Productivity
Higher productivity means more output per worker — lower per-unit costs. SRAS shifts right (more output at every price level).
Chapter 48: Aggregate Demand and Aggregate Supply
Core idea
The aggregate demand–aggregate supply (AD–AS) model scales up the familiar supply-and-demand framework from a single market to the entire economy. Instead of price and quantity for one good, it maps the price level (all prices in the economy) against real GDP (all output). Where AD meets AS, the economy sits in equilibrium — at a particular combination of output and price level.
The critical complexity is that aggregate supply has two versions: a short-run version (SRAS) that responds to price level changes because wages are temporarily sticky, and a long-run version (LRAS) that is vertical — real output is ultimately determined by the productive capacity of the economy (land, labor, capital, entrepreneurship), not by prices. Understanding which curve is doing the work explains almost every macroeconomic debate about stimulus, recession, and growth.
Why it matters
Aggregate demand: what shifts it and what doesn’t
AD is the total demand for all final domestic output from all sectors: households (consumption), businesses (private investment), government (spending), and the foreign sector (net exports). Like a single-market demand curve, it slopes downward — at higher price levels, less real GDP is demanded — but for different reasons:
- Wealth effect: Higher prices erode the real purchasing power of money and financial assets; people feel poorer and spend less.
- Interest rate effect: Higher prices push up interest rates, discouraging investment and big-ticket consumer purchases.
- Export effect: A higher domestic price level makes exports expensive for foreigners, reducing net exports.
What shifts AD (moves the whole curve, independent of the price level): changes in any spending component. A surge in consumer confidence, a business investment boom, an increase in government spending, or an export pickup all shift AD rightward. Tax increases, credit tightening, and export contractions shift it leftward.
Short-run aggregate supply: why wages matter
In the short run, firms respond to a rising price level by increasing output. Why? Because wages (and other input prices like rent and long-term contracts) are sticky — they don’t adjust instantly. When prices rise but wages haven’t yet, firms’ revenues climb while their labor costs stay flat. Profit margins expand, and firms respond by hiring more and producing more.
The reverse is equally true: if prices fall unexpectedly, firms’ revenues shrink while wage costs remain, squeezing margins and inducing output cuts and layoffs.
What shifts SRAS (makes the whole curve move):
Input prices
If raw material prices, energy, or wages rise independently of the price level, per-unit costs climb and SRAS shifts left.
Regulations & taxes
Compliance costs reduce productive capacity (SRAS left). Subsidies increase it (SRAS right).
Inflation expectations
If workers expect higher future inflation, they demand higher wages now. Higher wage costs shift SRAS left before actual inflation materializes.
Long-run aggregate supply: capacity, not prices
In the long run, wages and all other input prices adjust fully to whatever the price level is. When wages catch up to inflation, the profit-margin advantage firms enjoyed in the short run disappears. Firms have no incentive to produce more output than is warranted by their actual productive capacity. Long-run real GDP is therefore independent of the price level — LRAS is vertical.
LRAS shifts only when the economy’s underlying productive capacity changes:
- Rightward (growth): More land, more workers, more capital, better technology, more entrepreneurship — any increase in the quantity or quality of factors of production.
- Leftward (decline): Factor destruction — a pandemic that kills working-age people, a natural disaster that destroys capital, an emigration wave that depletes skilled labor.
Authors’ framing: The medieval Black Death wiped out a third of Europe’s population and dramatically reduced LRAS. The steam engine dramatically increased it. Both are examples of the same mechanism: changes in LRAS represent permanent changes to the economy’s productive ceiling.
Key takeaways
Key takeaways
- AD is total demand for domestic output from all sectors (C + I + G + NX). It slopes downward via the wealth effect, interest rate effect, and export effect.
- AD shifts when any spending component changes independent of the price level: confidence, government spending, taxes, investment, or net exports.
- SRAS slopes upward because wages are sticky in the short run: rising prices → higher firm margins → more output.
- SRAS shifts when per-unit production costs change: productivity, input prices, regulations, taxes, and inflation expectations.
- LRAS is vertical — long-run real GDP is determined by productive capacity (factors of production), not by prices.
- LRAS shifts only when the quantity or quality of factors of production changes. Rightward shifts are economic growth; leftward shifts are permanent decline.
- Inflationary expectations shift SRAS left (workers demand higher wages now, raising costs before inflation materializes).
Mental model
Read it as: Three distinct supply-and-demand building blocks feed into macroeconomic equilibrium. Blue (AD) is the spending side, shifting with all the things that change how much people want to buy. Amber (SRAS) is the short-run production side — sticky wages make it upward-sloping. Green (LRAS) is the long-run capacity ceiling — vertical because, eventually, prices don’t change how much the economy can produce. All three converge at the purple equilibrium node, which Chapter 49 examines in depth.
Practical application
Using AD–AS to predict policy effects
- Identify the shock. Is it a demand shock (changes spending) or a supply shock (changes production costs or capacity)?
- Identify the curve. Demand shocks shift AD. Supply shocks that affect costs shift SRAS. Permanent capacity changes shift LRAS.
- Trace the short-run effect. When AD rises, real GDP and price level both rise (short run). When SRAS falls, real GDP falls but price level rises (stagflation).
- Trace the long-run adjustment. Rising prices eventually raise wages, shifting SRAS left until LRAS is re-established at a higher price level. The economy returns to potential GDP but at a higher price level.
- Ask: is the gap positive or negative? If current output exceeds LRAS (inflationary gap), expect rising prices and eventual SRAS pullback. If output is below LRAS (recessionary gap), expect deflationary pressure and eventual SRAS recovery — or policy intervention.
Example
A supply chain shock vs. a stimulus package
Consider two different shocks hitting the same economy:
Shock A — A global chip shortage raises production costs for every manufacturer that uses semiconductors (cars, appliances, electronics). This is a SRAS leftward shift: per-unit costs rise. Result: prices rise while output falls simultaneously — stagflation in miniature. The correct response is to address the supply constraint (invest in domestic chip production, diversify supply chains), not to stimulate demand.
Shock B — A government infrastructure package adds $500 billion in spending on roads, bridges, and broadband. This is an AD rightward shift: government spending (G) rises. In the short run, real GDP rises and the price level rises modestly. In the long run, if the infrastructure genuinely improves productivity, LRAS shifts rightward too — a double benefit. If the infrastructure spending is pure demand stimulus without productivity gains, the economy simply returns to LRAS at a higher price level.
Same economy, two different curves, two completely different diagnoses and prescriptions. The AD–AS framework makes the distinction precise.
Related lessons
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