Intermediatemacroeconomics
AD-AS Equilibrium: Where Demand Meets Supply
Overview
The aggregate demand–aggregate supply model is macroeconomics' central workhorse: where AD meets AS, the economy's price level and output are determined. This lesson builds short-run and long-run equilibrium, traces how demand and supply shocks ripple through, and shows the economy's self-correcting path back to potential.
Learning objectives
- Identify short-run equilibrium at the AD–SRAS intersection.
- Identify long-run equilibrium where AD, SRAS, and LRAS all meet.
- Trace the effects of a demand shock in the short run and long run.
- Trace the effects of a supply shock in the short run and long run.
- Explain the self-correcting mechanism that returns output to potential.
Short-Run Equilibrium
Short-run equilibrium occurs where the aggregate demand curve intersects the short-run aggregate supply curve. That intersection pins down two things at once: the equilibrium price level and the equilibrium level of real GDP. At this point, the quantity buyers want equals the quantity firms will produce — plans are consistent. Crucially, short-run equilibrium output need not equal potential output. The economy can overheat above potential or languish below it — the gaps we call booms and recessions.
Long-Run Equilibrium
Long-run equilibrium is the special case where AD, SRAS, and LRAS all intersect at a single point. Output equals potential output, unemployment equals its natural rate, and the economy is at full employment. Here the price level is stable in the sense that no output gap is pushing it up or down. This is the economy's resting point — the destination toward which market forces pull it. Policy debates often reduce to how fast, and at what cost, the economy returns here after a shock.
Demand Shocks: Boom, Then Adjustment
Start in long-run equilibrium and shift AD right (say, a big tax cut). Short-run equilibrium moves to higher output and a higher price level — a boom with inflationary pressure. But output above potential strains labor markets: wages rise, pushing SRAS left as production costs climb. The economy slides up the AD curve until it returns to potential output — at a permanently higher price level. Lesson: demand stimulus raises output temporarily but prices permanently; in the long run, money is neutral.
A rightward AD shift: short-run boom (higher output + prices), then SRAS shifts left as wages adjust — back to potential output at a higher price level.
Supply Shocks: Stagflation, Then Recovery
Now shift SRAS left from long-run equilibrium (an energy crisis). The new short-run equilibrium has lower output and a higher price level — stagflation. With output below potential, unemployment exceeds its natural rate, putting downward pressure on wages. As wages fall (or grow slowly), production costs ease and SRAS drifts back right toward LRAS. The economy self-corrects to potential output — but at a higher price level than before, and the adjustment can take years of painful slack.
The Self-Correcting Economy — and Its Limits
The self-correction logic is elegant: output gaps move wages and prices, shifting SRAS until the gap closes. But 'eventually' can be a long time — wages are sticky downward, and prolonged slumps can damage potential output itself (hysteresis: laid-off workers lose skills, firms scrap investment). This is the Keynesian case for active policy: don't wait years for self-correction when stimulus can close the gap now. The classical reply: stimulus risks overshooting into inflation. AD-AS doesn't settle this debate — it gives both sides a common language.
Key points
- Short-run equilibrium: AD intersects SRAS; output can differ from potential.
- Long-run equilibrium: AD, SRAS, and LRAS intersect; output = potential.
- Demand shock: short-run output and price moves; long-run return to potential at a new price level.
- Supply shock: stagflation short-run; self-correction via wage adjustment long-run.
- Output gaps shift SRAS over time — the self-correcting mechanism.
- Sticky wages and hysteresis can make self-correction slow and costly — the case for policy.
Common misconceptions
✓ Truth: Short-run equilibrium just means plans are consistent — it can sit far below potential in a deep recession. Only long-run equilibrium implies full employment.
✓ Truth: Wage stickiness makes adjustment slow, and long slumps can reduce potential itself (hysteresis). 'Long run' can be painfully long.
✓ Truth: The model shows stimulus closes demand-driven gaps but risks inflation near potential — and can't fix supply constraints. It frames the debate; it doesn't end it.
Practice questions
Q1. Short-run equilibrium in the AD-AS model is found where…
- AD intersects SRAS
- AD intersects LRAS only
- SRAS intersects LRAS only
- demand equals zero
The AD–SRAS intersection determines the current price level and real GDP. LRAS enters only for the long-run equilibrium.
Q2. After a rightward AD shift, the long-run result is…
- permanently higher output
- output back at potential with a higher price level
- permanently lower prices
- a leftward shift of LRAS
Wage adjustment shifts SRAS left until output returns to potential — the demand shock leaves output unchanged long-run but prices permanently higher.
Q3. A leftward SRAS shift (supply shock) initially causes…
- higher output and lower prices
- lower output and higher prices
- higher output and higher prices
- no change
The new AD–SRAS intersection sits at lower real GDP and a higher price level — the stagflation combination.
Q4. Hysteresis means that…
- wages adjust instantly
- a prolonged slump can permanently reduce potential output
- the economy always self-corrects in months
- supply shocks are impossible
Skill decay and cancelled investment during long downturns can shift LRAS left — the recession scars the economy's capacity itself.
Click a question to reveal the answer and explanation.
Frequently asked questions
What is the difference between a movement and a shift in AD-AS?
Shifts of AD or SRAS change the curves and create new equilibria. Movements along the curves then trace the adjustment path — e.g., sliding up AD as SRAS shifts left during stagflation adjustment.
How does monetary policy appear in AD-AS?
Rate cuts shift AD right (cheaper borrowing lifts investment and consumption); rate hikes shift AD left. Policy works through the demand side in this model.
What is hysteresis?
When a prolonged slump damages the economy's capacity itself — workers' skills decay, investment is cancelled — so potential output (LRAS) shifts left. The downturn leaves a permanent scar.
Does AD-AS work for an open economy?
Yes, with extensions: exchange rates and foreign demand enter through net exports in AD, and import prices enter through SRAS costs. The core logic is unchanged.