## Learning Outcomes

- Understand what long-run macroeconomic equilibrium means in the AD–AS model
- Identify how AD, SRAS, and LRAS must be drawn and labeled on an AP graph
- Explain why LRAS is vertical and why long-run equilibrium occurs at potential output
- Distinguish clearly between short-run equilibrium and long-run equilibrium
- Explain how recessionary and inflationary gaps self-correct through SRAS shifts
- Apply this reasoning to AP multiple-choice and free-response questions

## AP Macroeconomics Syllabus

For the AP Macroeconomics, you are expected to understand long-run equilibrium in the AD–AS model, with a focus on the following syllabus points:

- Know that long-run macroeconomic equilibrium occurs where **AD, SRAS, and LRAS** all intersect
- Identify that long-run equilibrium output equals **full-employment output**, also called **potential output** or **Yf**
- Explain why **LRAS** is vertical and why it is not affected by the current price level
- Understand that full employment means the economy is at the **natural rate of unemployment**, not zero unemployment
- Distinguish a **recessionary gap** from an **inflationary gap** by locating short-run equilibrium relative to LRAS
- Explain how **long-run self-adjustment** works through changes in nominal wages and other input prices that shift SRAS
- Recognise that demand-side shocks change the price level in the long run but do not permanently change real GDP if LRAS is unchanged

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What conditions must be true for an economy to be in long-run equilibrium in the AD–AS model?
2. Why is the LRAS curve vertical?
3. What is the difference between full-employment output and zero unemployment?
4. How does the economy self-adjust from a recessionary gap in the long run?
5. If AD shifts right while LRAS stays fixed, what happens to real GDP in the short run and in the long run?

## Introduction

Long-run equilibrium on LRAS is one of the most tested ideas in AP Macroeconomics because it connects several core topics at once: aggregate demand, short-run aggregate supply, potential output, unemployment, inflation, and self-correction. If you can read this graph well, you can answer many multiple-choice questions and build strong free-response answers.

In the AD–AS model, students often notice where AD and SRAS cross, but miss the extra condition needed for long-run equilibrium. On the AP exam, that distinction matters. A short-run equilibrium can exist away from full employment. Long-run equilibrium is stricter: output must also equal potential output on LRAS.

> **Key Term: long-run macroeconomic equilibrium**
> The situation in which aggregate demand, short-run aggregate supply, and long-run aggregate supply all intersect at one point, so real GDP equals potential output.

### What long-run equilibrium looks like on the graph

A correct AP graph must show three curves:

- **AD** sloping downward
- **SRAS** sloping upward
- **LRAS** vertical

The vertical axis is the **price level**. The horizontal axis is **real GDP** or **output**.

At long-run equilibrium, all three curves intersect at the same point. The output at that point should be labeled **Yf**, **YF**, **potential output**, or **full-employment output**. These labels refer to the same basic idea in this context.

> **Key Term: long-run aggregate supply (LRAS)**
> The vertical curve showing the economy’s potential output, determined by resources, labor, capital, technology, and institutions rather than the current price level.
>
> **Key Term: potential output**
> The level of real GDP the economy can produce when resources are fully employed in a sustainable way; also called full-employment output or Yf.

Why is LRAS vertical? In the long run, changes in the overall price level are matched by changes in nominal wages and other input prices. If firms can charge higher prices but must also pay proportionally higher costs, there is no reason for total output to stay above or below capacity forever. So the economy’s long-run output depends on real productive capacity, not the price level alone.

This is why a rise in the price level does not move LRAS rightward. Students often mix up “higher prices” with “more output,” but that only fits the short run, not the long run.

A useful way to think about it is this:

- **SRAS** tells you what firms will produce at different price levels before all input prices fully adjust
- **LRAS** tells you what the economy can sustain once those input prices have adjusted
  > **Test Tip:** On an FRQ, if the prompt says “long-run equilibrium,” you must place equilibrium output on **LRAS** and label it **Yf** or **full-employment output**. If you only draw AD crossing SRAS, that is not enough.

