Core Concepts & Learning Goals
This topic explores the heart of short-run macroeconomic analysis: how the economy responds to sudden changes. The Aggregate Demand-Aggregate Supply (AD-AS) model is our primary tool for understanding these fluctuations. An economic shock is an unexpected event that shifts either the aggregate demand or the short-run aggregate supply curve, pushing the economy away from its initial equilibrium.
By the end of this section, you will be able to use the AD-AS model to explain how various economic shocks affect the key indicators of an economy's health in the short run: the overall price level, real output (real GDP), and the level of employment.
Key Concepts Breakdown
1. Shocks to Aggregate Demand (AD)
Aggregate Demand shocks are events that cause households, firms, the government, or foreign buyers to change their total spending on domestic goods and services. These shocks cause the AD curve to shift.
Positive AD Shock: An event that increases aggregate demand, shifting the AD curve to the right.
Cause: Examples include a sudden increase in consumer confidence, a tax cut, an increase in government spending, or a boom in a trading partner's economy.
Effect: When AD shifts right, there is more spending at every price level. This leads to a higher equilibrium price level, higher equilibrium real output, and higher employment as firms hire more workers to meet the increased demand.
Negative AD Shock: An event that decreases aggregate demand, shifting the AD curve to the left.
Cause: Examples include a fall in consumer confidence, a tax increase, a cut in government spending, or a recession in a trading partner's economy.
Effect: When AD shifts left, there is less spending at every price level. This leads to a lower equilibrium price level, lower equilibrium real output, and lower employment as firms scale back production.
2. Shocks to Short-Run Aggregate Supply (SRAS)
Short-Run Aggregate Supply shocks are events that change the costs of production for many firms in the economy. These shocks cause the SRAS curve to shift.
Positive SRAS Shock: An event that decreases the costs of production, increasing short-run aggregate supply and shifting the SRAS curve to the right.
Cause: Examples include a fall in economy-wide input prices (like oil or wages), a productivity boom from new technology, or beneficial weather for agriculture.
Effect: When SRAS shifts right, firms are willing and able to supply more output at every price level. This leads to a lower equilibrium price level, higher equilibrium real output, and higher employment.
Negative SRAS Shock: An event that increases the costs of production, decreasing short-run aggregate supply and shifting the SRAS curve to the left.
Cause: Examples include a sharp rise in input prices (an oil price shock), an increase in the nominal wage rate, or widespread natural disasters that disrupt production.
Effect: When SRAS shifts left, firms are willing and able to supply less output at every price level. This leads to a higher equilibrium price level, lower equilibrium real output, and lower employment. This specific, and particularly challenging, combination of rising prices (inflation) and falling output (stagnation) is known as stagflation.
3. Demand-Pull vs. Cost-Push Inflation
Inflation is a sustained increase in the general price level. The AD-AS model shows that it can arise from two distinct sources in the short run.
Demand-Pull Inflation: This type of inflation is caused by a positive shock to aggregate demand. As the AD curve shifts to the right, the excess demand "pulls" prices up. The economy experiences a rising price level alongside rising output.
Cost-Push Inflation: This type of inflation is caused by a negative shock to short-run aggregate supply. An increase in the costs of production "pushes" the SRAS curve to the left, leading to a higher price level for a smaller quantity of goods. The economy experiences a rising price level alongside falling output.
Summary of Short-Run Shocks
| Shock Type | Curve Shift | Impact on Price Level | Impact on Real GDP | Impact on Employment |
|---|---|---|---|---|
| Positive AD Shock | AD shifts right | Increases | Increases | Increases |
| Negative AD Shock | AD shifts left | Decreases | Decreases | Decreases |
| Positive SRAS Shock | SRAS shifts right | Decreases | Increases | Increases |
| Negative SRAS Shock | SRAS shifts left | Increases | Decreases | Decreases |
Graphical Analysis (Text-Only)
Let's analyze a negative shock to aggregate demand using a text-based description of the AD-AS graph.
