Difficulty: Intermediate | Prerequisites: Aggregate demand (Topic 3.1), production possibilities curve (Unit 1).
Topics 3.1 and 3.2 covered the demand side. Now you are looking at the supply side: how much are all firms in the economy collectively willing and able to produce? The critical distinction here is between the short run and the long run, which in macroeconomics hinges on whether input prices (especially wages) can adjust to changes in the overall price level. This distinction drives the shape of two different supply curves and underpins everything that follows in the AD-AS model. If you studied the production possibilities curve in Unit 1, the long-run aggregate supply curve is essentially the same idea expressed differently.
Short-run aggregate supply (SRAS) slopes upward because input prices are sticky, so firms produce more when the price level rises. Long-run aggregate supply (LRAS) is a vertical line at full-employment output because, given enough time, all input prices adjust and output returns to its maximum sustainable level. Different factors shift each curve.
Aggregate Supply
The total quantity of final goods and services that all firms in an economy are willing and able to produce at each price level. Think of it as "everything every firm is prepared to sell, at various price levels."
Short-Run Aggregate Supply (SRAS)
The aggregate supply curve in a period where wages and resource prices are sticky, meaning they do not adjust quickly to changes in the price level. The SRAS slopes upward: when the price level rises but input costs stay fixed, firms earn higher profit margins and produce more.
Long-Run Aggregate Supply (LRAS)
The aggregate supply curve in a period where wages and resource prices have fully adjusted to the price level. The LRAS is a vertical line at the economy's full-employment output (also called potential output or Yf). In simple terms, once wages catch up to prices, the economy produces the same quantity regardless of the price level.
Sticky Wages / Sticky Prices
The observation that wages and some input prices do not adjust immediately when the overall price level changes. Contracts, menu costs, and negotiation lags keep them fixed in the short run. This stickiness is the entire reason the SRAS slopes upward rather than being vertical.
Full-Employment Output (Yf or Q_Y)
The level of real GDP the economy produces when all resources are being used at their normal, sustainable rate. This is not zero unemployment; it is the natural rate of unemployment (frictional + structural only). The LRAS curve sits at this output level.
In the short run, if the price level rises, firms receive higher prices for their output.
Because wages and resource costs are sticky (locked in by contracts or slow to renegotiate), firms' costs do not rise immediately.
The gap between rising revenue and fixed costs means higher profits, which encourages firms to produce more.
The reverse holds: a falling price level squeezes profit margins against sticky costs, and firms cut production.
In the long run, all input prices adjust. Workers renegotiate wages, suppliers adjust resource prices.
Once input costs have fully caught up with the price level, firms are back to the same real profit margins they had before.
Output returns to the full-employment level regardless of whether the price level is high or low.
The LRAS is drawn as a vertical line at Yf (full-employment GDP).
Anything that changes production costs for firms shifts the SRAS curve:
Change in resource prices: A rise in oil prices, wages, or raw material costs shifts SRAS left (decreases supply). A fall shifts it right.
Change in government actions: New regulations or taxes on producers increase costs (shift left). Subsidies or deregulation reduce costs (shift right).
Change in productivity: If firms can produce more output per unit of input (e.g. through better technology or processes), SRAS shifts right. A decline in productivity shifts it left.
The LRAS shifts when the economy's productive capacity changes. These are the same factors that shift the production possibilities curve (PPC):
Change in resource quantity or quality: More workers, more capital, discovery of new natural resources, or better-educated labour all shift LRAS right.
Change in technology: Technological advances allow more output from the same inputs, shifting LRAS right.
A rightward shift in LRAS represents long-run economic growth, meaning the economy can sustainably produce more than before.
There are no formulas for this section, but diagram literacy is essential.
SRAS: Upward-sloping curve. Price level on the vertical axis, real GDP on the horizontal axis.
LRAS: Vertical line at Yf. Same axes.
