1. Aggregate expenditure
Autonomous versus induced expenditure
The consumption function
The theory of investment
Government purchase function
The net export function

2. Autonomous versus induced Expenditure
Autonomous expenditure: The components of aggregate expenditure that do not change when real GDP changes.
Induced expenditure: The components of aggregate expenditure that change when real GDP changes.

3. The consumption function
The consumption function reveals the relationship between consumption and disposable income, other things constant.

4. Disposable income, consumption, and saving in US

5. US consumption depends on disposable income

6. Keynes’s fundamental law of consumption
People show a tendency, as a rule and on average, to increase their consumption when their income increases—but not by as much as the increase in income.

7. Marginal propensities to consume (MPC) and save (MPS)
MPC: The fraction of the change in income that is spent on consumption.
Keynes’s fundamental law implies that:
MPS: The fraction of the change in income that is saved

8. The consumption function1 2 3 4 5 7 6 8 9 10 11
Real consumption (trillions of dollars) C
The consumption function, C, shows the relationship between consumption and disposable income, other things constant. 1 2 3 4 5 7 6 8 9 10 14 13 12 11
Real disposable income (trillions of dollars)

9. Marginal propensities to consume and save
Real consumption (trillions of dollars)
(a) Consumption function
(b) Saving function
Real saving (trillions of dollars)
MPC=∆C/∆DI=0.4/0.5=4/5
MPS=∆S/∆DI=0.1/0.5=1/5 b d
∆C=0.4 a c
∆S=0.1
∆DI=0.5
∆DI=0.5
Real disposable income (trillions of dollars)
The slope of the C function equals the marginal propensity to consume.
For the straight-line C function in (a), the slope is the same at all levels of income and is given by the change in consumption divided by the change in disposable income that causes it: MPC=4/5.
The slope of the S function in (b) equals the marginal propensity to save, MPS=1/5.

10. Determinants of consumption
Disposable Income
Net Wealth
Consumption
Interest Rates
Expectations

11. Shifts of the consumption function
Real consumption C
An upward shift, such as from C to C’’, can be caused by an increase in net wealth, a decrease in the price level, an favorable change in consumer expectations, or a decrease in the interest rate. C’
Real disposable income
A downward shift of the consumption function, such as from C to C’, can be caused by a decrease in net wealth, an increase in the price level, an unfavorable change in consumer expectations, or an increase in the interest rate.

12. The crash of ‘08

13. Let’s take out a loan so we can “cash out” some home equity.
Rising home values have stimulated household borrowing and consumption in the past decade.

14. Consumer Confidence has fluctuated lately

15. Debtor nation

16. Theory of Investment
Why do firms purchase things like new offshore drilling platforms, food processing plants, or bulldozers? Because they expect they can make a profit by doing so.

17. Investment Function Let Where: I is gross investment
 is the expected profit of investment; and
r is the interest rate.

18. The Investment Decision
Acquisition cost of a tractor–trailer rig $150,000.00
Useful life years
Expected (extra) sales revenue per year from the use of the asset ,000.00
Expected costs per year to operate the asset . . $180,000.00
Diesel fuel $38, Driver salary & benefits , Depreciation , Repairs (including tires) , Misc. (fees, fines, etc.) 5,000
Expected net sales revenue per year from the use of the asset $20,000.00

19. Computing expected profit ()
To compute expected profit in percentage terms:
Thus we have:

20. We would consider the tractor-trailer rig a sound investment if the interest rate were less than 13.7 percent.

21. Rates of return on golf carts and the opportunity cost of funds5 8 10 15 20 25
Nominal interest rate (percent)
An individual firm invests in any project with a rate of return that exceeds the market interest rate.
At an interest rate of 8%, Hacker Haven purchases three golf carts, investing $6,000.
Expected rate
of return
Market rate
of interest
$2,000
$4,000
$6,000
$8,000
$10,000
Investment

22. Investment demand curve for the economy6 8 10
Nominal interest rate (percent)
The investment demand curve for the economy sums the investment demanded by each firm at each interest rate.
At lower interest rates, more investment projects become profitable for individual firms, so total investment in the economy increases. D
0.9
1.0
1.1
Investment
(trillions of dollars)

23. Investment function
A decrease in the interest rate or more upbeat business expectations would increase investment at every level of income, as shown by the upward shift from I to I’’.
0.9
1.0
1.1
Investment (trillions of dollars)
I’’ I
An increase in the interest rate or less favorable business expectations would decrease investment at every level of income, as shown by the downward shift from I to I’. I’
2.0
4.0
6.0
8.0
10.0
Real disposable income
(trillions of dollars)
12.0
14.0
Investment is assumed to be independent of income, as shown by horizontal lines. Thus, investment is assumed to be autonomous.

24. Annual percentage change in US real GDP, consumption, and investment

25. Government purchase function
Because government purchases are controlled by public officials, we treat them as autonomous—that is, determined independent of income

26. Net export function
-420
-400
-380
Net exports (billions of dollars)
2.0
4.0
6.0
8.0
10.0
Real disposable income
(trillions of dollars)
12.0
14.0
A decrease in the value of the dollar would increase net exports at each level of income, as shown by the shift up to X’’-M’’.
X’’-M’’
X-M
X’-M’
An increase in the value of the dollar relative to other currencies would decrease net exports at each level of income, as shown by the shift down to X’-M’.
Net exports here are assumed to be independent of disposable income, as shown by the horizontal lines. X-M is the net export function when autonomous net exports equal -$400 billion.

27. US spending components as percentages of GDP since 1959