AP Macroeconomics Unit 3 Notes: The AD-AS Model for Output and the Price Level

Aggregate Demand

What aggregate demand is (and what it is not)

Aggregate demand (AD) is the total amount of real output (real GDP) that all buyers in an economy want to purchase at each possible price level, holding everything else constant. In the AD-AS model, the price level is usually measured by a broad index (like the GDP deflator), and real GDP is inflation-adjusted output.

A common confusion is to treat aggregate demand as “total spending.” Spending is part of the story, but AD is specifically a relationship between the price level and real GDP demanded—it’s a curve showing many possible combinations.

The AD curve is typically drawn downward sloping: as the price level falls, the quantity of real GDP demanded rises.

Why aggregate demand matters

Aggregate demand is one of the two big “forces” (the other is aggregate supply) that determines the economy’s current real GDP and price level in the AD-AS model. When AD changes, it helps explain:

  • Recessions (falling real GDP and rising unemployment) when AD decreases.
  • Demand-pull inflation (rising price level) when AD increases rapidly.
  • How fiscal policy (taxes and government spending) and monetary policy (interest rates and the money supply) transmit through the economy.

In Unit 3, the key skill is using AD shifts and supply curves to interpret what happens to real GDP and the price level, and connecting that to unemployment and inflation.

How the AD curve is built: the spending components

In AP Macroeconomics, AD is commonly linked to total planned spending:

AD = C + I + G + (X - M)

Where:

  • C = consumption spending by households
  • I = investment spending by firms (and new residential construction)
  • G = government purchases of goods and services (not transfer payments)
  • X = exports
  • M = imports

This identity is not saying spending literally equals AD at every price level by itself; it’s a useful way to organize what shifts AD. If one of these components rises (holding the price level constant), AD tends to shift right.

Why the AD curve slopes downward

You’ll often be expected to explain the negative slope using three mechanisms:

  1. Wealth effect (real balances effect): When the price level falls, the purchasing power of money holdings rises. People feel wealthier in real terms and may consume more.

  2. Interest rate effect: A lower price level reduces the amount of money people need for transactions. This can push interest rates down, encouraging borrowing and increasing C and especially I.

  3. International trade effect (exchange rate effect): If the domestic price level falls relative to foreign price levels, domestic goods become relatively cheaper. Exports rise and imports fall, increasing X - M.

A major misconception: students sometimes say “AD slopes down because prices are lower so people buy more.” That logic is fine for a single market demand curve, but for AD you need one of the three AD-slope effects above (wealth, interest rate, net exports). The AD curve is not just “market demand scaled up.”

What shifts aggregate demand (the shifters)

A shift in AD means that at the same price level, the total quantity of real GDP demanded changes. Common right-shifters (increase AD) include:

  • Higher consumer confidence (raises C)
  • Lower interest rates (raises I and some C)
  • Higher expected future income/profits (raises C and I)
  • Expansionary fiscal policy: higher G or lower taxes (raises C via disposable income)
  • A weaker domestic currency (tends to increase X - M)
  • Higher foreign income (raises demand for exports)

Common left-shifters (decrease AD) reverse those ideas.

AD in action: a concrete example

Suppose households become pessimistic about the future and cut consumption. At every price level, planned C is lower. That means:

  • AD shifts left.
  • In the short run (with an upward-sloping SRAS), equilibrium real GDP falls and the price level typically falls.

Notice what did not happen: the economy did not “move along AD” just because consumers got pessimistic. Pessimism is not a change in the price level; it is a shift factor.

Exam Focus
  • Typical question patterns:
    • Given a scenario (e.g., interest rates fall, consumer confidence rises, currency appreciates), identify whether AD shifts left/right and predict effects on real GDP and price level.
    • Explain why AD slopes downward using the wealth, interest rate, and international trade effects.
    • Distinguish between a movement along AD (price level changes) and an AD shift (component spending changes).
  • Common mistakes:
    • Using “lower prices increase quantity demanded” without referencing the AD-specific mechanisms.
    • Treating transfer payments (like Social Security) as G; they affect C indirectly, not G.
    • Confusing a change in M (imports) with a change in net exports X - M; imports rising by itself reduces X - M.

