Long-Run Aggregate Supply

Section 3.2 built SRAS on a single critical assumption: that wages and other input costs are sticky — slow to adjust in the short run. That stickiness is what makes SRAS slope upward, and it's what gives firms a temporary profit-margin boost when prices rise unexpectedly.

But "sticky" isn't the same as "permanent." Eventually, labor contracts expire. Workers see that prices have risen and demand wage increases. Suppliers renegotiate raw material contracts. Rents reset. Every input cost gets a chance to catch up. When all that adjustment is finished, we've entered what economists call the long run — a period not defined by a specific number of years, but by the fact that every price and wage in the economy has had time to fully adjust.

This shift changes everything about supply behavior. The profit-margin advantage that made SRAS upward-sloping disappears. If prices doubled, wages also doubled, and the relationship between cost and revenue is back where it started. Firms have no reason to produce more or less than they did before the price level changed. Output settles at a level determined not by prices, but by the economy's real productive capacity — its workers, machines, technology, and resources.

The result is the Long-Run Aggregate Supply (LRAS) curve — vertical instead of upward-sloping, and pinned to a specific level of output called potential GDP. This section explains why LRAS looks the way it does, what determines its position, and why it turns out to be the same concept as the PPC from Unit 1.

Long-Run Aggregate Supply tells us how much total output the economy can produce when every input price has had time to fully adjust. The answer turns out to be straightforward: the economy produces exactly the amount determined by its real productive capacity, no more and no less, regardless of the price level.

The Three Names for the Same Idea

The AP exam uses several phrases that all describe the exact same level of output. Don't let the vocabulary trip you up — they're interchangeable:

• Potential GDP (Y_f) — The level of real output produced at full employment.
• Full-employment output — The output level when only frictional and structural unemployment exist (recall the natural rate of unemployment from Section 2.2).
• Long-run equilibrium output — The level the economy gravitates toward over time, regardless of short-run fluctuations.

All three refer to the position of the LRAS curve. When economists say "the economy is at potential," "operating at full employment," or "in long-run equilibrium," they're describing the same thing: real GDP equals the level marked by the LRAS curve.

The defining feature of LRAS is its shape: a vertical line. To see why, follow what happens to the economy when the price level changes and we let enough time pass for all costs to adjust.

The Step-by-Step Argument

Imagine the economy is initially producing at potential GDP, with all prices in equilibrium. Now suppose the price level rises by 10% (perhaps because of an AD increase). Walk through what happens over time:

1. Step 1 (short run). Prices rise by 10%, but wages and other input costs are sticky. Firms see higher revenue with the same costs, so profit margins expand. Production rises temporarily above potential — this is the upward-sloping SRAS at work.
2. Step 2 (medium run). Workers notice prices have risen. They renegotiate contracts and demand higher wages. Suppliers raise their prices too. Input costs begin catching up to the higher price level.
3. Step 3 (long run). Wages and all other input costs have risen by the same 10% as the price level. Profit margins return to their original size. Firms have no incentive to produce more or less than they did before the price-level change.
4. Result. Output returns to exactly the level determined by the economy's real resources (workers, capital, technology). The 10% price change had zero permanent effect on real output. That level of output is potential GDP — the same regardless of whether the price level is high or low.

The same logic works in reverse for a falling price level: temporary profit-margin squeeze in the short run, but wages and costs eventually adjust downward too, and output returns to potential.

The Key Insight

In the long run, the price level is a nominal variable — it just affects how many dollars are exchanged. Real output depends on real things: how many workers the economy has, how much capital it possesses, what technology it can use, what natural resources are available. Doubling all prices doesn't double the number of workers or invent new factories. The economy's productive capacity is unaffected.

That's why LRAS is vertical. Any change in the price level moves us up or down along the same vertical line, but output stays pinned at Y_f. To change long-run output, you have to change the underlying real resources — and that's exactly what shifts the LRAS curve, which we cover next.

With AD and SRAS, the distinction between movement along and shifts of the curve was central. With LRAS, the distinction is much simpler — but still tested on the AP exam.

Why This Matters

The vertical shape has a powerful implication: no demand-side policy can permanently change long-run output. The Federal Reserve printing money, Congress passing a stimulus bill, foreign trade fluctuations — all of these shift AD, which moves the economy along the LRAS line (changing the price level) but doesn't shift LRAS itself. To increase long-run output, you have to change something on the supply side: invest in workers, build more capital, advance technology, find new resources.

This is the foundation of every long-run policy debate you'll encounter in Units 4 and 5. It's also why economists distinguish so carefully between short-run stabilization (managing AD around a fixed potential) and long-run growth (shifting potential itself).

Since LRAS is the economy's productive capacity, anything that changes that capacity shifts the curve. The shifters fall into four categories — and they should look familiar, because they're the four factors of production from Unit 1: labor, capital, land (natural resources), and entrepreneurship (which manifests largely through technology and innovation).

The LRAS Shift Visualized

Summary Table

Here's a payoff that ties together two units of material. The LRAS curve, the Production Possibilities Curve (PPC) from Unit 1, and "potential GDP" all describe the same economic concept — the maximum sustainable level of output the economy can produce when all resources are fully employed. They're just shown on different graphs and emphasized for different purposes.

Side-by-Side Visualization

The Same Causes Move Both

Every shifter we listed for LRAS also shifts the PPC. The mapping is exact:

• "LRAS slopes upward like SRAS, just less steeply." No — LRAS is strictly vertical. The upward slope of SRAS depends entirely on sticky wages; once wages adjust fully (the long run), the slope disappears completely. Drawing LRAS as anything other than vertical will lose you points on FRQs.
• "Monetary or fiscal policy can shift LRAS." No. Monetary policy (Fed actions) and fiscal policy (government spending and taxes) shift AD, not LRAS. They affect prices and short-run output, but they don't add workers, capital, technology, or resources to the economy. Only changes to real productive capacity shift LRAS.
• "LRAS is the unemployment-equals-zero level of output." No. LRAS is the output level when the economy operates at the natural rate of unemployment (frictional + structural, typically 4–5%). Zero unemployment isn't possible — people are always between jobs and skills are always evolving. This trap was already tested heavily in Unit 2 and reappears in Unit 3.
• "Higher prices increase long-run output." No — that's the SRAS story, and it only works because wages are sticky in the short run. In the long run, every cost catches up, profit margins normalize, and output returns to potential. The price level has zero effect on long-run output. That's literally what "vertical LRAS" means.
• "LRAS and PPC are different concepts." Wrong. They're two graphical representations of the same economic reality — the maximum sustainable output of the economy. Whatever shifts one shifts the other.
• "Population growth always shifts LRAS right." Usually, but watch the context. If population growth outpaces capital accumulation, output per worker can actually fall. For the AP exam, treat population growth as a rightward LRAS shifter unless the question explicitly contrasts it with capital growth.
• "A change in AD eventually shifts LRAS." No. AD changes the price level and short-run output, but the economy self-corrects back to the same LRAS. AD shifts move along LRAS; they don't shift it. The only way to permanently raise output is to shift LRAS itself by adding to productive capacity.
• "Low real interest rates shift LRAS right." Be careful. Low rates encourage investment, which builds capital, which does shift LRAS right — but the channel is the increase in capital stock, not the rates themselves. The interest rates affect AD directly and LRAS only indirectly (and only over years).