Consumer Behaviour, Demand and Elasticity

1. Needs, wants and the consumer's problem of choice

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Needs vs wants (Class 7, Understanding Markets)

• A need is something a person requires to survive: food, water, clothing, shelter.
• A want is something a person desires but does not need to survive, such as a new phone model or a restaurant meal.
• Both are met mostly through markets. People buy what they cannot make themselves (Adam Smith's epigraph in that chapter).

Who is a consumer

• A consumer buys goods or uses services for her own or her family's use (or as a gift), not for resale or business use. A wholesaler buying rice to resell is not a consumer in this sense.
• Consumption means buying goods to satisfy needs. Studying it asks one question: with a given income and known prices, what does a household buy?
• Class 12 uses "goods" to mean both goods and services.

The problem of choice (Class 12, Theory of Consumer Behaviour)

• Income is limited and wants are many. The consumer must decide how to split her income across goods to get the most satisfaction. Economists call this the problem of choice.
• The best combination depends on two things:
• Preferences: her likes over the available bundles.

• What she can afford: set by the prices of goods and her income.

• Price has two definitions:

• Class 12: the amount of money the consumer pays per unit of the good (₹p per kg of bananas).
• Class 7: the amount at which a buyer is willing to buy and a seller is willing to sell. It comes out of bargaining, as in the guava example.

The set-up used throughout

• Two goods: bananas (x₁) and mangoes (x₂). Using only two goods keeps the diagrams simple.
• A consumption bundle is any combination (x₁, x₂). For example, (5, 10) means 5 bananas and 10 mangoes, and (10, 5) means 10 bananas and 5 mangoes. Quantities can be positive or zero.
• A rational consumer knows what is good or bad for her. From the bundles available to her, she always picks the one that gives the most satisfaction.

Utility

• Utility is a good's want-satisfying capacity. The stronger the need or desire, the higher the utility.
• It is subjective:
• It varies across persons: a chocolate lover gets more utility from a chocolate than someone who does not care for chocolate.
• It varies with place: a room heater gives more utility in Ladakh than in Chennai.
• It varies with time: the same heater gives more utility in winter than in summer.

Consumer sovereignty

• Consumer sovereignty means that what consumers spend on decides what gets produced in a market economy.
• Class 7 example: when many buyers ask for refrigerators that use less electricity, the signal passes back through retailers and wholesalers to manufacturers. Manufacturers then make energy-efficient models (BEE star ratings), and society benefits.

Two ways to model choice

• Cardinal approach (section 2): utility is measured in numbers.
• Ordinal approach (sections 3-5): bundles are only ranked.

2. Cardinal utility: total utility, marginal utility and the law of diminishing marginal utility

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Cardinal utility analysis assumes utility can be measured and expressed in numbers, for example "this shirt gives me 50 units of utility".

Two measures

• Total utility (TU): the total satisfaction from consuming a given amount of a good. TUₙ is the utility from n units.
• Marginal utility (MU): the change in TU from consuming one more unit.
• MUₙ = TUₙ − TUₙ₋₁
• Example: if 4 bananas give TU of 28 and 5 bananas give 30, then MU₅ = 30 − 28 = 2.
• TUₙ = MU₁ + MU₂ + … + MUₙ, so total utility is the sum of the marginal utilities.

Table 2.1 (Class 12)

Units	TU	MU
1	12	12
2	18	6
3	22	4
4	24	2
5	24	0
6	22	−2

• TU rises, but at a diminishing rate, because MU keeps falling (12 → 6 → 4 → 2).
• TU is at its maximum when MU = 0 (5th unit). This point is called satiety.
• After that, MU turns negative and TU falls (6th unit).

Law of diminishing marginal utility (DMU)

• The law of diminishing marginal utility says that the MU of each extra unit of a good falls as you consume more of it, holding consumption of other goods constant.
• Reason: once she has some of the good, her desire for more grows weaker.
• Class 9 example: the first mango tastes delicious, the second is good, and by the third you are barely interested.
• It is also called Gossen's first law (Hermann Gossen, 1854).