Multipliers

What the multiplier is

The multiplier effect is the idea that an initial change in spending can lead to a larger final change in real GDP, because one person’s spending becomes another person’s income, which then generates further spending.

For example, if the government increases purchases of goods and services, that directly raises income for firms and workers. Those households then spend part of the new income, creating more income for others, and so on. The process “ripples” through the economy.

Why the multiplier matters

Multipliers explain why relatively small changes in autonomous spending (spending not directly dependent on current income) can have noticeable effects on real GDP in the short run. This is central for understanding:

  • The potential strength of fiscal policy (changes in G and taxes).
  • Why economies can experience amplified booms or downturns.
  • How marginal propensities (spending vs saving behavior) shape the size of the response.

The key behavioral idea: MPC and MPS

The multiplier is tied to how much of additional income people spend.

  • Marginal propensity to consume (MPC): the fraction of an extra dollar of income that is spent.
  • Marginal propensity to save (MPS): the fraction of an extra dollar of income that is saved.

They add to 1:

MPC + MPS = 1

If MPC is high, people spend most additional income, so the ripple continues strongly and the multiplier is larger. If MPC is low, the ripple dies out faster.

The simple spending multiplier formula

In the basic Keynesian model used in AP Macroeconomics, the spending multiplier is:

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

Then the change in equilibrium real GDP is:

\Delta Y = k \Delta A

Where:

  • k = spending multiplier
  • \Delta Y = change in real GDP (output/income)
  • \Delta A = initial change in autonomous spending (often \Delta G or \Delta I)

It’s also common to see the multiplier written using MPS:

k = \frac{1}{MPS}

This is equivalent because MPS = 1 - MPC.

How the multiplier works (step-by-step intuition)

Imagine the government increases G by 100.

  1. Round 1: Government buys 100 more output. Firms receive 100 more revenue, households earn 100 more income.
  2. Round 2: Households spend MPC of that extra income. If MPC = 0.8, consumption rises by 80.
  3. Round 3: That 80 becomes someone else’s income; they spend 0.8 of it, adding 64.
  4. The additional spending continues, each round smaller than the last.

The total increase in real GDP is the sum of that series, which the formula captures.

Worked example: government spending multiplier

Suppose MPC = 0.75 and the government increases purchases by 40.

  1. Compute the multiplier:

k = \frac{1}{1 - 0.75} = \frac{1}{0.25} = 4

  1. Apply it to the change in autonomous spending:

\Delta Y = 4 \times 40 = 160

Interpretation: the AD shift from higher G leads to a short-run equilibrium real GDP increase of 160 (assuming the price level is held constant for the spending-model calculation and there is no crowding out built into this simplified setup).

A note on taxes (common AP extension)

AP questions sometimes compare spending changes to tax changes. A tax cut increases disposable income, which increases consumption by only the fraction MPC. That’s why, in the simplest model, tax changes usually have a smaller effect than equal-sized spending changes.

A common formula used for the tax multiplier in the simple model is:

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

And:

\Delta Y = k_T \Delta T

Where \Delta T is the change in taxes. The negative sign reflects that higher taxes reduce output, while lower taxes increase output.

What can weaken the multiplier in the real world

The simple multiplier assumes the main “leakage” is saving. In reality, additional leakages can reduce the ripple:

  • Some extra spending goes to imports (money leaves the domestic circular flow).
  • Higher income can increase tax payments.
  • Interest rates may rise as income rises, potentially reducing I (often discussed as “crowding out” in later units).

You don’t need a complicated model to get AP questions right, but you do need the core logic: bigger MPC means bigger multiplier.

Exam Focus
  • Typical question patterns:
    • Given MPC (or MPS), compute the multiplier and the change in real GDP from a change in G or I.
    • Explain verbally why the multiplier exists using the spending-income feedback loop.
    • Compare the effectiveness of a change in G versus an equal-sized change in taxes.
  • Common mistakes:
    • Forgetting that MPC + MPS = 1, or miscomputing 1 - MPC.
    • Applying the spending multiplier directly to a tax change without accounting for the fact that only MPC of the tax change becomes consumption.
    • Treating the multiplier as a long-run growth tool; it is mainly a short-run demand-side amplification mechanism.

