Unit 3 Deep Dive: Using AD-AS to Understand Equilibrium and Fiscal Policy

Equilibrium in the AD-AS Model

The aggregate demand–aggregate supply (AD-AS) model is the main tool you use in AP Macroeconomics to explain how the overall economy determines real GDP (output) and the price level. Think of it as the “market for everything”—instead of one good, you’re looking at the total quantity of final goods and services produced (real GDP) and the overall level of prices.

What “equilibrium” means in AD-AS

An equilibrium in the AD-AS model occurs at the point where aggregate demand (AD) intersects short-run aggregate supply (SRAS). At that intersection, the economy’s planned total spending equals the amount of real output firms are willing to produce at the given price level.

Graphically:

  • The vertical axis is the price level (not the inflation rate; it’s an index of overall prices).
  • The horizontal axis is real GDP.
  • The intersection determines an equilibrium price level and equilibrium real GDP.

Why this matters: in Unit 3, you’re constantly explaining changes in output and the price level using shifts of AD and SRAS. Equilibrium is your “starting point” before any shock or policy.

Aggregate Demand (AD): what it is and why it slopes downward

Aggregate demand (AD) is the total quantity of real GDP demanded at different price levels.

AD slopes downward for three classic reasons (you should be able to explain these, not just name them):

  1. Wealth effect: If the price level rises, the real purchasing power of money balances falls, so consumers feel less “wealthy” in real terms and buy less.
  2. Interest rate effect: A higher price level tends to increase interest rates (as households and firms need more money for transactions), which reduces interest-sensitive spending like investment and some consumption.
  3. Foreign purchases effect: If domestic prices rise relative to foreign prices, exports tend to fall and imports rise, reducing net exports.

AD shifts when components of spending change. In the expenditures approach, total spending is:

Y = C + I + G + NX

Where:

  • Y is real GDP (output/income)
  • C is consumption
  • I is investment spending
  • G is government purchases of goods and services
  • NX is net exports

A common misconception: a change in the price level causes a movement along AD, not a shift of AD. A shift requires something besides the price level changing (consumer confidence, taxes, interest rates driven by monetary policy, foreign income, etc.).

Short-Run Aggregate Supply (SRAS): why it slopes upward

Short-run aggregate supply (SRAS) is the quantity of real GDP firms produce at different price levels in the short run, when some input prices (especially wages) are “sticky.”

SRAS slopes upward because when the price level rises faster than input costs, producing becomes more profitable, so firms increase output.

SRAS shifts when production costs or productivity change, such as:

  • changes in nominal wages
  • changes in commodity prices (oil, electricity)
  • supply shocks (natural disasters, pandemics)
  • changes in productivity or technology

A key distinction: SRAS is about firms’ willingness to produce at different price levels, not about demand.

Showing equilibrium “in action”

To analyze any scenario, you typically follow this sequence:

  1. Identify the initial equilibrium (intersection of AD and SRAS).
  2. Decide what changes (a determinant of AD or SRAS shifts).
  3. Shift the correct curve in the correct direction.
  4. Conclude how equilibrium real GDP and price level change.
Example 1: Consumer confidence rises
  • Mechanism: Higher confidence increases consumption C.
  • AD shifts right.
  • New equilibrium: real GDP rises and price level rises.

Interpretation: This is demand-pull inflation if the economy is near potential output.

Example 2: Oil prices spike
  • Mechanism: Higher input costs reduce SRAS.
  • SRAS shifts left.
  • New equilibrium: price level rises and real GDP falls.

Interpretation: This is a negative supply shock and often produces stagflation-like outcomes (higher prices and lower output).

Exam Focus

  • Typical question patterns:
    • Given a shock (confidence, taxes, oil prices), draw AD-AS and identify changes in equilibrium PL and Y.
    • Explain whether the scenario causes demand-pull or cost-push inflation.
    • Connect the graph to components in Y = C + I + G + NX.
  • Common mistakes:
    • Shifting AD when the prompt describes a change in the price level (that should be movement along AD).
    • Confusing SRAS shifts (cost conditions) with AD shifts (spending conditions).
    • Labeling the vertical axis as “inflation” instead of “price level.”

