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Theory of Consumer Behaviour – Class 12 Economics Chapter 2 Notes & NCERT PDF

Class 12EconomicsChapter 2NCERT book: Introductory Microeconomics - Economics Class 12

Notes and a simple summary of Chapter 2 of the NCERT Class 12 Economics book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 2: Theory of Consumer Behaviour

Introduction

Chapter 2 studies the behaviour of a consumer who must decide how to spend a limited income on different goods. We first measure satisfaction (utility), then describe what the consumer can buy (the budget set) and what they prefer (indifference curves), and combine the two to find the optimal choice. From this we derive the demand curve, study what shifts it, add individual demands to get market demand, and finally measure responsiveness through price elasticity of demand.

Key Concepts

2.1 Utility

Utility is the want-satisfying capacity of a commodity. It is subjective and differs across people.

Cardinal utility analysis assumes utility can be measured in numbers (utils).

  • Total utility (TU): total satisfaction from consuming a given amount of a good.
  • Marginal utility (MU): change in TU from one more unit: MUn = TUn − TUn−1. If TU rises from 28 to 30 when the 5th unit is consumed, MU5 = 30 − 28 = 2.
  • Law of diminishing marginal utility: MU from each additional unit falls as consumption increases. When TU reaches its maximum, MU = 0; after that MU becomes negative.
  • Deriving demand: a consumer buys a good up to the point where MU (in money terms) equals the price. Since MU falls as quantity rises, the consumer buys more only at a lower price, so the demand curve slopes downward.

    • Ordinal utility analysis only ranks bundles and uses indifference curves.

  • An indifference curve (IC) joins all bundles that give the consumer the same satisfaction.
  • Marginal rate of substitution (MRS) = |ΔY/ΔX|, the amount of good Y the consumer is willing to give up for one more unit of X. MRS diminishes along an IC (for example 3:1, then 2:1, then 1:1), making the IC convex to the origin.
  • Perfect substitutes (such as ₹5 notes and ₹5 coins) give a straight-line IC.
  • Monotonic preferences: a bundle with more of at least one good and no less of the other is preferred.
  • Features of ICs: they slope downward; a higher IC gives higher utility; two ICs never intersect (the book shows that intersection would make A(7,10) and C(9,5) equally good even though B(9,7) has more than C and lies on the same curve as A, a contradiction).

    • 2.2 The Consumer's Budget

  • Budget set: all bundles the consumer can afford: p1x1 + p2x2 ≤ M.
  • Budget line: bundles costing exactly M: p1x1 + p2x2 = M. Its intercepts are M/p1 on the x1-axis and M/p2 on the x2-axis. Its slope is −p1/p2, the rate at which the market allows good 2 to be exchanged for good 1.
  • Changes: a change in income shifts the budget line parallel; a change in the price of one good rotates it about the intercept of the other good.

    • 2.3 Optimal Choice

    The consumer chooses the bundle on the budget line that lies on the highest attainable IC. At the optimum, the IC is tangent to the budget line: MRS = p1/p2. If MRS > p1/p2, the consumer values good 1 more than the market price and buys more of it; if MRS < p1/p2, they buy less.

    2.4 Demand

  • Demand function: X = f(P), with other factors (income, prices of related goods, tastes) constant.
  • Law of demand: other things equal, quantity demanded falls when price rises. A fall in price works through the substitution effect (the good becomes relatively cheaper) and the income effect (real income rises).
  • Linear demand: d(p) = a − bp.
  • Normal goods: demand rises with income. Inferior goods: demand falls with income. A Giffen good is an inferior good whose demand rises with its price.
  • Substitutes (tea and coffee): a rise in the price of one raises demand for the other. Complements (tea and sugar): a rise in the price of one lowers demand for the other.
  • Movement vs shift: a change in the good's own price causes a movement along the curve; a change in income, related prices or tastes shifts the whole curve.

    • 2.5 Market Demand

    Market demand is the horizontal sum of individual demands. With d1 = 10 − p and d2 = 15 − p: for p ≤ 10, market demand = 25 − 2p; for 10 < p ≤ 15, it is 15 − p; for p > 15 it is zero.

    2.6 Price Elasticity of Demand

    eD = (percentage change in quantity) / (percentage change in price) = (ΔQ/Q) / (ΔP/P).

  • |eD| < 1: inelastic; = 1: unitary elastic; > 1: elastic.
  • On a linear demand curve, elasticity is 0 where price is zero, 1 at the midpoint (p = a/2b) and infinite where quantity is zero. Geometrically, eD at a point = lower segment / upper segment.
  • Constant elasticity curves: vertical demand (eD = 0), horizontal demand (eD = ∞), rectangular hyperbola (eD = 1).
  • Determinants: necessities are inelastic, luxuries elastic; goods with close substitutes are elastic.
  • Elasticity and expenditure: if demand is elastic, a price rise lowers expenditure; if inelastic, a price rise raises expenditure; if unitary, expenditure is unchanged.

