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28.3 Aggregate Expenditures and Aggregate Demand

Learning Objectives

  1. Explain and illustrate how a change in the price level affects the aggregate expenditures curve.
  2. Explain and illustrate how to derive an aggregate demand curve from the aggregate expenditures curve for different price levels.
  3. Explain and illustrate how an increase or decrease in autonomous aggregate expenditures affects the aggregate demand curve.

We can use the aggregate expenditures model to gain greater insight into the aggregate demand curve. In this section we shall see how to derive the aggregate demand curve from the aggregate expenditures model. We shall also see how to apply the analysis of multiplier effects in the aggregate expenditures model to the aggregate demand–aggregate supply model.

Aggregate Expenditures Curves and Price Levels

An aggregate expenditures curve assumes a fixed price level. If the price level were to change, the levels of consumption, investment, and net exports would all change, producing a new aggregate expenditures curve and a new equilibrium solution in the aggregate expenditures model.

A change in the price level changes people’s real wealth. Suppose, for example, that your wealth includes $10,000 in a bond account. An increase in the price level would reduce the real value of this money, reduce your real wealth, and thus reduce your consumption. Similarly, a reduction in the price level would increase the real value of money holdings and thus increase real wealth and consumption. The tendency for price level changes to change real wealth and consumption is called the wealth effectThe tendency for price level changes to change real wealth and consumption..

Because changes in the price level also affect the real quantity of money, we can expect a change in the price level to change the interest rate. A reduction in the price level will increase the real quantity of money and thus lower the interest rate. A lower interest rate, all other things unchanged, will increase the level of investment. Similarly, a higher price level reduces the real quantity of money, raises interest rates, and reduces investment. This is called the interest rate effectThe tendency for a higher price level to reduce the real quantity of money, raise interest rates, and reduce investment..

Finally, a change in the domestic price level will affect exports and imports. A higher price level makes a country’s exports fall and imports rise, reducing net exports. A lower price level will increase exports and reduce imports, increasing net exports. This impact of different price levels on the level of net exports is called the international trade effectThe impact of different price levels on the level of net exports..

Panel (a) of Figure 28.16 "From Aggregate Expenditures to Aggregate Demand" shows three possible aggregate expenditures curves for three different price levels. For example, the aggregate expenditures curve labeled AEP=1.0 is the aggregate expenditures curve for an economy with a price level of 1.0. Since that aggregate expenditures curve crosses the 45-degree line at $6,000 billion, equilibrium real GDP is $6,000 billion at that price level. At a lower price level, aggregate expenditures would rise because of the wealth effect, the interest rate effect, and the international trade effect. Assume that at every level of real GDP, a reduction in the price level to 0.5 would boost aggregate expenditures by $2,000 billion to AEP = 0.5, and an increase in the price level from 1.0 to 1.5 would reduce aggregate expenditures by $2,000 billion. The aggregate expenditures curve for a price level of 1.5 is shown as AEP=1.5. There is a different aggregate expenditures curve, and a different level of equilibrium real GDP, for each of these three price levels. A price level of 1.5 produces equilibrium at point A, a price level of 1.0 does so at point B, and a price level of 0.5 does so at point C. More generally, there will be a different level of equilibrium real GDP for every price level; the higher the price level, the lower the equilibrium value of real GDP.

Figure 28.16 From Aggregate Expenditures to Aggregate Demand

Because there is a different aggregate expenditures curve for each price level, there is a different equilibrium real GDP for each price level. Panel (a) shows aggregate expenditures curves for three different price levels. Panel (b) shows that the aggregate demand curve, which shows the quantity of goods and services demanded at each price level, can thus be derived from the aggregate expenditures model. The aggregate expenditures curve for a price level of 1.0, for example, intersects the 45-degree line in Panel (a) at point B, producing an equilibrium real GDP of $6,000 billion. We can thus plot point B′ on the aggregate demand curve in Panel (b), which shows that at a price level of 1.0, a real GDP of $6,000 billion is demanded.

Panel (b) of Figure 28.16 "From Aggregate Expenditures to Aggregate Demand" shows how an aggregate demand curve can be derived from the aggregate expenditures curves for different price levels. The equilibrium real GDP associated with each price level in the aggregate expenditures model is plotted as a point showing the price level and the quantity of goods and services demanded (measured as real GDP). At a price level of 1.0, for example, the equilibrium level of real GDP in the aggregate expenditures model in Panel (a) is $6,000 billion at point B. That means $6,000 billion worth of goods and services is demanded; point B' on the aggregate demand curve in Panel (b) corresponds to a real GDP demanded of $6,000 billion and a price level of 1.0. At a price level of 0.5 the equilibrium GDP demanded is $10,000 billion at point C', and at a price level of 1.5 the equilibrium real GDP demanded is $2,000 billion at point A'. The aggregate demand curve thus shows the equilibrium real GDP from the aggregate expenditures model at each price level.

The Multiplier and Changes in Aggregate Demand

In the aggregate expenditures model, a change in autonomous aggregate expenditures changes equilibrium real GDP by the multiplier times the change in autonomous aggregate expenditures. That model, however, assumes a constant price level. How can we incorporate the concept of the multiplier into the model of aggregate demand and aggregate supply?

