Economists agree on the short run. They disagree, quite sharply, about the long run. Classical economists draw the long-run supply curve as a vertical line; Keynesians draw it as an L on its side. That difference is not just drawing style — it decides whether governments should step in during a recession or stay out of the way.
📚 What you need to know
The classical (monetarist / new classical) LRAS is vertical at the full employment level of output, YFE.
Classical view: markets self-correct, so in the long run only the average price level changes, never long-run output.
The Keynesian AS curve has three sections: perfectly elastic (flat), then upward sloping, then perfectly inelastic (vertical).
Keynesian view: an economy can get stuck in equilibrium well below full employment for years.
The disagreement comes down to one thing: are wages flexible downwards?
Classical thinking favours supply-side policy; Keynesian thinking favours government spending to lift AD.
The two shapes, side by side
Both curves become vertical at full employment. The argument is about everything to the left of that point.
The classical view
Classical economists start from one assumption: wages and prices are flexible, both up and down. If output falls below capacity, workers are laid off, unemployed workers accept lower pay, firms’ costs fall, SRAS shifts right, and the economy slides back to YFE on its own. All that has changed is a lower price level.
The same works in reverse. If AD is pushed above capacity, resources become scarce, wages get bid up, costs rise, SRAS shifts left, and output returns to YFE at a higher price level.
🧩 The classical chain of reasoning
AD falls and output drops below YFE.
Firms need fewer workers, so they lay people off.
Unemployed workers accept lower wages to get hired again.
Lower wages mean lower costs of production, so SRAS shifts right.
Output returns to YFE, but at a lower average price level.
Conclusion: unemployment is temporary, so no government intervention is needed.
Because output always ends up at YFE, classical economists argue that demand-side policy only moves prices around. If you want real growth, you must move the vertical line itself — that means supply-side policy.
The Keynesian view
Keynes accepted the top of that story but rejected the middle of it. His point was blunt: wages are sticky downwards. Minimum wage laws, trade unions and long contracts stop pay from falling. If wages will not fall, costs will not fall, SRAS will not shift right, and the economy simply sits there with high unemployment.
Section of the Keynesian curve
What is happening in the economy
Effect of a rise in AD
1. Perfectly elastic (flat)
Deep spare capacity: idle factories, high unemployment
Output rises, prices do not
2. Upward sloping
Spare capacity is running out; firms start bidding for scarce resources
Output rises and prices start to rise
3. Perfectly inelastic (vertical)
Full employment: every resource is already in use
Prices rise, output cannot
The three sections are really one sensible idea. When there are lots of unused workers and machines, extra demand is easy to meet. When there are none left, extra demand can only push prices up. Everything between the two is a mix of the two.
Why the flat bit exists. Prices cannot fall below a floor because wages cannot. That floor is set by minimum wage laws, union agreements and long-term contracts — and it is the whole reason the Keynesian curve is L shaped rather than vertical.
Why the disagreement matters
Question
Classical answer
Keynesian answer
Are wages flexible downwards?
Yes, given time
No, they are sticky
Does the economy self-correct?
Yes, automatically
Not reliably, and possibly not for years
What should the government do in a recession?
Little — focus on the supply side
Spend, to shift AD right and restore confidence
Can demand-side policy raise long-run output?
No, only the price level
Yes, while spare capacity exists
What is the main risk of doing nothing?
Little risk; markets fix it
Years of lost output and long-term unemployment
Keynes’s famous line about the long run — that we are all dead before it arrives — captures the practical objection. Even if markets do fix themselves eventually, a decade of mass unemployment is a real cost paid by real people.
Worked example
WORKED EXAMPLE
Same policy, two predictions
A government increases spending by 4% of GDP during a deep recession. Explain how the classical and Keynesian models predict different outcomes.
Step 1: what the policy doesHigher G raises AD, so the AD curve shifts right in both models.Step 2: the classical predictionAD shifts right along a vertical LRASOutput was heading back to YFE anyway, so the main lasting effect is a higher average price level — and possibly higher government debt for nothing.Step 3: the Keynesian predictionIn a deep recession the economy sits on the flat sectionSpare capacity means output can rise with almost no rise in prices, so unemployment falls sharply.Same shift right of AD, opposite conclusionsEvaluation point: the answer depends on how much spare capacity there really is.
💡 Exam tip
Read the command term. If it says “using a Keynesian diagram”, a vertical LRAS scores zero for the diagram.
On a Keynesian diagram, where you draw AD matters. Put it on the flat section for a recession, near the vertical section for an overheating economy.
The strongest evaluation line is: it depends how much spare capacity there is.
Label the vertical section YFE on both diagrams — both models agree there is a maximum.
Link the model to the policy. Classical thinking leads to supply-side policy; Keynesian thinking leads to demand-side policy.
Mention why wages are sticky — minimum wages, unions, contracts. That is the heart of the debate.
⚠️ Common mix-up
Thinking Keynesians deny a maximum output. They do not — their curve also goes vertical at full employment.
Drawing SRAS on a Keynesian diagram. The Keynesian AS curve already contains the short run in its flat and rising sections.
Saying classical economists want no government at all. They want government focused on the supply side, not absent.
Confusing “long run” with “a long time”. In this model the long run is defined by wage flexibility, not by a calendar.
Putting AD on the vertical section then claiming output rises. On that section, output cannot rise — only prices.
Treating one model as correct. Examiners want you to weigh both.
Up next: What Shifts Long-Run Aggregate Supply — the four things that genuinely raise an economy’s productive capacity.
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