Aggregate Supply and Aggregate Demand

The whole-economy version of supply and demand explains how output, prices, and the business cycle all move together.

The supply and demand model covered in an earlier module explains a single market - the market for coffee, or wheat, or concert tickets. This lesson scales that same basic idea up to the entire economy at once, using aggregate demand and aggregate supply.

Demand and supply for everything, all at once

Aggregate demand represents the total quantity of goods and services that all buyers in an economy - households, businesses, government, and foreign buyers combined - are willing to purchase at each overall price level. Like an individual demand curve, it generally slopes downward: at a lower overall price level, the total quantity of goods and services demanded across the economy tends to be higher.

Aggregate supply represents the total quantity of goods and services that all producers in an economy are willing to supply at each overall price level. In the short run, it generally slopes upward, similar to an individual firm’s supply curve, covered in an earlier module - a higher overall price level makes production more profitable across the economy, encouraging more total output, at least until the economy runs into capacity constraints.

Just as an individual market settles at the price where its supply and demand curves intersect, the whole economy settles at a macroeconomic equilibrium - a combination of the overall price level and total real GDP where aggregate demand equals aggregate supply.

Tracing a demand shock through the whole economy

Imagine consumer confidence drops sharply, and households across the economy cut back spending simultaneously - a leftward shift of the entire aggregate demand curve, not a movement along it, echoing the shift-versus-movement distinction from the supply and demand module. With less total spending chasing the same productive capacity, businesses across many industries see falling sales, output falls, and unemployment rises - the beginning of a recession, covered in the previous lesson. This single shift in aggregate demand ripples through nearly every individual market in the economy at once, which is exactly the kind of broad, simultaneous effect that distinguishes macroeconomic analysis from analyzing one market in isolation.

What shifts the aggregate curves

Aggregate demand shifts with changes in consumer and business confidence, interest rates, government spending, and international demand for a country’s exports. Aggregate supply shifts with changes in production costs (like energy or wage costs), available technology, and the size and productivity of the workforce. Distinguishing which curve has shifted - and in which direction - is essential for correctly diagnosing what kind of inflation or downturn an economy is experiencing.

Assuming every downturn and every inflation has the same underlying cause

A common oversimplification is treating every recession, or every bout of inflation, as though it has the same underlying cause and therefore calls for the same policy response. A downturn driven by falling aggregate demand looks different, and calls for different tools, than one driven by a negative aggregate supply shock, like a sudden spike in energy costs. Similarly, demand-pull and cost-push inflation, covered in the earlier lesson on inflation, map directly onto shifts in aggregate demand versus aggregate supply respectively - and the two can require genuinely different, sometimes even opposite, policy responses, which is exactly what makes stagflation, covered later in this module, such a difficult case for policymakers.

Why this model ties the module together

The aggregate supply and demand model is the macroeconomic engine behind most of the concepts covered elsewhere in this module - it explains why the business cycle happens, why inflation and unemployment sometimes move in opposite directions and sometimes don’t, and why economic indicators, covered in a later lesson, are watched so closely for early signs of a shift in either curve.

Key takeaways

Aggregate demand is the total quantity of goods and services all buyers want at each overall price level.
Aggregate supply is the total quantity of goods and services all producers offer at each overall price level.
Macroeconomic equilibrium occurs where aggregate demand equals aggregate supply, setting the overall price level and total output.
Aggregate demand shifts with confidence, interest rates, government spending, and export demand.
Aggregate supply shifts with production costs, technology, and workforce size and productivity.
Whether a downturn stems from a demand shift or a supply shift affects which policy response actually makes sense.