# Aggregate Supply

> IB Economics Higher Level · IB Economics HL
> Source: https://www.owlsprep.com/study/ib-economics-hl-u3-aggregate-supply/

This subtopic explains aggregate supply (AS), the key difference between short-run and long-run AS, factors that shift AS curves, and the role of AS in the AS-AD macroeconomic model. It is core for all IB HL macroeconomics analysis.

**Prerequisites:** [Aggregate Demand](https://www.owlsprep.com/study/ib-economics-hl-u3-aggregate-demand/); [AS-AD Model Basics](https://www.owlsprep.com/study/ib-economics-hl-u3-as-ad-model/)

## Learning objectives

- Distinguish between short-run and long-run aggregate supply
- Explain determinants of aggregate supply shifts
- Draw and interpret aggregate supply curves
- Analyze how AS shifts impact macroeconomic outcomes

## Core Definition of Aggregate Supply

**Aggregate Supply** — The total quantity of goods and services that all firms in an economy are willing and able to produce at each possible price level, over a given time period.

*Notation:* AS

*Example:* At higher price levels, ceteris paribus, firms are generally willing to supply more output in the short run.

Aggregate supply is an economy-wide concept, not a simple sum of all individual microeconomic supply curves. It describes the relationship between the overall price level and total national output, and its shape depends on the time horizon we consider: short run vs long run.

**Worked example:** A student claims 'Aggregate supply is just the sum of all individual supply curves for all goods in the economy.' Is this correct? Explain.

1. First, recall that aggregate supply accounts for economy-wide interactions that do not exist at the micro level.
2. A price increase for one good increases its supply, but a price increase for *all* goods in the economy involves proportional changes in input prices that change the relationship between price and output.
3. Conclusion: The claim is incorrect. Aggregate supply describes the overall relationship between the aggregate price level and total output, not a simple sum of micro supply curves.

## Short-Run Aggregate Supply (SRAS)

**Short-Run Aggregate Supply** — The relationship between the price level and total output when nominal wages and other input prices are sticky (slow to adjust to changes in the price level). The SRAS curve is upward sloping.

*Notation:* SRAS

The upward slope of SRAS is explained by sticky wage theory: when the price level rises, nominal wages do not adjust immediately, so production costs rise more slowly than output prices. Firms earn higher profits per unit, so they increase production, leading to higher total output.

- Changes in nominal wages
- Changes in commodity/input prices
- Temporary productivity shocks
- Changes in business taxes and subsidies

**Worked example:** A major global oil price increase raises production costs across all sectors of the economy. How does this impact SRAS? What is the effect on equilibrium output and price level if AD is unchanged?

1. An increase in input prices reduces the quantity of output firms are willing to produce at every price level. This causes a leftward shift of the SRAS curve, not a movement along it.
2. $$\text{Original: } SRAS_1, \text{New: } SRAS_2 \text{ left of } SRAS_1$$
3. With unchanged AD, the new equilibrium intersects at a lower level of real GDP ($Y_2 < Y_1$) and a higher price level ($PL_2 > PL_1$).

## Long-Run Aggregate Supply (LRAS)

**Long-Run Aggregate Supply** — The relationship between the price level and total output when all nominal prices and wages are fully flexible, and the economy operates at full employment (potential output). LRAS is vertical at potential GDP $Y_p$.

*Notation:* LRAS

In the long run, wages and prices adjust fully to changes in the price level. If prices double, wages also double, so production costs rise proportionally, so there is no incentive for firms to change output. Potential output depends on the quantity and quality of factors of production, not the price level.

- Changes in the size of the labor force
- Changes in the stock of physical/human capital
- Improvements in technology
- Institutional changes (e.g. labor market reform)
- Discovery of new natural resources

**Worked example:** A country invests heavily in primary education, leading to a permanent increase in the skill level of its labor force. How does this impact LRAS? Explain the effect on long-run output.

1. An increase in the quality of labor (human capital) permanently increases the economy's potential output $Y_p$, the maximum sustainable output it can produce.
2. This causes a rightward shift of the vertical LRAS curve:
3. $$Y_{p2} > Y_{p1}, \text{ LRAS}_2 \text{ right of } \text{ LRAS}_1$$
4. At the new long-run equilibrium, the economy operates at a higher sustainable level of real output, with no permanent increase in inflation if AD remains unchanged. This represents long-run economic growth.