Another exam trap is labeling equilibrium output just as “Y1” when the economy is explicitly at full employment. AP scoring often expects you to make clear that **Y1 = Yf** in that case.

### Full employment, natural unemployment, and what students get wrong

Students often hear “full employment” and assume it means every single worker has a job. That is not correct in macroeconomics.

> **Key Term: full-employment output**
> The level of real GDP produced when the economy is at the natural rate of unemployment, meaning there is no cyclical unemployment.

At full employment, the economy still has:

- **frictional unemployment** — people between jobs
- **structural unemployment** — mismatch of skills or location

What it does **not** have is **cyclical unemployment**. So in long-run equilibrium:

- actual unemployment = natural rate of unemployment
- cyclical unemployment = 0

This matters because AP questions may ask whether the economy is at full employment, at the natural rate of unemployment, or in long-run equilibrium. Those ideas point to the same graph position: output at **Yf** on LRAS.

Suppose the economy is producing exactly at Yf. That does not mean factories are under impossible strain or that every worker is constantly working overtime. It means resources are being used at a sustainable level consistent with the economy’s capacity.

Now compare that with an economy producing to the right of LRAS. That is above potential, but only temporarily. Firms may run overtime and unemployment may fall below the natural rate, but this cannot last because tight labor markets push wages up. The economy then self-corrects.

> **Key Term: natural rate of unemployment**
> The unemployment rate that exists when the economy is at full-employment output; it includes frictional and structural unemployment but not cyclical unemployment.

A common AP mistake is to write “full employment means zero unemployment.” That can lose points because it shows the wrong concept. The better phrase is: “full employment means unemployment is at the natural rate.”

### Short-run equilibrium versus long-run equilibrium

In the AD–AS model, **AD and SRAS intersect** at the current equilibrium price level and real GDP. But that point is not always on LRAS.

So there are two levels of equilibrium to track:

1. **Short-run equilibrium**: where AD crosses SRAS
2. **Long-run equilibrium**: where AD, SRAS, and LRAS all cross together

If the AD–SRAS intersection lies left of LRAS, the economy is in a recessionary gap. If it lies right of LRAS, the economy is in an inflationary gap.

> **Key Term: recessionary gap**
> A situation in which short-run equilibrium real GDP is below potential output, shown when AD and SRAS intersect to the left of LRAS.
>
> **Key Term: inflationary gap**
> A situation in which short-run equilibrium real GDP is above potential output, shown when AD and SRAS intersect to the right of LRAS.

This distinction is central to AP reasoning. A question may show an economy in equilibrium, but that does not automatically mean long-run equilibrium. Always check the position relative to LRAS.

Worked example:

Suppose the economy begins in long-run equilibrium at point E1, where:

- AD, SRAS, and LRAS all intersect
- output is Yf
- unemployment is at the natural rate

Now aggregate demand decreases, perhaps because consumers and firms cut spending.

Step 1:
AD shifts left.

Step 2:
The new short-run equilibrium is where the new AD crosses SRAS.

Step 3:
Real GDP falls below Yf and the price level falls.

Step 4:
Because output is below potential, unemployment rises above the natural rate.

At this stage, the economy is in **short-run equilibrium**, but not **long-run equilibrium**.

The key exam insight is that lower output creates slack in the labor market. Unemployed workers and underused resources put downward pressure on nominal wages and other input prices. Lower production costs make firms willing to supply more at each price level.

So:

Step 5:
SRAS shifts right.

Step 6:
The economy returns to Yf where AD, SRAS, and LRAS intersect again.

Step 7:
The long-run result is lower price level, but real GDP returns to potential.

This is why a fall in AD causes a recession only in the short run if LRAS does not change.

> **Test Tip:** For self-adjustment questions, do not say LRAS shifts after a demand shock. In standard AP analysis, a demand shock changes **AD** first, then **SRAS** adjusts over time. **LRAS stays fixed** unless resources, technology, or institutions change.

