Aggregate Demand and Aggregate Supply analysis

Aggregate Demand (AD) is the total planned spending on an economy's output at each price level: AD = C + I + G + (X − M). The AD curve slopes downward because a lower price level raises real spending (through the wealth, interest-rate and international-trade effects). Aggregate Supply (AS) is the total output firms are willing to supply at each price level: the short-run AS curve (SRAS) slopes upward, while the long-run AS curve (LRAS) is vertical at the economy's productive capacity (or has three sections).

On the diagram, the price level is measured on the vertical axis and real output (real GDP) on the horizontal axis. The economy is in macroeconomic equilibrium where the AD curve intersects the AS curve. At this point, the total planned spending in the economy exactly equals the total output firms supply.

This intersection determines the equilibrium price level (P*) and the equilibrium level of real output (Y*). Because producing this output requires labour, it also determines the associated level of employment: a higher equilibrium output is associated with higher employment, and a lower output with higher unemployment.

If AD or AS shifts, the equilibrium moves: for example, a rightward shift in AD raises both the price level and real output (the split depending on how close the economy is to full capacity), while a leftward shift in SRAS raises the price level but lowers output. The AD/AS model therefore shows how the interaction of total demand and total supply determines the price level, real output and employment.

The two events affect different curves and pull the price level and output in partly opposing ways.

Lower interest rates make borrowing cheaper and saving less attractive, raising consumption (C) and investment (I) — components of AD. So AD shifts to the right (AD₁ → AD₂). On its own this raises both the price level and real output (and employment).

A doubling of oil prices sharply raises firms' costs of production, so SRAS shifts to the left (SRAS₁ → SRAS₂). On its own this raises the price level but lowers real output (cost-push inflation).

Combining the two: both the rightward AD shift and the leftward SRAS shift push the price level upwards, so the price level will almost certainly rise — possibly sharply (inflation). The effect on real output is ambiguous: the AD rise increases output while the SRAS fall decreases it, so the net change depends on the relative size of the two shifts. If the AD boost outweighs the cost shock, output rises; if the oil shock dominates, output falls. The most likely outcome is higher inflation with an uncertain effect on output — a difficult situation that could resemble stagflation if output falls.

An increase in aggregate demand (a rightward shift of AD, from higher C, I, G or net exports) raises total planned spending. Whether this causes inflation (a sustained rise in the price level) depends critically on the state of the economy.

When a rise in AD does cause inflation. If the economy is at or near full capacity (on the steep/vertical section of AS, near LRAS), firms cannot easily produce more, so the extra demand mainly bids up prices — this is demand-pull inflation. With output already close to its maximum, an AD rise translates largely into a higher price level rather than more goods. If AD keeps rising at full capacity, inflation can be significant.

When a rise in AD does not (much) cause inflation. If the economy has substantial spare capacity — for example in a recession with high unemployment (on the elastic, near-horizontal section of AS) — firms can raise output using idle resources without much increase in costs or prices. Here a rise in AD mainly increases real output and employment with little inflation. The same applies if the AD rise is accompanied by a rightward shift in SRAS/LRAS (e.g. from supply-side improvements), which can absorb higher demand without inflation.

Other considerations. The effect also depends on the size of the AD increase, expectations (if firms and workers expect inflation, it can become self-fulfilling), and the time period. A one-off rise in AD may raise the price level once without causing sustained inflation.

Judgement. An increase in AD will not always cause inflation. The key determinant is the degree of spare capacity: near full capacity, an AD rise is likely to be inflationary (demand-pull); with significant spare capacity, it mainly raises output with little inflation. Therefore the statement is conditional — a rise in AD is inflationary chiefly when the economy is close to its productive capacity, and much less so when there is spare capacity. The most defensible conclusion is that whether AD causes inflation depends on where the economy is on the AS curve (and on supply-side conditions and expectations), so it is wrong to say it always causes inflation.

Investment (I) is firms' spending on capital goods and is a component of AD. A large rise in investment affects the economy through both aggregate demand and aggregate supply.

The demand-side (short-run) effect. Higher investment is an increase in AD, shifting the AD curve to the right. This raises real output and employment (and the price level), as firms producing capital goods and their suppliers earn more income, which is then spent. If the economy has spare capacity, the main effect is higher output with little inflation; near full capacity, the main effect is demand-pull inflation.

The supply-side (long-run) effect. Crucially, investment also raises the economy's capital stock and, where it embodies new technology, productivity. This shifts the LRAS curve to the right, increasing productive capacity — i.e. long-run economic growth. This means the economy can produce more without generating inflation, and can sustain higher living standards over time.

Qualifications. The benefits are not guaranteed. The investment may be poorly chosen or left idle if demand is weak. There may be a time lag before the supply-side effects appear. If investment is financed by borrowing, it could raise interest rates or debt. And if the economy is already at full capacity, the short-run demand effect may be inflationary before the capacity benefits arrive. The effect on the current account depends on whether the capital goods are imported.

Judgement. A large increase in investment is generally beneficial because it is almost unique in boosting both AD (raising output and employment now) and LRAS (raising capacity and growth later), making it a powerful driver of sustainable growth. However, the size and balance of these effects depend on the state of the economy (spare capacity vs full capacity), the quality of the investment, and how it is financed. The most defensible conclusion is that higher investment is one of the most desirable sources of growth because of its dual demand- and supply-side benefits, provided it is well-directed and the economy can absorb the short-run demand without excessive inflation.