Short-Run Aggregate Supply

What SRAS is

Short-run aggregate supply (SRAS) is the relationship between the price level and the quantity of real GDP produced by firms in the short run, when some input prices (especially wages) are sticky or slow to adjust.

SRAS is usually drawn upward sloping: when the price level rises, firms are willing to produce more real output in the short run.

Why SRAS matters

SRAS is what allows the AD-AS model to explain short-run fluctuations. If SRAS were vertical in the short run, changes in AD would only change the price level. But because SRAS slopes upward, changes in AD can change both real GDP and the price level—matching what we observe during booms and recessions.

Why SRAS slopes upward (short-run frictions)

AP Macroeconomics typically emphasizes that SRAS is upward sloping because in the short run:

  • Nominal wages are sticky: Many workers have contracts or wage norms that adjust slowly. If the price level rises unexpectedly, firms’ output prices rise faster than wages, so hiring and producing more becomes more profitable.
  • Some input prices are sticky: Costs for materials or long-term supplier contracts may adjust with a lag.
  • Misperceptions can occur: Firms may temporarily mistake a rise in their output price for a rise in their relative price and increase production.

The key idea: in the short run, a higher price level can mean higher profit margins (at least temporarily), inducing more production.

Movements along SRAS vs shifts of SRAS

  • A movement along SRAS happens when the price level changes (usually because AD changes). Firms change output supplied because the price level changes relative to sticky costs.
  • A shift of SRAS happens when production costs or productive conditions change at every price level.

Students often mix this up: if wages rise, that is not “moving along SRAS.” It is a shift because the cost environment changed.

What shifts SRAS

SRAS shifts when per-unit production costs change or when firms’ short-run ability to produce changes.

SRAS shifts right (increase SRAS) when costs fall or productivity rises:

  • Lower nominal wages (or slower wage growth)
  • Lower input prices (oil, raw materials)
  • Lower business taxes or more production subsidies
  • Improved productivity (better technology, better management)
  • Lower expected inflation (so wage demands and price-setting behavior are less aggressive)

SRAS shifts left (decrease SRAS) when costs rise:

  • Higher nominal wages
  • Higher input prices (a classic example is an oil price spike)
  • Higher business taxes or fewer subsidies
  • Supply chain disruptions that raise costs
  • Higher expected inflation (workers and firms build in higher future costs)

SRAS in action: negative supply shock

Suppose a major increase in oil prices raises transportation and production costs across the economy.

  • SRAS shifts left (or up).
  • In the short-run equilibrium, the price level rises and real GDP falls.

This is a big deal because it creates stagflation: higher inflation (higher price level) combined with lower output (and typically higher unemployment). AD-only stories cannot produce that combination as cleanly, so supply shocks are a common exam theme.

Worked example: AD increase with upward-sloping SRAS

Assume the economy starts at short-run equilibrium. Consumer confidence rises, increasing C and shifting AD right.

  • Real GDP rises (firms expand production in the short run).
  • The price level rises (demand-pull inflation).

Be careful with wording: the AD shift increases spending; the higher price level is an equilibrium outcome, not the initial cause.

Exam Focus
  • Typical question patterns:
    • Given a scenario (oil price change, wage change, productivity improvement), identify SRAS shift direction and predict effects on real GDP and price level.
    • Distinguish demand-pull inflation (AD right) from cost-push inflation (SRAS left).
    • Explain stagflation using SRAS shifts.
  • Common mistakes:
    • Claiming SRAS shifts right when the price level falls (that is a movement along SRAS driven by AD, not a shift).
    • Mixing up “higher productivity” (SRAS right) with “higher price level” (movement along SRAS).
    • Forgetting that a leftward SRAS shift usually raises the price level while lowering real GDP.

Long-Run Aggregate Supply

What LRAS is

Long-run aggregate supply (LRAS) represents the level of real GDP the economy can produce when wages and other input prices are fully flexible and the economy is producing at potential output (also called full-employment output).

