1. The Short – Run Macro Model
Chapter 23
The Short – Run Macro Model

2. The Short-Run Macro Model
In short-run, spending depends on income, and income depends on spending.
The more income households have, the more they will spend.
The more households spend, the more output firms will produce
More income they will pay to their workers.
Many ideas behind the model were originally developed by British economist John Maynard Keynes in 1930s.
Short-run macro model focuses on spending in explaining economic fluctuations.

3. Review the Categories of Spending
Macroeconomists have found that the most useful approach is to divide those who purchase the GDP into four broad categories
Households --- consumption spending (C)
Business firms --- planned investment spending (IP)
Government agencies --- government purchases (G)
Foreigners --- net exports (NX)
Nominal or real spending?
Real terms

4. Consumption What factors affect households’ spending?
Disposable income: Yd (= Y – T)
Wealth (= total assets – total liability)
Price level
Interest rate
When interest rate falls, consumption rises
Expectations about future

5. Figure: U.S. Consumption and Disposable Income, 1985-2002

6. Figure: The Consumption Function

7. Consumption and Disposable Income
Autonomous consumption spending
Consumption spending when disposable income is zero
Marginal propensity to consume, or MPC
The slope of the consumption function
MPC = Δ Consumption ÷ Δ Disposable Income
MPC measures by how much consumption spending rises when disposable income rises by one dollar
Logic and empirical evidence suggest that the MPC should be larger than zero, but less than 1
So, we assume that 0 < MPC < 1

8. Representing Consumption with an Equation
C = a + bYd
Where C is consumption spending
And a is the autonomous consumption spending
And b is the marginal propensity to consume (MPC)
Equation between consumption and total income
Since Yd = Y – T,
C = a + b(Y – T)
So, C = (a- bT) + bY

9. Figure: The Consumption-Income Line

10. Shifts in the Consumption-Income Line
When a change in income causes consumption spending to change, we move along consumption-income line.
When a change in anything else besides income causes consumption spending to change, the line will shift.

11. Figure: A Shift in the Consumption-Income Line

12. IP, G and NX
For now, in the short-run macro model, planned investment spending, government purchases, and net exports are all treated as given or fixed values.

13. Summing Up: Aggregate Expenditure
Aggregate expenditure (AE)
Sum of spending by households, businesses, government, and foreign sector on final goods and services produced in United States
Aggregate expenditure = C + IP + G + NX
AE plays a key role in explaining economic fluctuations
Why?
Because over several quarters or even a few years, business firms tend to respond to changes in aggregate expenditure by changing their level of output.

14. Finding Equilibrium GDP
When aggregate expenditure is less than GDP, inventories will increase and output will decline in future.
When aggregate expenditure is greater than GDP, inventories will decrease and output will rise in future.
In short-run, equilibrium GDP is level of output at which output and aggregate expenditure are equal.

15. Inventories and Equilibrium GDP
When firms produce more goods than they sell, what happens to unsold output?
Added to their inventory stocks
Find output level at which change in inventories is equal to zero.
AE < GDP  ΔInventories > 0  GDP↓ in future periods
AE > GDP  ΔInventories < 0  GDP↑ in future periods
AE = GDP  ΔInventories = 0  No change in GDP
Equilibrium output level is the one at which change in inventories equals zero.

16. Figure: Deriving the Aggregate Expenditure Line

17. Figure: Using a 45° to Translate Distances

18. Figure: Determining Equilibrium Real GDP

19. Equilibrium GDP and Employment
When economy operates at equilibrium, will it also be operating at full employment?
Not necessarily
For instance, insufficient spending causes business firms to decrease their demand for labor.
Remember, in the long run (classical) macro model, it takes time for labor market to achieve full employment.
In the short-run model, it would be quite a coincidence if our equilibrium GDP happened to be the full employment output level.
In short-run macro model, output can be lower or higher than the full employment output level.

20. Figure: Short-Run Equilibrium GDP < Full Employment GDP

21. Figure: Short-Run Equilibrium GDP > Full-Employment GDP

22. A Change in Investment Spending
Suppose the initial equilibrium GDP in an economy is $6,000 billion.
Now, business firms increase their investment spending on plant and equipment by $1,000 billion.
Then, firms that sell investment goods receive $1,000 billion as income, which is to be distributed as salary, rent, interest, and profit.
What will households do with their $1,000 billion in additional income?
Spend the money !
How much to spend depends crucially on marginal propensity to consume (MPC): let’s assume MPC = 0.6

23. A Change in Investment Spending
When households spend an additional $600 billion, firms that produce consumption goods and services will receive an additional $600 billion in sales revenue.
Which will become income for households that supply resources to these firms. At this point, total income has increased by $1,000+$600=$1,600billion
With an MPC of 0.6, consumption spending will further rise by 0.6 x $600 billion = $360 billion, creating still more sales revenue for firms, and so on and so on…
At end of process, when economy has reached its new equilibrium.
Total spending and total output are considerably higher.

24. Figure: The Effect of a Change in Investment Spending

25. The Expenditure Multiplier
Whatever the rise in investment spending, equilibrium GDP would increase by a factor of 2.5, so we can write
ΔGDP = 2.5 x ΔIP
Value of expenditure multiplier depends on value of MPC
So, when the increase in planned investment spending is
ΔIP, the increase in total income (GDP) is calculated as:

26. The Expenditure Multiplier
A sustained increase in investment spending will cause a sustained increase in GDP.
Multiplier process works in both directions.
Just as increases in investment spending cause equilibrium GDP to rise by a multiple of the change in spending.
Decreases in investment spending cause equilibrium GDP to fall by a multiple of the change in spending.

27. Spending Shocks
Shocks to economy can come from other sources besides investment spending.
Government purchases (G)
Net exports (NX)
Autonomous consumption (a)
Changes in planned investment, government purchases, net exports, or autonomous consumption lead to a multiplier effect on GDP.

28. Spending Shocks
The effect of a change in spending on total income through expenditure multiplier

29. Figure: A Graphical View of the Multiplier

30. Automatic Stabilizers and the Multiplier
Automatic stabilizers reduce size of multiplier and therefore reduce impact of spending shocks.
With milder fluctuations, economy is more stable.
Some real-world automatic stabilizers we’ve ignored in the simple, short-run macro model of this chapter
Taxes
Transfer payments
Interest rates
Imports
Forward-looking behavior

31. The Role of Saving
In long-run, saving has positive effects on economy.
But in short-run, automatic mechanisms of classical model do not keep economy operating at its potential.
In long-run, an increase in desire to save leads to faster economic growth and rising living standards.
In short-run, however, it can cause a recession that pushes output below its potential.
Two sides to the “saving coin”
Impact of increased saving is positive in long-run and potentially dangerous in short-run.

32. The Effect of Fiscal Policy
In classical model fiscal policy—changes in government spending or taxes designed to change equilibrium GDP—is completely ineffective – crowding out effect.
In short-run, an increase in government purchases causes a multiplied increase in equilibrium GDP.
Therefore, in short-run, fiscal policy can actually change equilibrium GDP.
Observation suggests that fiscal policy could, in principle, play a role in altering path of economy.
Indeed, in 1960s and early 1970s, this was the thinking of many economists.
But very few economists believe this today.