    • Worked Numericals

  • Example 2.1 (budget set). Income ₹20, both prices ₹5. Bundles with whole units that satisfy 5x1 + 5x2 ≤ 20 number 15; exactly 5 of them (such as (0,4), (1,3), (2,2), (3,1), (4,0)) cost ₹20.
  • Exercise 4. Prices ₹4 and ₹5, income ₹20. Budget line: 4x1 + 5x2 = 20. Maximum of good 1 = 20/4 = 5; of good 2 = 20/5 = 4; slope = −4/5.
  • Example 2.2 (elasticity). Price rises from ₹5 to ₹7; quantity falls from 15 to 12. %ΔQ = −3/15 × 100 = −20%; %ΔP = 2/5 × 100 = 40%. eD = 20/40 = 0.5, so demand is inelastic.
  • Exercise 22. Price rises from ₹4 to ₹5; quantity falls from 25 to 20. %ΔQ = −20%, %ΔP = 25%; eD = 0.8.
  • Exercise 24. eD = −0.2 and price rises 5%: quantity falls 0.2 × 5 = 1%.
  • Exercise 25. eD = −0.2 and price rises 10%: quantity falls 2%. New expenditure = 1.10 × 0.98 = 1.078 of the old, so expenditure rises (inelastic demand).

    • Formula Sheet

  • MUn = TUn − TUn−1
  • MRS = |ΔY/ΔX|
  • Budget set: p1x1 + p2x2 ≤ M; budget line slope = −p1/p2; intercepts M/p1 and M/p2
  • Consumer optimum: MRS = p1/p2
  • Linear demand: d(p) = a − bp; eD = bp/q (in magnitude)
  • eD = (ΔQ/Q) ÷ (ΔP/P)
  • Geometric eD at a point = lower segment / upper segment
  • ΔE ≈ Δp · q (1 + eD), where eD is negative

    • Board Exam Focus

  • Budget line equation, its slope and the effect of income or price changes (Exercises 2 to 7).
  • Indifference curve properties and why two ICs cannot intersect.
  • Conditions for the consumer's equilibrium (MRS = price ratio) with a diagram.
  • Normal, inferior and Giffen goods; substitutes and complements; movement versus shift.
  • Numericals on elasticity (Exercises 22 to 26) and the expenditure method.

    • Key Terms

  • Utility (उपयोगिता)
  • Marginal utility (सीमांत उपयोगिता)
  • Law of diminishing marginal utility (ह्रासमान सीमांत उपयोगिता का नियम)
  • Indifference curve (अनधिमान वक्र)
  • Marginal rate of substitution (सीमांत प्रतिस्थापन दर)
  • Budget line (बजट रेखा)
  • Consumer's equilibrium (उपभोक्ता संतुलन)
  • Law of demand (माँग का नियम)
  • Normal good (सामान्य वस्तु)
  • Inferior good (निम्नस्तरीय वस्तु)
  • Substitutes (स्थानापन्न वस्तुएँ)
  • Complements (पूरक वस्तुएँ)
  • Market demand (बाज़ार माँग)
  • Price elasticity of demand (माँग की कीमत लोच)

    • Common Mistakes

  • Adding individual demand curves vertically; market demand is a horizontal sum of quantities.
  • Writing the budget line slope as −p2/p1; with good 1 on the x-axis it is −p1/p2.
  • Thinking a price change shifts the demand curve. The own price change causes a movement along it.
  • Reporting elasticity as −0.5 and calling it "less than zero, so elastic". Compare the magnitude with 1.
  • Treating every inferior good as a Giffen good. All Giffen goods are inferior, not all inferior goods are Giffen.

    • 💡 Key Learning Points

    • ✓Marginal utility diminishes as consumption rises, and this explains why a consumer buys more only at a lower price.
    • ✓The budget line p1x1 + p2x2 = M has slope −p1/p2; income changes shift it parallel, price changes rotate it.
    • ✓Indifference curves are downward sloping, convex (diminishing MRS) and never intersect; the optimum is at tangency where MRS = p1/p2.
    • ✓Demand depends on own price, income, related prices and tastes; own price moves along the curve, other factors shift it; market demand is the horizontal sum.
    • ✓Price elasticity eD = %ΔQ/%ΔP measures responsiveness; it varies along a linear demand curve and determines how expenditure changes with price.

    👨‍🏫 Teaching Tips

    • →Let students eat biscuits one by one and rate satisfaction to experience diminishing marginal utility first-hand.
    • →Draw the budget line for ₹20 with ₹5 prices on graph paper and have students count the 15 affordable bundles.
    • →Show why ICs cannot intersect by labelling A, B and C on two crossing curves and asking students to find the contradiction.
    • →Use the tea-coffee and tea-sugar pairs to make substitutes and complements memorable.
    • →Use two colours on the board: one for movements along a demand curve, one for shifts of the curve.
    • →Build market demand from two tables (10 − p and 15 − p) so students see the kink at p = 10.
    • →Practise elasticity with the percentage method first, then connect it to the expenditure method.
    • →Mark eD = 0, 1 and ∞ on a straight-line demand curve so the geometric method is visual.

    📋 Assessment Questions

    1. Can the student compute MU from a TU schedule and state the law of diminishing marginal utility?
    2. Can the student write the budget line equation, find intercepts and slope, and show the effect of price and income changes?
    3. Can the student explain consumer equilibrium using MRS = p1/p2 and the properties of indifference curves?
    4. Can the student distinguish normal, inferior, substitute and complementary goods and movements versus shifts?
    5. Can the student calculate price elasticity by the percentage method and relate it to total expenditure?

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    All chapters in Introductory Microeconomics - Economics Class 12

    1. 1. Introduction
    2. 2. Theory of Consumer Behaviour
    3. 3. Production and Costs
    4. 4. The Theory of the Firm under Perfect Competition
    5. 5. Market Equilibrium