DMU explains the downward-sloping demand curve

• Each extra unit is worth less to her, so she will pay less for it.
• Class 12 example: at ₹40 per unit she buys 5 units. The 6th unit is worth less than the 5th, so she buys it only if the price falls below ₹40.

Extensions beyond NCERT

• Law of equi-marginal utility (Gossen's second law): a consumer gets the most satisfaction when the MU per rupee is the same for every good she buys.
• MUx/Px = MUy/Py = MU of money
• If MUx/Px > MUy/Py, she moves a rupee from y to x until the two ratios are equal.

• This is the cardinal version of the ordinal optimum MRS = p₁/p₂ (section 5).

• Consumer surplus (Alfred Marshall, Principles, 1890): the extra benefit a buyer gets over what she pays.

• Consumer surplus = what she is willing to pay − what she actually pays.
• On a graph, it is the area below the demand curve and above the market price.
• Example: she would pay ₹60 for the 1st unit and ₹50 for the 2nd, but the price is ₹40. Her surplus is 20 + 10 = ₹30.

• Its use in measuring welfare loss (deadweight loss) belongs to the market-equilibrium-price-controls topic.

• Diamond-water paradox (Adam Smith, 1776): water is essential but cheap, while diamonds are inessential but costly.

• Resolution: price follows marginal utility and scarcity, not total utility.
• Water is plentiful, so the MU of one more litre is tiny even though its TU is huge. Diamonds are scarce, so their MU is high.
• The marginalists (Jevons, Menger, Walras) solved this in the 1870s.

Weakness of the cardinal approach

• In real life nobody measures utility in numbers. At most we rank bundles.
• This drawback led to the ordinal approach (section 3).

3. Ordinal utility: indifference curves, MRS and preferences

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Ordinal utility analysis (Hicks and Allen, 1934) does not measure utility. The consumer only ranks bundles as better, worse or equally good.

Indifference curve (IC)

• An indifference curve joins all bundles that give the consumer equal satisfaction. She is indifferent between any two points on it (A, B, C, D in Fig 2.3).
• To get one more banana and stay on the same curve, she must give up some mangoes. So the IC slopes downward.

Marginal rate of substitution (MRS)

• The MRS is the number of mangoes she is willing to give up for one extra banana while keeping total utility the same.
• MRS = |ΔY/ΔX|, the size of the slope ignoring the minus sign. If ΔY/ΔX = −3/1, then MRS = 3.

Table 2.2: diminishing MRS

Bundle	Bananas (X)	Mangoes (Y)	MRS
A	1	15	–
B	2	12	3:1
C	3	10	2:1
D	4	9	1:1

• Law of diminishing MRS: as bananas increase, she gives up fewer and fewer mangoes for each extra banana.

• Why: bananas become plentiful, so the MU of another banana falls. Mangoes become scarce, so the MU of each remaining mango rises.

• Convexity of the IC: diminishing MRS makes the IC convex to the origin, bowed inward. This is the most common shape.

Special shapes

• Perfect substitutes: goods that can replace each other and give exactly the same utility, such as ₹5 notes and ₹5 coins (Table 2.3: (1,8), (2,7), (3,6), (4,5)). The MRS stays constant at 1:1, so the IC is a straight line.
• Perfect complements (beyond NCERT): goods used only in a fixed ratio, such as left and right shoes. An extra left shoe adds nothing, so the IC is L-shaped.

Monotonic preferences

• Monotonic preferences: of two bundles, she prefers the one with more of at least one good and no less of the other.
• Examples:
• (10, 10) is preferred to (10, 9), which is preferred to (9, 9).
• A monotonic consumer cannot be indifferent between (10, 8) and (8, 6).
• If a friend is indifferent between (5, 6) and (6, 6), the friend's preferences are not monotonic.

Indifference map

• An indifference map is a family of ICs showing all of a consumer's preferences. With monotonic preferences, a higher IC is preferred.

