The Aggregate Demand-Aggregate Supply Model
Two Views on What Drives the Macroeconomy
Economists have long debated whether the size of an economy is mainly determined by how much it can supply or by how much people want to spend. One long-standing view, often summarized as the claim that supply creates its own demand, holds that an economy’s total output is set by its productive capacity, meaning the labor, capital, and technology available to it, and that spending will naturally follow whatever gets produced. A contrasting view, developed most forcefully during a period of prolonged mass unemployment in the early twentieth century, holds that in the short run the level of total spending in the economy, not its productive capacity, determines how much actually gets produced, because factories and workers can sit idle if demand is too weak even when the physical capacity to produce more clearly exists. Neither view fully explains macroeconomic behavior on its own: the supply-focused view fits the long run reasonably well, once wages and prices have time to adjust, while the demand-focused view fits the short run, when many firms are constrained more by weak orders than by any physical limit on what they could produce. The aggregate demand-aggregate supply model combines both forces into a single framework for analyzing how real output and the overall price level are jointly determined.
The Aggregate Supply Curve
The aggregate supply curve shows the total quantity of real output, meaning GDP adjusted for inflation, that firms across the economy are willing to produce and sell at each possible price level, holding the prices of inputs like labor and energy fixed. The curve slopes upward because, when the price level for finished goods rises while input costs stay the same, firms find it more profitable to expand production, so a higher price level for outputs is associated with a greater quantity of real GDP supplied. The curve is drawn to eventually cross a vertical line marking potential GDP, the maximum sustainable output the economy can produce when labor and capital are fully employed; short-run output can temporarily exceed potential GDP if firms push workers and machinery to unsustainable extremes, such as running double shifts around the clock, but such intensity cannot be maintained indefinitely. Near potential GDP, the aggregate supply curve becomes steep, since even sharply higher prices cannot draw out much additional output once labor and capital are already fully employed; this segment is often called the short-run aggregate supply curve, in contrast to the vertical long-run aggregate supply curve positioned exactly at potential GDP, which reflects the economy’s productive limits once wages and prices have fully adjusted.
The Aggregate Demand Curve
The aggregate demand curve shows the total planned spending on domestic goods and services at each price level, combining consumption, investment, government spending, and net exports, the difference between exports and imports. The curve slopes downward for three main reasons. First, a wealth effect: as the price level rises, the real purchasing power of savings people hold in bank accounts erodes, so consumption spending tends to fall. Second, an interest rate effect: as prices rise, more money is needed to finance the same purchases, which pushes interest rates higher and discourages borrowing for investment and big household purchases. Third, a foreign price effect: if domestic prices rise while prices abroad stay fixed, domestically made goods become relatively more expensive to foreign buyers, reducing exports, while imports become relatively cheaper and rise, both of which reduce net exports. Suppose a small island economy called Meridia sees its overall price level climb sharply in a single year; households there would likely feel poorer, borrowing would become costlier, and Meridian exporters would find it harder to compete abroad, all three effects pulling total spending down as the price level rises, which is exactly the downward-sloping relationship the aggregate demand curve captures.
Macroeconomic Equilibrium
Macroeconomic equilibrium in this model occurs where the aggregate demand and aggregate supply curves intersect, determining both the equilibrium level of real GDP and the equilibrium price level simultaneously. At a low price level, firms have little incentive to produce even though buyers would gladly purchase a large quantity of output, so as the price level rises from that low starting point, the quantity firms are willing to supply rises while the quantity buyers are willing to demand falls, until the two curves meet at a single equilibrium point. Where this equilibrium falls relative to potential GDP carries important information: if equilibrium output sits well below potential GDP, in the relatively flat portion of the aggregate supply curve, the economy is likely to show high unemployment alongside a fairly stable price level, since firms have plenty of spare capacity and little reason to raise prices sharply. If equilibrium output sits close to or at potential GDP, in the steep portion of the aggregate supply curve, the economy is more likely to show low unemployment alongside rising prices, since firms facing capacity constraints respond to additional demand mainly by raising prices rather than expanding output further. This link between where equilibrium falls and the resulting mix of unemployment and inflationary pressure is one of the model’s most useful applications for evaluating an economy’s current condition.
What Shifts the Curves
The aggregate supply curve shifts to the right, allowing greater output at every price level, mainly due to productivity growth, meaning workers producing more output per hour worked over time thanks to better skills, equipment, or technology; it shifts to the left when input prices such as wages or energy costs rise broadly across the economy, or after a supply shock such as a severe crop-destroying storm or a sudden loss of available workers, both of which raise production costs or reduce what firms can produce at any given price level. Note that productivity growth shifts both the short-run and the long-run aggregate supply curve, since it genuinely raises the economy’s sustainable capacity, while a temporary change in input prices typically shifts only the short-run curve. The aggregate demand curve shifts to the right when any of its components rises: stronger consumer or business confidence tends to raise consumption and investment spending, increased government spending directly adds to demand, and tax cuts that leave households or firms with more after-tax income tend to raise consumption or investment as well. The same factors working in reverse, weaker confidence, reduced government spending, or higher taxes, shift the aggregate demand curve to the left. Suppose a manufacturing region called Dellworth sees a wave of new business optimism after a major employer announces expansion plans; if that optimism raises investment spending broadly across the region, the local aggregate demand curve shifts right, raising both regional output and the regional price level toward a new equilibrium at a point closer to potential GDP.
Quick Revision Summary
The aggregate demand-aggregate supply model combines a supply-side view, in which productive capacity mainly explains output over the long run, with a demand-side view, in which total spending mainly explains output over the short run. The upward-sloping aggregate supply curve shows the real GDP firms will produce at each price level, becoming steep as the economy approaches potential GDP, the maximum sustainable output level. The downward-sloping aggregate demand curve shows total planned spending at each price level, sloping down due to wealth, interest rate, and foreign price effects. Macroeconomic equilibrium occurs where the two curves intersect, and where that equilibrium falls relative to potential GDP indicates whether the economy is more likely to show high unemployment with stable prices or low unemployment with rising prices. Aggregate supply shifts with productivity growth and input price changes, while aggregate demand shifts with consumer and business confidence, government spending, and tax policy, and either type of shift moves the economy to a new equilibrium level of output and prices.