When both are drawn together with AD, you get the full AD-AS model (covered in Topics 3.5–3.6).
The 1970s oil crises are the textbook example of an SRAS shift. When OPEC restricted oil supply, production costs surged across nearly every industry, shifting SRAS to the left. Output fell and prices rose simultaneously, a combination called stagflation. On the LRAS side, investment in education, infrastructure, and technology is how countries shift their long-run productive capacity outward over decades.
Students often think "short run" and "long run" refer to specific calendar periods (e.g. one year vs. five years). They do not. The short run is defined by sticky wages and resource prices; the long run is when those prices have fully adjusted. The actual time this takes varies.
Students sometimes believe the LRAS curve can slope. It cannot, by definition. If wages and resource prices have fully adjusted, output is determined by real productive capacity, not the price level.
Confusing SRAS shifters with AD shifters is a common exam error. A change in consumer confidence shifts AD. A change in oil prices shifts SRAS. These are different curves, different shifters.
Students forget that the LRAS shifters are the same as the PPC shifters. If it would shift the PPC outward (more resources, better technology), it shifts LRAS right.
⚠️ The distinction between short-run and long-run is the backbone of the AD-AS model. Expect at least one free-response question that depends on whether wages are sticky or flexible.
⚠️ You must be able to list the shifters of SRAS and LRAS separately, and explain why they are different. SRAS shifts with changes in production costs; LRAS shifts with changes in productive capacity.
⚠️ Correctly identifying whether a given event shifts AD, SRAS, or LRAS is one of the most common multiple-choice question types.
⚠️ When the exam asks about economic growth, it is asking about a rightward shift of LRAS (and PPC), not a movement along a curve.
True or False: The SRAS curve slopes upward because wages adjust quickly to price level changes.
The LRAS curve is a ______ line at ______.
True or False: An increase in oil prices shifts the LRAS curve to the left.
Name two factors that shift LRAS to the right.
Fill in the blank: In the short run, wages and resource prices are ______.
Answers: 1. False (it slopes upward because wages are sticky and do NOT adjust quickly). 2. Vertical; full-employment output (Yf). 3. False (it shifts SRAS to the left; LRAS is shifted by changes in productive capacity, not input costs). 4. Any two of: increase in resource quantity/quality, improvement in technology. 5. Sticky.
Q: Explain why the short-run aggregate supply curve slopes upward.
A: In the short run, wages and resource prices are sticky and do not change when the price level changes. When the price level rises, firms receive higher revenue while their input costs remain fixed, increasing profit margins and incentivising greater production. This positive relationship between the price level and real output gives SRAS its upward slope.
Q: Why is the long-run aggregate supply curve vertical?
A: In the long run, all wages and resource prices fully adjust to changes in the price level. Once input costs have caught up, real profit margins return to normal and firms produce the same quantity of output regardless of the price level. Output is determined solely by the economy's productive capacity.
Q: A major earthquake destroys a significant portion of a country's infrastructure. Which curve(s) shift, and in which direction?
A: The LRAS shifts to the left because the economy's productive capacity (capital stock) has decreased. SRAS may also shift left if the destruction raises production costs. AD could also be affected, but the primary and most testable impact is on LRAS.
Q: Distinguish between a factor that shifts SRAS and a factor that shifts LRAS.
A: A rise in energy prices increases production costs for firms, shifting SRAS to the left (supply-side cost change). An increase in the labour force expands the economy's productive capacity, shifting LRAS to the right (capacity change). Cost changes shift SRAS; capacity changes shift LRAS.
The SRAS/LRAS distinction feeds directly into the AD-AS equilibrium model (Topic 3.5) and explains how the economy self-corrects in the long run (Topic 3.7). The LRAS shifters are identical to the PPC shifters from Unit 1, so if you understand one, you understand both. Monetary policy (Unit 4) also affects the economy through its impact on AD relative to SRAS and LRAS.
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