Chapter 9 · Aggregate demand and aggregate supply
Keynesian aggregate supply
Explore the horizontal, upward-sloping and vertical sections of Keynesian AS. Explain how spare capacity changes the effects of higher AD.
The economic idea
The Keynesian model emphasises that an economy can remain below full employment because aggregate demand is insufficient. Nominal wages and prices may be sticky downward, so market forces do not necessarily restore full employment quickly or automatically.
The Keynesian aggregate supply curve shows the relationship between the average price level and the real output firms produce. It has three stylised sections because the availability of idle resources changes as production expands.
On the horizontal section, extensive spare capacity allows firms to raise output without bidding strongly against one another for scarce labour and inputs. An increase in aggregate demand therefore raises real output with little or no increase in the price level.
On the upward-sloping section, some industries approach capacity before others. Bottlenecks, overtime and less-suitable resources raise marginal costs, so stronger aggregate demand increases both real output and the price level.
On the vertical section, the economy is at maximum full-employment output. All suitable resources are fully employed at normal sustainable rates, so an increase in aggregate demand raises the price level but cannot increase real output.
Full-employment output is the maximum sustainable output shown by the vertical section. It is a capacity benchmark, not a claim that every person has a job or that structural and frictional unemployment are zero.
Reading recessionary equilibrium on the upward-sloping section
“Price level” measures the economy-wide price level and “Real GDP” measures real GDP.
“AD” shows total planned expenditure at each price level. “Keynesian AS” shows how much real output firms can produce across different degrees of spare capacity.
“E” is where “AD” intersects “Keynesian AS”, determining “PLₑ” and “Yₑ” in the current graph.
Why Keynesian AS changes shape
The horizontal part of “Keynesian AS” represents extensive spare capacity, the curved middle represents increasingly binding bottlenecks, and the vertical part represents maximum full-employment output. The curve therefore embeds a changing output response rather than one constant slope.
Current equilibrium: E
“E” lies on the upward-sloping section of “Keynesian AS” and to the left of maximum capacity. The economy has a recessionary gap because actual output “Yₑ” is below full-employment output.
What a change in aggregate demand would do here
Near “E”, a rightward shift of “AD” would raise both real output and the price level because spare capacity remains but bottlenecks are becoming important.
Why the same spending increase has different effects
With many idle resources, extra orders can be met by using existing workers and equipment more fully. As spare capacity disappears, firms compete for scarce inputs and costs rise. At the vertical capacity limit, extra spending cannot add real output in this model.
When comparing outcomes, identify the section containing the equilibrium first. The shape of AS explains why the balance between output growth and price pressure changes.
Trace demand through the section of AS that is actually visible
Locate “E” where “AD” intersects “Keynesian AS”; do not infer the state from the diagram title alone.
Compare “Yₑ” with the vertical capacity section. Because equilibrium is to its left, a recessionary gap and involuntary cyclical unemployment are present.
A change in planned expenditure is divided between real output and the price level as sectoral bottlenecks appear.
Evaluate the result using the size of spare capacity, the multiplier, import and tax leakages, confidence, time lags and possible changes in productive capacity.
Common mistakes to avoid
- The horizontal section of “Keynesian AS” does not mean every individual price is fixed; it represents little change in the average price level while aggregate output expands from deep spare capacity.
- Full employment does not mean zero unemployment. Frictional and structural unemployment may remain.
- A point on the upward-sloping section can still show a recessionary gap because output remains below the vertical capacity line.
- An increase in aggregate demand at full employment creates inflationary pressure, not long-run economic growth.
- The Keynesian AS curve is a stylised model. Real economies do not have perfectly sharp boundaries between all three ranges.
How to draw this diagram
- Title the diagram “Keynesian aggregate supply”. Label the horizontal axis “Real GDP” and the vertical axis “Price level”.
- Draw AD first and label it clearly.
- Draw Keynesian AS next and label it clearly.
- Mark each relevant intersection and distinguish the initial, market and socially optimum or policy outcomes where applicable.
- Project important intersections to the axes with dashed guide lines and label the outcomes.
- Check that every symbol used in the explanation appears on the diagram and add arrows showing the direction of change.
Assumptions and limitations
- The three sections are stylised and their boundaries cannot be observed with certainty in real time.
- Spare capacity can coexist with sector-specific shortages, so inflation may appear before national full employment.
- The curve does not show how expectations, exchange rates or imported inflation alter the price response.
- Demand stimulus can change productive capacity through investment or hysteresis, rather than moving only along a fixed AS curve.
- The model aggregates distributional effects: households can experience very different employment and inflation outcomes.