# National Income

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• Published : March 19, 2013

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National Income Determination

Introduction
• A key objective of Macroeconomics is to explain GDP growth and its fluctuations • Therefore, need to understand the forces that determine GDP (“National Income”)

• John Maynard Keynes in his “General Theory of Employment, Interest and Money” (1936) developed a model of income determination • Known as Keynesian Theory of Income Determination • Aggregate spending / demand determines the level of aggregate output

Concepts and Functions
Actual vs. Planned Expenditure
• Actual expenditures are expenditures actually incurred by the economic entities in the economy • Planned expenditures are expenditures intended to undertake by the economic entities • Macroeconomic equilibrium (as proposed by Keynes) occurs when the actual Aggregate Demand (expenditure) equals the planned Aggregate Demand (expenditure) • i.e., the planned (intended) AD should equal the actual level of output, which is the equilibrium level of output

Components of Aggregate Demand Two-sector Model
Consumption Function (C)
• A functional statement of the relationship between disposable income (Y) and consumption expenditure (C)

C = f(Yd)
• Consumption is a positive linear function of income

C = a + bYd
Note: In a two-sector model Y = Yd. (Why?)

a is a positive constant, (a>0) showing the level of consumption at zero level of income, also known as autonomous consumption b represents the slope of the consumption function 0 excess demand (ED) If AD < Output => excess supply (ES)

Equilibrium Output
E 

45o

Output, Income

Planned Investment = Planned Savings
Saving, Investment S = -a + (1-b)Y

-a

E

I
Income (Y)
Planned investment < planned saving i.e. households planned consumption is less than firms’ investment plans; firms build up stocks and/or reduce output

Planned investment > planned saving i.e. households planned consumption is greater than firms’ investment plans; firms deplete stocks and/or raise output

Equilibrium Output, Income
Desired spending

E 

C = a + bY

(a  I )
a
0

S = -a + (1-b)Y E 
45o

I
Output, Income
Planned investment < planned saving

-a
Planned investment > planned saving

 Y*

Equilibrium level of Income, Output
Value of output should equal aggregate spending Planned savings should equal planned investment

Y CI

SI
Substituting in for S

substituting in for C

Y  a  bY  I
and solving for Y

(a  (1  b)Y )  I
and solving for Y

Y  bY  a  I

a  I  Y
1  b 

(1  b)Y  a  I (a  I ) Y  (1  b)

a I Y  (1  b) (1  b)

Exercise 1

Suppose the household sector’s planned consumption is C = 50 + 0.80Y, and intended investment is Rs.50. (a) Derive an equation for the saving function? (b) What is the equilibrium level of income and aggregate consumption?

Aggregate Spending – The Multiplier

Effect of Changes in Autonomous

Changes in autonomous aggregate spending (a  I ) causes parallel shifts of the consumption function

Thus, a change in investment demand leads to a shift in the aggregate demand schedule

Aggregate Spending – The Multiplier
A change in investment demand

Effect of Changes in Autonomous

Suppose there is an increase in the autonomous investment (I ) because firms become optimistic about future demand conditions for their products –

What happens to our analysis?

A change in Investment demand

I

 E

45o
Y

Output, Income

A change in Investment demand

We know that

a I Y  (1  b) (1  b)

If investment (I) changes, the effect on output is given by: Y 1 b = marginal propensity  to consume (MPC) I (1  b) E.g. if investment were to change by one unit, and if the MPC=0.75, then Y 1  4 1 (1  0.75) Thus, a one unit change in...