policies primarily address short-run fluctuations, while supply-side policies primarily expand long-run productive capacity.