Initial State: Short-Run Equilibrium
Axes: The vertical axis is the Price Level (PL). The horizontal axis is Real GDP (Y).
Curves:
The Aggregate Demand curve (AD1) is downward-sloping.
The Short-Run Aggregate Supply curve (SRAS1) is upward-sloping.
Equilibrium (E1): The economy is initially at equilibrium point E1, where AD1 intersects SRAS1. This determines the initial equilibrium price level, PL1, and the initial equilibrium real GDP, Y1. The level of employment is associated with producing Y1.
The Shock: A Decrease in Aggregate Demand
Event: A major stock market crash reduces household wealth, causing a sharp drop in consumer confidence and spending.
Curve Shift: This is a negative AD shock. The AD1 curve shifts to the left to a new position, AD2. The SRAS1 curve does not change in the short run.
New Equilibrium (E2): The new short-run equilibrium, E2, is found at the intersection of the new AD2 curve and the original SRAS1 curve.
Outcome Analysis:
The new equilibrium price level, PL2, is below the original PL1.
The new equilibrium real GDP, Y2, is below the original Y1.
Because real GDP (output) has fallen, the level of employment has also fallen.
The result of a negative AD shock is a recessionary period with lower prices, lower output, and higher unemployment.
Step-by-Step Example
Let's walk through a scenario involving a negative supply shock.
Scenario: The economy is in short-run equilibrium. Suddenly, international tensions cause the price of imported oil, a key resource for transportation and manufacturing, to double.
Step 1: Identify the Shock and the Affected Curve.
The doubling of oil prices is an economy-wide increase in the cost of production. This is a negative shock to short-run aggregate supply. Therefore, the SRAS curve will be affected.
Step 2: Determine the Direction of the Shift.
Because production costs have risen, firms will produce less output at any given price level. This causes the SRAS curve to shift to the left.
Step 3: Analyze the Impact on the Economy.
Starting from an initial equilibrium (E1) with price level PL1 and output Y1, the leftward shift of the SRAS curve (to SRAS2) creates a new equilibrium (E2). At E2, the new price level (PL2) is higher than PL1, and the new output level (Y2) is lower than Y1. Since output has decreased, employment has also decreased. This outcome—rising prices and falling output—is stagflation.
AP Exam Tips & Common Pitfalls
[FRQ Task]: A common free-response question will describe a scenario and ask you to draw a correctly labeled AD-AS graph to show the short-run effect. You must show the initial equilibrium, the shift of the correct curve in the correct direction, and clearly indicate the resulting changes to the price level and real GDP on your graph's axes.
[MCQ Task]: Multiple-choice questions often test your ability to connect a specific event (e.g., "an increase in government spending") to the correct shift and the resulting outcome for price level and output. You may also be given an outcome (e.g., "the price level falls while real GDP rises") and asked to identify the shock that caused it.
[Common Pitfall ①]: Misidentifying the Shifter. Students often confuse which events shift AD versus SRAS. For example, a change in income taxes affects disposable income and thus shifts AD, while a change in business taxes affects costs of production and shifts SRAS.
- Fix: Create a clear list of shifters for AD (related to spending) and SRAS (related to production costs/productivity) and memorize them.
[Common Pitfall ②]: Forgetting the Link Between Output and Employment. It's easy to focus only on price level and real GDP and forget to mention employment.
- Fix: Always remember that in the short run, real GDP and employment move in the same direction. If output (Y) increases, employment increases. If output (Y) decreases, employment decreases.
Key Vocabulary
Economic Shock: An unexpected event that causes a shift in the aggregate demand or short-run aggregate supply curve, moving the economy away from its equilibrium.
Demand-Pull Inflation: An increase in the general price level resulting from a positive shock to aggregate demand (a rightward shift of the AD curve).
Cost-Push Inflation: An increase in the general price level resulting from a negative shock to short-run aggregate supply (a leftward shift of the SRAS curve), typically caused by an increase in production costs.
Stagflation: An economic condition of both rising prices (inflation) and falling real output (stagnation), caused by a negative shock to short-run aggregate supply.