### How long-run self-adjustment restores output to Yf

The self-adjustment process is really about input prices.

> **Key Term: long-run self-adjustment**
> The process by which changes in nominal wages and other input prices shift SRAS until the economy returns to potential output on LRAS.

There are two main cases.

#### Recessionary gap: output below Yf

If output is below potential:

- firms are producing less
- unemployment is above the natural rate
- labor and other resources are underused
- nominal wages and input prices tend to fall

As wages and costs fall, firms’ profitability improves at each price level, so **SRAS shifts right**. Output rises back to Yf.

Chain of reasoning:

AD falls → short-run real GDP falls below Yf → unemployment rises → wages fall → SRAS shifts right → output returns to Yf

The long-run result:

- lower price level
- real GDP back at Yf

#### Inflationary gap: output above Yf

If output is above potential:

- labor markets are very tight
- unemployment is below the natural rate
- firms compete for workers
- nominal wages and input prices rise

As wages and costs rise, firms supply less at each price level, so **SRAS shifts left**. Output falls back to Yf.

Chain of reasoning:

AD rises → short-run real GDP rises above Yf → unemployment falls below natural rate → wages rise → SRAS shifts left → output returns to Yf

The long-run result:

- higher price level
- real GDP back at Yf

A simple memory rule:

- left of LRAS = recessionary gap = **SRAS right**
- right of LRAS = inflationary gap = **SRAS left**

Students often reverse these directions. The best way to avoid that error is to ask: what happens to wages? High unemployment pushes wages down; low unemployment pushes wages up.

### Why demand shocks change prices in the long run but not output

One of the biggest AP conclusions in this topic is this: if only aggregate demand changes and LRAS stays fixed, then in the long run real GDP returns to Yf.

That means demand-side policy or shocks can affect:

- real GDP in the **short run**
- price level in the **short run and long run**

But they do **not** permanently change potential output.

For example:

- If AD increases, short-run GDP rises above Yf, but long-run GDP returns to Yf and the price level ends up higher.
- If AD decreases, short-run GDP falls below Yf, but long-run GDP returns to Yf and the price level ends up lower.

This is also why LRAS is such an important dividing line between short-run macro fluctuations and long-run productive capacity.

By contrast, if resources or technology improve, then LRAS itself shifts right. That is true economic growth. In that case, potential output increases.

So be careful not to confuse:

- **movement back to Yf after a demand shock** with
- **an increase in Yf from growth**

The first is self-correction. The second is expansion of productive capacity.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Long-run macroeconomic equilibrium occurs where AD, SRAS, and LRAS all intersect
- At long-run equilibrium, real GDP equals potential output or full-employment output, labeled Yf
- LRAS is vertical because long-run output depends on productive capacity, not the current price level
- Full employment does not mean zero unemployment; it means unemployment is at the natural rate
- The natural rate of unemployment includes frictional and structural unemployment but not cyclical unemployment
- Short-run equilibrium occurs where AD and SRAS intersect, even if that point is not on LRAS
- A recessionary gap exists when short-run equilibrium is left of LRAS
- An inflationary gap exists when short-run equilibrium is right of LRAS
- In a recessionary gap, falling nominal wages and input prices shift SRAS right
- In an inflationary gap, rising nominal wages and input prices shift SRAS left
- Demand shocks change real GDP in the short run, but if LRAS is unchanged, long-run output returns to Yf
- In AP graphs, long-run equilibrium output should be clearly labeled Yf or full-employment output

## Key Terms and Concepts

- long-run macroeconomic equilibrium
- long-run aggregate supply (LRAS)
- potential output
- full-employment output
- natural rate of unemployment
- recessionary gap
- inflationary gap
- long-run self-adjustment


Equilibrium in the Aggregate Demand–Aggregate Supply (AD–AS) Model - Long-run equilibrium on LRAS