Consider the aggregate expenditures curves given in Panel (a) of Figure 28.17 "Changes in Aggregate Demand", each of which corresponds to a particular price level. Suppose net exports rise by $1,000 billion. Such a change increases aggregate expenditures at each price level by $1,000 billion.

A $1,000-billion increase in net exports shifts each of the aggregate expenditures curves up by $1,000 billion, to AEP=1.0 and AEP=1.5. That changes the equilibrium real GDP associated with each price level; it thus shifts the aggregate demand curve to AD2 in Panel (b). In the aggregate expenditures model, equilibrium real GDP changes by an amount equal to the initial change in autonomous aggregate expenditures times the multiplier, so the aggregate demand curve shifts by the same amount. In this example, we assume the multiplier is 2. The aggregate demand curve thus shifts to the right by $2,000 billion, two times the $1,000-billion change in autonomous aggregate expenditures.

Figure 28.17 Changes in Aggregate Demand

The aggregate expenditures curves for price levels of 1.0 and 1.5 are the same as in Figure 28.16 "From Aggregate Expenditures to Aggregate Demand", as is the aggregate demand curve. Now suppose a $1,000-billion increase in net exports shifts each of the aggregate expenditures curves up; AEP=1.0, for example, rises to AEP=1.0. The aggregate demand curve thus shifts to the right by $2,000 billion, the change in aggregate expenditures times the multiplier, assumed to be 2 in this example.

In general, any change in autonomous aggregate expenditures shifts the aggregate demand curve. The amount of the shift is always equal to the change in autonomous aggregate expenditures times the multiplier. An increase in autonomous aggregate expenditures shifts the aggregate demand curve to the right; a reduction shifts it to the left.

Key Takeaways

  • There will be a different aggregate expenditures curve for each price level.
  • Aggregate expenditures will vary with the price level because of the wealth effect, the interest rate effect, and the international trade effect. The higher the price level, the lower the aggregate expenditures curve and the lower the equilibrium level of real GDP. The lower the price level, the higher the aggregate expenditures curve and the higher the equilibrium level of real GDP.
  • A change in autonomous aggregate expenditures shifts the aggregate expenditures curve for each price level. That shifts the aggregate demand curve by an amount equal to the change in autonomous aggregate expenditures times the multiplier.

Try It!

Sketch three aggregate expenditures curves for price levels of P1, P2, and P3, where P1 is the lowest price level and P3 the highest (you do not have numbers for this exercise; simply sketch curves of the appropriate shape). Label the equilibrium levels of real GDP Y1, Y2, and Y3. Now draw the aggregate demand curve implied by your analysis, labeling points that correspond to P1, P2, and P3 and Y1, Y2, and Y3. You can use Figure 28.16 "From Aggregate Expenditures to Aggregate Demand" as a model for your work.

Case in Point: Predicting the Impact of Alternative Fiscal Policies in 2008

Figure 28.18

Economists are often asked to simulate the effects of policy changes on the economy. In 2008, as the economy weakened and Congress and the president debated a stimulus package, Mark M. Zandi, an economist at Moody’s Economy.com, produced a paper assessing the impact of various possible stimulus packages. His research produced the following table.

Table 28.2 Fiscal Economic Bang for the Buck

Tax cuts
Nonrefundable lump-sum tax rebate 1.02
Refundable lump-sum tax rebate 1.26
Temporary tax cuts
     Payroll tax holiday 1.29
     Across-the-board tax cut 1.03
     Accelerated depreciation 0.27
Permanent tax cuts
     Extend alternative minimum tax patch 0.48
     Make Bush income tax cuts permanent 0.29
     Make dividend and capital gains tax cuts permanent 0.37
Spending increases
Extending UI benefits 1.64
Temporary increase in food stamps 1.73
General aid to state governments 1.36
Increased infrastructure spending 1.59

The $100-billion tax rebate for households that was actually passed was of the nonrefundable lump-sum type. “Nonrefundable” refers to the fact those not earning enough to pay income taxes would not receive a rebate, meaning that many households received zero or partial refunds. As shown, he estimated that a $100-billion tax rebate would shift the aggregate demand curve by $102 billion, assuming a constant price level. As part of the analysis, in the article he mentioned that he assumed an MPC of about two-thirds, since many households at the time were living paycheck-to-paycheck. So, consumption would increase by about $67 billion. The implied multiplier is thus 1.54 (= 102/67).

The table shows that other stimuli would have smaller or larger effects. He reasoned that making various tax cuts permanent would have little impact on consumption now, since households in 2008 were cash-strapped. The table shows an estimated bigger bang for the buck from various kinds of government spending increases. That follows, since the change in aggregate expenditure is the full amount of the spending increase instead of the portion of a tax rebate of the same magnitude that consumers decide to spend.

Answer to Try It! Problem

The lowest price level, P1, corresponds to the highest AE curve, AEP = P1, as shown. This suggests a downward-sloping aggregate demand curve. Points A, B, and C on the AE curve correspond to points A′, B′, and C′ on the AD curve, respectively.

Figure 28.19