## Distinguishing SRAS and LRAS Shifts

A common exam confusion is identifying which AS curve shifts for a given change. Temporary changes only impact SRAS, while permanent changes to potential output impact both SRAS and LRAS. For example, a temporary drought shifts only SRAS, while permanent new technology shifts both SRAS and LRAS.

| Property | SRAS | LRAS |
| --- | --- | --- |
| Curve Shape | Upward sloping | Vertical at $Y_p$ |
| Key Assumption | Nominal wages are sticky | All wages are fully flexible |
| Output Level | Can be above/below $Y_p$ | Always equal to $Y_p$ |
| Time Horizon | 1-2 years | 3+ years |

**Worked example:** A country introduces a permanent cut in corporate income taxes for all firms. Will this shift SRAS, LRAS, both, or neither? Explain.

1. A permanent corporate tax cut increases firms' incentives to invest in capital, improving productivity permanently.
2. The cut also reduces production costs for firms in the short run, so it shifts SRAS right.
3. Because the change permanently increases potential output, it also shifts LRAS right.
4. Conclusion: Both SRAS and LRAS shift rightward.

**Exam command terms**

- **Distinguish between SRAS and LRAS** — State at least two clear differences in assumptions and shape, do not just define both *(One difference: SRAS assumes sticky wages, while LRAS assumes fully flexible wages.)*

- **Draw an AS shift** — Label axes, original/new curves clearly, and explicitly state if it is SRAS or LRAS *(Always label potential output $Y_p$ when drawing LRAS.)*

## Common pitfalls

- **Wrong:** Confusing movement along SRAS with a shift of SRAS
  - Why it fails: A change in the price level causes a movement along SRAS, only changes in non-price determinants cause shifts
  - Correct: Always check if the change affects the price level (movement) or production capacity/costs (shift)
- **Wrong:** Claiming LRAS is vertical because potential output never changes
  - Why it fails: LRAS is vertical because output does not depend on the price level, not because potential output is fixed
  - Correct: LRAS is vertical at the *current* level of potential output, which shifts right as the economy grows
- **Wrong:** Assuming a change that shifts LRAS never shifts SRAS
  - Why it fails: Any permanent change that shifts LRAS almost always shifts SRAS in the same direction too
  - Correct: If a change shifts LRAS, it shifts SRAS the same direction unless told otherwise
- **Wrong:** Drawing an upward-sloping LRAS curve
  - Why it fails: This confuses short-run and long-run assumptions, and will lose marks in IB exams
  - Correct: Always draw LRAS as a vertical line at potential output $Y_p$
- **Wrong:** Claiming a negative supply shock increases output and reduces prices
  - Why it fails: Negative supply shocks shift SRAS left, not right, reversing the outcome
  - Correct: Negative (adverse) shocks shift AS left; positive shocks shift AS right

## Cheatsheet

| Concept | Shape | Key Impact |
| --- | --- | --- |
| SRAS | Upward-sloping | Shifts from input price changes, temporary shocks |
| LRAS | Vertical at $Y_p$ | Shifts from changes in potential output |
| Adverse SRAS shock | SRAS left | Lower output, higher price level (stagflation) |
| Positive LRAS shift | LRAS right | Higher $Y_p$, sustainable long-run growth |

## What's next

Aggregate supply is a core foundation for analyzing all key IB HL macroeconomics topics, from supply-side policies and long-run economic growth to business cycles and inflation. Aggregate supply interacts with aggregate demand to determine all macroeconomic equilibrium outcomes. After mastering AS, you will next explore how combined shifts of AS and AD create different macroeconomic outcomes, then learn how supply-side policies are designed to shift LRAS right to increase long-run growth and reduce inflation.

- [Macroeconomic Equilibrium](https://www.owlsprep.com/study/ib-economics-hl-u3-macroeconomic-equilibrium/)
- [Economic growth](https://www.owlsprep.com/study/ib-economics-hl-u3-economic-growth/)
- [Unemployment and employment](https://www.owlsprep.com/study/ib-economics-hl-u3-unemployment-and-employment/)

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