In the AD-AS model, LRAS is drawn as a vertical line at potential real GDP. Vertical means that in the long run, the economy’s output is determined by resources and technology, not by the price level.

Why LRAS matters

LRAS is what anchors the model to the economy’s productive capacity. It helps you reason about:

  • Recessionary gaps (real GDP below potential) and inflationary gaps (real GDP above potential).
  • Why AD increases can raise real GDP in the short run but mainly raise the price level in the long run if potential output doesn’t change.
  • The role of long-run growth factors (capital, labor, technology) versus short-run demand fluctuations.

Why LRAS is vertical (long-run neutrality of the price level)

In the long run, if the overall price level rises, wages and other input prices eventually rise too. Firms’ costs adjust along with output prices, so there is no permanent incentive to produce more just because the price level is higher.

So changes in AD primarily affect the price level in the long run, while real GDP returns to potential.

What shifts LRAS (changes in potential output)

LRAS shifts only when the economy’s capacity to produce changes. Major right-shifters include:

  • More labor (population growth, increased labor force participation, immigration)
  • More capital (investment that increases the capital stock)
  • Improved technology (innovation, better production methods)
  • More natural resources or better access to them
  • Institutional improvements that raise efficiency (for example, better infrastructure or well-designed regulations)

A left shift is less common but can occur if the economy loses productive capacity (natural disasters destroying capital, a large decline in labor force participation, war, severe institutional breakdown).

Connecting LRAS to unemployment and the “natural rate” idea

Potential output corresponds to an unemployment rate consistent with normal labor market turnover and frictions (often discussed as the natural rate of unemployment). You don’t need to compute this rate in the AD-AS section, but you should understand the linkage:

  • If real GDP is below potential (LRAS), unemployment tends to be higher.
  • If real GDP is above potential, the economy is “overheating,” and inflation pressure tends to build.

Self-correction mechanism (how the economy returns to LRAS)

A core AP idea is long-run self-adjustment: when the economy is not at potential output, wages and expectations adjust over time, shifting SRAS until the economy returns to LRAS.

Case 1: Recessionary gap (short-run equilibrium left of LRAS)

  • Real GDP is below potential; unemployment is high.
  • Over time, weak labor demand puts downward pressure on nominal wages.
  • Lower wage costs shift SRAS right.
  • The economy returns toward potential output with a lower price level than at the start of the adjustment.

Case 2: Inflationary gap (short-run equilibrium right of LRAS)

  • Real GDP is above potential; unemployment is very low.
  • Over time, tight labor markets push nominal wages up.
  • Higher costs shift SRAS left.
  • The economy returns toward potential output with a higher price level.

A frequent misconception is that LRAS shifts during self-correction. In the basic story, LRAS stays fixed; it’s SRAS that adjusts as wages and expectations change.

Worked example: AD increase and the long-run outcome

Suppose AD increases substantially (for example, due to expansionary policy). In the short run:

  • Real GDP rises above potential.
  • Price level rises.

In the long run:

  • Workers and firms adjust wage and price expectations upward.
  • SRAS shifts left as nominal wages and input costs rise.
  • Real GDP returns to potential (LRAS), but the price level ends up higher than initially.

This is the conceptual bridge between “stimulus can reduce unemployment in the short run” and “persistent demand increases can create inflation over time.”

Exam Focus
  • Typical question patterns:
    • Identify whether the economy is in a recessionary or inflationary gap based on a graph description and predict the direction of SRAS adjustment over time.
    • Explain why LRAS is vertical and what determines its position.
    • Given a change (technology, capital stock, labor force), predict how LRAS shifts and how that affects long-run output and the price level.
  • Common mistakes:
    • Saying LRAS shifts due to a change in the price level or AD; LRAS shifts due to capacity changes, not demand changes.
    • Confusing SRAS long-run adjustment (wage/expectations changes) with fiscal or monetary policy actions.
    • Claiming long-run output permanently rises from an AD increase without any change in productivity, labor, or capital.