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February 5, 2026

Class 12th ch-5 Aggregate Demand and Its Related Concepts

Class 12th ch-5 Aggregate Demand and Its Related Concepts

NOTES

Introduction – Keynes & Great Depression

1️⃣ INTRODUCTION (KEYNES & GREAT DEPRESSION)

This chapter highlights the constructive role of J. M. Keynes (John Maynard Keynes) during the Great Depression (1929–1933) in America.

Background

  • Classical economics failed to explain mass unemployment
  • Keynes introduced a demand-side approach

Key Emphasis by Keynes

  • Aggregate Demand
  • Aggregate Supply
  • Consumption and Saving
  • Psychological Law of Consumption

His theory explains how changes in demand affect income, output, and employment.

Aggregate Demand (AD)

2️⃣ AGGREGATE DEMAND (AD)

Meaning

Aggregate Demand refers to the total demand for final goods and services in an economy during an accounting year.

Keynesian View

According to Keynes:

  • Aggregate demand is the total amount of money buyers are willing to spend
  • It is measured in money terms, not in physical units
  • It is that part of national income which society is ready to spend

Definition

Aggregate Demand is the aggregate expenditure on ex-ante (planned) consumption and ex-ante (planned) investment that all sectors of the economy are willing to incur at each level of income.

COMPONENTS OF AGGREGATE DEMAND

(i) Private Consumption Demand (C)

  • Total expenditure by households on consumption
  • Depends on:
  • Disposable income
  • Propensity to consume

(ii) Private Investment Demand (I)

  • Demand for capital goods by private investors
  • Adds to real capital stock (machines, buildings, tools)
  • Depends on:
  • Marginal Efficiency of Capital (MEC)
  • Rate of Interest

Types of Investment:

  • Autonomous Investment
  • Induced Investment

➡ In Keynesian theory, investment is assumed to be autonomous.

(iii) Government Demand for Goods & Services (G)

  • Government is a major buyer in modern economies
  • Expenditure on:
  • Roads, schools, hospitals
  • Defence, law and order
  • Income inelastic
  • Autonomous in nature

    • The Aggregate Demand curve is a straight line parallel to the X-axis.

    (iv) Net Export Demand (X − M)

    • Difference between exports and imports
    • Positive when exports exceed imports
    • Influenced by:
    • Exchange rate
    • Trade policy
    • Prices and quality of goods

    SIMPLIFIED KEYNESIAN MODEL

    For simplicity, Keynes considered only:

    • Consumption Demand (C)
    • Investment Demand (I)

    AD = C + I

    Aggregate Supply (AS)

    3️⃣ AGGREGATE SUPPLY (AS)

    Meaning

    Aggregate Supply refers to the total supply of goods and services by producers in an economy.

    Keynesian Definition

    Aggregate supply is the total money value of goods and services, paid as factor income for factor services.

    ➡ It is equal to Net National Product at Factor Cost (NNPFC).

    FACTOR PAYMENTS

    • Rent → Land
    • Wages → Labour
    • Interest → Capital
    • Profit → Entrepreneur

    CLOSED ECONOMY ASSUMPTION

    Keynes assumed:

    • Closed capitalist economy
    • Net Factor Income from Abroad (NFIA) = 0

    Aggregate Supply = NNPFC = NDPFC

    AS = Rent + Wages + Interest + Profit

    SIMPLIFICATION

    Income is either:

    • Consumed (C)
    • Saved (S)

    AS = C + S

    Consumption Function (Propensity to Consume)

    4️⃣ CONSUMPTION FUNCTION (PROPENSITY TO CONSUME)

    Meaning

    The consumption function shows the functional relationship between consumption and income.

    C = f(Y)

    Where:

    • C = Consumption
    • Y = Disposable income

    CONSUMPTION FUNCTION EQUATION

    C = c + bY

    Where:

    • c = Autonomous consumption
    • b = Marginal Propensity to Consume (MPC)
    Keynes’ Psychological Law of Consumption

    5️⃣ KEYNES’ PSYCHOLOGICAL LAW OF CONSUMPTION

    According to Keynes:

    • As income increases, consumption also increases
    • Increase in consumption is less than the increase in income
    • Consumption lags behind income
    • Propensity to consume remains stable in the short run
    Break-Even Point

    6️⃣ BREAK-EVEN POINT

    Meaning

    The break-even point (BEP) is the level of income where:

    C = Y


    S = 0

    Key Observations

    • Below BEP → C > Y → Negative saving
    • At BEP → C = Y → Zero saving
    • Above BEP → Y > C → Positive saving
    Propensity to Consume – Types

    7️⃣ PROPENSITY TO CONSUME – TYPES

    (a) Average Propensity to Consume (APC)

    APC = C / Y

    Important Points:

    • APC > 1 → Before Break-Even Point (BEP)
    • APC = 1 → At Break-Even Point (BEP)
    • APC < 1 → After Break-Even Point (BEP)

    (b) Marginal Propensity to Consume (MPC)

    MPC = ΔC / ΔY

    Important Points:

    • MPC lies between 0 and 1
    • MPC falls with rising income
    • Income is either consumed or saved
    Saving Function (Propensity to Save)

    8️⃣ SAVING FUNCTION (PROPENSITY TO SAVE)

    Meaning

    The saving function shows the functional relationship between saving and income.

    S = f(Y)

    Saving Function Equation

    S = −c + (1 − b)Y

    SAVING BEHAVIOUR

    • Below BEP → Negative saving
    • At BEP → Zero saving
    • Above BEP → Positive saving
    Propensity to Save – Types

    9️⃣ PROPENSITY TO SAVE – TYPES

    (a) Average Propensity to Save (APS)

    APS = S / Y

    Key Points:

    • APS can be negative
    • APS = 0 at Break-Even Point (BEP)
    • APS < 1 always
    • APS rises with income

    (b) Marginal Propensity to Save (MPS)

    MPS = ΔS / ΔY

    IMPORTANT RELATION

    MPC + MPS = 1

    Words That Matter – Keynesian Theory

    🔟 WORDS THAT MATTER (VERY IMPORTANT FOR EXAM)

    • Aggregate Demand – Planned consumption + planned investment
    • Induced Investment – Profit-motivated investment
    • Autonomous Investment – Income-independent investment
    • Aggregate Supply – Total factor income
    • Consumption Function – C = f(Y)
    • Autonomous Consumption – Minimum consumption at zero income
    • Psychological Law – Consumption increases less than income
    • Break-even Point – C = Y
    • APC – C / Y
    • MPC – ΔC / ΔY
    • Saving Function – S = f(Y)
    • APS – S / Y
    • MPS – ΔS / ΔY

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