Short-Run and Long-Run Equilibrium

AD-AS becomes much more powerful when you add the long run and introduce the economy’s potential output.

Potential output and Long-Run Aggregate Supply (LRAS)

Potential output (also called full-employment output) is the level of real GDP the economy can produce when resources are fully employed at their normal rates (this does not mean zero unemployment; it means natural rate of unemployment).

Long-run aggregate supply (LRAS) is a vertical line at potential output. It is vertical because in the long run, nominal wages and other input prices adjust—so the price level does not permanently change real output.

What determines LRAS (and shifts it) are real factors:

  • quantity and quality of labor
  • capital stock
  • technology
  • productivity
  • institutions and incentives

A useful mental model: LRAS is the economy’s “capacity.” AD can push the economy away from capacity in the short run, but capacity itself changes only when fundamentals change.

Short-run equilibrium vs long-run equilibrium

  • Short-run equilibrium is where AD intersects SRAS.
  • Long-run equilibrium occurs when the short-run equilibrium output also equals potential output—meaning the AD–SRAS intersection lies on LRAS.

In long-run equilibrium:

  • output equals potential output
  • unemployment is at the natural rate
  • there is no automatic pressure for wages to adjust up or down due to output being above/below potential

Output gaps: recessionary and inflationary

When real GDP differs from potential output, the economy has an output gap:

  • Recessionary gap: equilibrium real GDP is below potential output.

    • Typically associated with higher cyclical unemployment.
    • Price level pressure tends to be downward (or inflation slows).

  • Inflationary gap: equilibrium real GDP is above potential output.

    • Labor markets are very tight; firms compete for workers.
    • Price level pressure tends to be upward (inflation accelerates).

A common misconception: students sometimes think “inflationary gap” means the price level is already high. It really means output is above potential, creating upward pressure on wages and prices.

Self-correction: how the economy moves back to long-run equilibrium

The AD-AS model includes a built-in mechanism called self-correction, driven mainly by wage and input price adjustments.

If there is a recessionary gap
  1. Output is below potential; unemployment is above natural.
  2. Workers have less bargaining power; nominal wages tend to grow more slowly or fall.
  3. Firms’ costs fall, so SRAS shifts right.
  4. Output rises back toward potential, and the price level tends to fall.
If there is an inflationary gap
  1. Output is above potential; unemployment is below natural.
  2. Workers have more bargaining power; nominal wages rise.
  3. Firms’ costs rise, so SRAS shifts left.
  4. Output falls back toward potential, and the price level rises further.

Why this matters: policy debates often revolve around whether to “wait for self-correction” or use discretionary fiscal policy (next section) to close gaps faster.

Worked AD-AS gap example (conceptual)

Suppose the economy starts in long-run equilibrium. Then investment spending falls sharply (firms become pessimistic).

  • Step 1: I falls, so AD shifts left.
  • Step 2: In the short run, new equilibrium has lower real GDP and lower price level.
  • Step 3: Now real GDP is below potential (recessionary gap).
  • Step 4: Over time, wages and input prices adjust downward, shifting SRAS right.
  • Step 5: The economy returns to potential output at a lower price level.

Notice the key pattern: with a negative demand shock, the long-run outcome restores potential output but does not restore the original price level.

Exam Focus

  • Typical question patterns:
    • Identify whether the economy is in short-run equilibrium only or also long-run equilibrium.
    • Given a recessionary or inflationary gap, show self-correction using an SRAS shift.
    • Explain the direction of wage changes and how they shift SRAS.
  • Common mistakes:
    • Shifting LRAS in response to a demand shock (LRAS shifts only from real growth factors).
    • Mixing up which way SRAS shifts during self-correction.
    • Assuming the economy always returns to the original price level (it returns to potential output, not necessarily the original PL).

Fiscal Policy

Fiscal policy is the use of government spending and taxation to influence aggregate demand, with the goal of stabilizing output, employment, and the price level. In the AD-AS model, fiscal policy mainly works by shifting AD.

