NIOS Economics • Module 10

Lesson 26: Consumption, Saving and Investment

Lesson 26 Summary • Module 10

Consumption, Saving and Investment

Master macro concepts of aggregate consumption, saving, and investment. Study Keynes' Psychological Law of Consumption, linear equations \(C = a + bY\) and \(S = -a + (1-b)Y\), Break-Even Analysis (\(C = Y, S = 0\)), Propensity identities (\(APC+APS=1, MPC+MPS=1\)), and types of investment.

Section 1

1. Consumption Function & Keynes' Psychological Law

Core Macro Foundation

The functional relationship between aggregate consumption expenditure and national disposable income is called the Consumption Function: \(C = f(Y)\).

Disposable Income (\(Y\)): Income remaining after compulsory payments of personal direct taxes and fines. \(Y = \text{Total Income} - \text{Taxes}\). In short-run macro model, income and disposable income are assumed equal.
Keynes' Psychological Law: As income increases over time, aggregate consumption also increases, but at a slower rate than the increase in income (\(\Delta C < \Delta Y\)). Thus, \(MPC < 1\).

Linear Consumption Function Equation: \(C = a + bY\)

  • \(a\) (Autonomous Consumption): Fixed minimum expenditure on bare necessities (food, shelter) needed to survive when income is zero (\(Y=0\)). Financed by borrowing or dissaving.
  • \(b\) (\(MPC\)): Marginal Propensity to Consume—the slope of the consumption line (\(0 < b < 1\)).
  • \(Y\): Level of aggregate disposable income.
Section 2

2. Propensity to Consume & Propensity to Save

Mathematical Ratios
Measure Definition Formula Property
APC Ratio of total consumption to total income \(APC = \frac{C}{Y}\) Falls as income rises
MPC Ratio of change in consumption to change in income \(MPC = \frac{\Delta C}{\Delta Y}\) \(0 < MPC < 1\)
APS Ratio of total saving to total income \(APS = \frac{S}{Y}\) Can be negative at low income
MPS Ratio of change in saving to change in income \(MPS = \frac{\Delta S}{\Delta Y}\) \(0 < MPS < 1\)
Identity 1: APC + APS = 1

Since \(Y = C + S\), dividing by \(Y\): \(\frac{Y}{Y} = \frac{C}{Y} + \frac{S}{Y} \implies 1 = APC + APS\).
Therefore: \(APC = 1 - APS\) and \(APS = 1 - APC\).

Identity 2: MPC + MPS = 1

Since \(\Delta Y = \Delta C + \Delta S\), dividing by \(\Delta Y\): \(\frac{\Delta Y}{\Delta Y} = \frac{\Delta C}{\Delta Y} + \frac{\Delta S}{\Delta Y} \implies 1 = MPC + MPS\).
Therefore: \(MPC = 1 - MPS\) and \(MPS = 1 - MPC\).

Section 3

3. Saving Function & Break-Even Analysis

Equilibrium Concept

Saving is the unconsumed portion of disposable income (\(S = Y - C\)).

Derivation of Linear Saving Function Equation:

Starting from \(S = Y - C\) and substituting \(C = a + bY\):

\(S = Y - (a + bY) = -a + Y - bY\)

\(S = -a + (1 - b)Y\)

Where \(-a\) is dissaving at zero income, and \((1 - b) = MPS\) is the slope of the saving curve.

The Break-Even Point (BEP):

The Break-Even Point occurs where aggregate consumption equals aggregate income (\(C = Y\)). At this point, saving is exactly zero (\(S = 0\)) and \(APC = 1\).

  • Below BEP (\(Y < C\)): Consumption exceeds income (\(C > Y\)), resulting in dissaving (\(S < 0\)).
  • Above BEP (\(Y > C\)): Income exceeds consumption (\(Y > C\)), resulting in positive saving (\(S > 0\)).
Section 4

4. Determinants of Consumption & Investment Types

4 Non-Income Determinants of Consumption:
  • 1. Rate of Interest: High interest encourages saving over immediate consumption. (Less critical in short run for urgent necessities).
  • 2. Wealth: Holding physical/financial assets increases capacity to consume.
  • 3. Distribution of Income: Equal distribution increases aggregate consumption as poor have higher MPC than rich.
  • 4. Consumer Credit: Easy availability of loans boosts spending on consumer durables (cars, appliances).
Autonomous vs. Induced Investment:
  • Autonomous Investment (\(I_0\)): Fixed investment independent of income level or profit. Represented by a horizontal line.
  • Induced Investment: Driven by income level and profit expectations. Slopes upwards as national income grows.
  • Gross vs. Net Investment: \(\text{Gross Investment} = \text{Net Investment} + \text{Depreciation}\).