What counts as fiscal policy (and what doesn’t)

In AP Macro, discretionary fiscal policy refers to deliberate changes in:

  • government purchases (G)
  • taxes (which affect consumption through disposable income)

Transfer payments (like unemployment benefits) matter for disposable income and consumption, but in national income accounting they are not counted in G because G includes only purchases of final goods and services.

A common confusion: “government spending” in everyday language includes many things, but in the AD-AS and GDP identity context, G is specifically government purchases.

Expansionary vs contractionary fiscal policy

Fiscal policy is typically described by what problem it’s trying to fix.

Expansionary fiscal policy

Used when the economy is in a recessionary gap (output below potential).

Tools:

  • Increase G
  • Decrease taxes

Effect in AD-AS:

  • AD shifts right
  • real GDP increases
  • price level tends to increase
Contractionary fiscal policy

Used when the economy is in an inflationary gap (output above potential).

Tools:

  • Decrease G
  • Increase taxes

Effect in AD-AS:

  • AD shifts left
  • real GDP decreases
  • price level tends to decrease

The multiplier: why fiscal policy can have a magnified effect

Fiscal policy’s impact can be larger than the initial change because of the multiplier effect: one person’s spending becomes another person’s income, which can lead to further spending.

The key parameter is the marginal propensity to consume (MPC)—the fraction of an additional dollar of income that households spend.

A standard multiplier relationship used in AP Macro is:

k = \frac{1}{1 - MPC}

Where:

  • k is the spending multiplier
  • MPC is marginal propensity to consume

If government purchases increase, a simple relationship is:

\Delta Y = k\Delta G

Where:

  • \Delta Y is the change in real GDP
  • \Delta G is the change in government purchases

Taxes influence output too because changing taxes changes disposable income, which changes consumption. A common simplified expression for the tax multiplier is:

k_T = -\frac{MPC}{1 - MPC}

Then:

\Delta Y = k_T\Delta T

Where:

  • \Delta T is the change in taxes
  • k_T is negative because higher taxes reduce output

Important nuance students miss: in many simplified models, the absolute value of the tax multiplier is smaller than the spending multiplier because taxes change consumption indirectly (people save part of the tax cut), while G directly purchases goods and services.

Worked multiplier problem

Suppose:

  • MPC = 0.8
  • Government increases purchases by 50 billion dollars.

Step 1: Find the multiplier.

k = \frac{1}{1 - 0.8} = \frac{1}{0.2} = 5

Step 2: Multiply.

\Delta Y = 5\times 50 = 250

So real GDP increases by about 250 billion dollars in this simplified framework.

If instead taxes increase by 50 billion dollars:

k_T = -\frac{0.8}{1 - 0.8} = -\frac{0.8}{0.2} = -4

\Delta Y = -4\times 50 = -200

So output falls by about 200 billion dollars.

Tradeoffs and limitations of fiscal policy

Fiscal policy is powerful, but it’s not “free.” AP Macro expects you to understand the main limitations conceptually.

Time lags

Fiscal policy can be slow because:

  • it takes time to recognize the problem (data come with delays)
  • it takes time for legislation to pass
  • it takes time for spending/tax changes to affect behavior

These lags matter because the economy may already be changing by the time the policy hits.

Crowding out (especially in the loanable funds framework)

When the government increases deficit spending, it often needs to borrow more. Increased borrowing can raise interest rates, which can reduce private investment. This effect is called crowding out.

In AD-AS terms, crowding out can partially offset the rightward shift in AD by discouraging interest-sensitive spending (especially I).

A common mistake: treating crowding out as automatic and total. In reality, its size depends on economic conditions and monetary policy responses, but for AP you should be able to explain the direction of the effect.

Budget deficits and national debt

Expansionary fiscal policy (higher G or lower taxes) can increase the budget deficit, adding to the national debt over time. AP questions may ask you to weigh stabilization benefits against long-run fiscal concerns.

Fiscal policy in the AD-AS gap framework

The most common AP setup is:

  • Identify a recessionary or inflationary gap.
  • Choose a fiscal policy that shifts AD the right way.
  • Show the shift closing the gap (moving equilibrium output back to potential output).

If the economy is below potential, expansionary fiscal policy shifts AD right to close the recessionary gap. If the economy is above potential, contractionary fiscal policy shifts AD left to close the inflationary gap.

Exam Focus

  • Typical question patterns:
    • Given an output gap, choose expansionary or contractionary fiscal policy and show the AD shift.
    • Compute the multiplier effect given MPC and a change in G or T.
    • Explain tradeoffs like crowding out and time lags.
  • Common mistakes:
    • Confusing a change in taxes with a change in G on the expenditure identity (taxes affect C, not G directly).
    • Using the spending multiplier formula for a tax change (mixing up multipliers).
    • Claiming fiscal policy shifts SRAS directly (it mainly shifts AD; SRAS shifts are cost/productivity shocks).

Automatic Stabilizers

Not all fiscal policy requires new laws. Automatic stabilizers are features of the tax and spending system that automatically increase or decrease aggregate demand in a way that reduces the size of economic fluctuations.

What automatic stabilizers are

An automatic stabilizer is a fiscal mechanism that:

  • kicks in automatically when income and employment change
  • does not require new legislation
  • tends to smooth the business cycle by supporting spending in recessions and restraining spending in booms

The two most commonly emphasized automatic stabilizers in AP Macroeconomics are:

  1. Progressive income taxes
  2. Unemployment insurance and other need-based transfers

How progressive taxes stabilize the economy

With a progressive tax system, tax liabilities rise more than proportionally as income rises.

  • In an expansion, incomes rise, so households pay more in taxes. Disposable income rises by less than it otherwise would, so consumption increases more slowly. This dampens the rightward pressure on AD.
  • In a recession, incomes fall, so households pay less in taxes. Disposable income falls by less than it otherwise would, which cushions consumption and reduces the fall in AD.

In AD-AS terms: progressive taxes reduce the size of AD shifts caused by changes in income.

A common misconception: thinking stabilizers “prevent recessions.” They don’t eliminate cycles; they reduce amplitude.

How unemployment insurance and transfers stabilize the economy

When the economy weakens and unemployment rises:

  • more people qualify for unemployment benefits and other transfers
  • transfer payments increase household income relative to what it would be without benefits
  • consumption falls less, which supports AD

When the economy strengthens:

  • fewer people qualify for benefits
  • government transfer spending falls automatically
  • this reduces upward pressure on AD

Important accounting note: transfers are not part of G in the GDP identity, but they still affect AD indirectly by changing disposable income and consumption.

Automatic stabilizers vs discretionary fiscal policy

Automatic stabilizers are often contrasted with discretionary policy:

  • Automatic stabilizers respond immediately to changes in income/unemployment and avoid legislative delays.
  • Discretionary fiscal policy can be targeted and larger in scale, but it faces time lags and political constraints.

A helpful analogy: automatic stabilizers are like the shock absorbers on a car (they smooth the bumps automatically), while discretionary fiscal policy is like actively steering around potholes (potentially helpful but slower and requires decisions).

“In action” example: recession with stabilizers

Imagine the economy enters a recession due to a fall in investment.

  • Without stabilizers, falling incomes reduce consumption sharply and AD falls a lot.
  • With stabilizers, tax payments fall and transfers rise, so disposable income does not fall as much.
  • Consumption falls less, so the AD shift is smaller than it would have been.

Graphically, you’d still show AD shifting left, but the key idea is: automatic stabilizers make the shift smaller.

What goes wrong: common misconceptions

Students often make two predictable errors:

  1. They draw automatic stabilizers as shifting SRAS. Stabilizers work through spending, so they affect AD.
  2. They think stabilizers always increase the deficit “by choice.” Stabilizers change the budget balance automatically: deficits tend to rise in recessions and shrink in expansions even without new policy.

Exam Focus

  • Typical question patterns:
    • Identify an automatic stabilizer and explain how it affects AD during a recession or expansion.
    • Contrast automatic stabilizers with discretionary fiscal policy, especially regarding time lags.
    • Explain how tax revenues and transfer payments change over the business cycle.
  • Common mistakes:
    • Treating transfer payments as G in the GDP identity.
    • Claiming stabilizers shift LRAS or change potential output (they do not).
    • Mixing up directions: in recessions, tax revenues fall and transfers rise; in booms, tax revenues rise and transfers fall.