Properties of indifference curves

1. They slope downward from left to right. More bananas must be offset by fewer mangoes. Otherwise she would move to a higher IC.
2. A higher IC gives higher utility. Table 2.4: A (1, 10), B (2, 10) and C (3, 10) have the same mangoes and more bananas, so C > B > A, and each lies on a higher IC. This holds as long as MU is positive.
3. Two ICs never intersect. Suppose IC₁ and IC₂ cross at A (Fig 2.8): - A = B (both on IC₁) and A = C (both on IC₂), so B = C. - But B has more mangoes than C with the same bananas, so B must be better than C. - This is a contradiction, so the curves cannot cross.

Other representations

• Utility function: preferences can also be written as a function U(x₁, x₂) that gives higher numbers to preferred bundles. It is an alternative to the indifference map. Only the ranking of the numbers matters.
• Revealed preference (Paul Samuelson, 1938): infer preferences from what people actually buy, not from asking about utility. If she buys bundle A when B was also affordable, A is revealed preferred to B.

4. The consumer's budget: budget set, budget line and its shifts

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Budget constraint

• Income M, prices p₁ (bananas) and p₂ (mangoes). The bundle (x₁, x₂) costs p₁x₁ + p₂x₂.
• She can buy it only if p₁x₁ + p₂x₂ ≤ M. This inequality is the budget constraint.

Budget set

• The budget set is every bundle she can buy with her income at current market prices.
• Example 2.1: M = ₹20, both goods at ₹5, sold only in whole units.
• There are 15 affordable bundles: (0,0), (0,1), (0,2), (0,3), (0,4), (1,0), (1,1), (1,2), (1,3), (2,0), (2,1), (2,2), (3,0), (3,1), (4,0).
• Five cost exactly ₹20: (0,4), (1,3), (2,2), (3,1), (4,0).

• (3,3) and (4,5) cost too much.

• Divisible goods come in non-whole units, such as half a kg of rice or a quarter litre of milk. Oranges and bananas are sold only in whole units. With divisible goods, the budget set is the whole triangle on and under the line.

Budget line

• The budget line is all bundles that cost exactly M: p₁x₁ + p₂x₂ = M.
• Rewritten: x₂ = M/p₂ − (p₁/p₂)x₁, which has the form y = c + mx.
• Horizontal intercept = M/p₁ (all income spent on bananas).
• Vertical intercept = M/p₂ (all income spent on mangoes).

• Slope = −p₁/p₂.

• Points below the line cost less than M. Points above it cannot be bought.

• Why the slope is −p₁/p₂: take two points on the line.
• p₁x₁ + p₂x₂ = M and p₁(x₁+Δx₁) + p₂(x₂+Δx₂) = M.
• Subtract: p₁Δx₁ + p₂Δx₂ = 0, so Δx₂/Δx₁ = −p₁/p₂.
• The line slopes down because, with the whole budget spent, more of one good means less of the other.

Price ratio vs MRS

• Price ratio p₁/p₂: the rate at which the market lets her swap bananas for mangoes. One extra banana costs p₁, which is the price of p₁/p₂ mangoes.
• MRS: the rate at which she is willing to swap.
• Comparing the two gives the optimum (section 5).

Shift of the budget line

• Income changes (prices fixed): a parallel shift. The slope −p₁/p₂ stays the same.
• M rises: both intercepts rise and the line moves outward.

• M falls: the line moves inward.

• One price changes (p₁ changes; p₂ and M fixed): the line pivots about the vertical intercept M/p₂.

• p₁ rises: the line gets steeper and the horizontal intercept falls.
• p₁ falls: the line gets flatter and the horizontal intercept rises.
• A change in p₂ has the mirror effect, pivoting about M/p₁.

Class 12 exercise drills

• Prices ₹4 and ₹5, M = ₹20:
• Budget line: 4x₁ + 5x₂ = 20. Intercepts 5 and 4. Slope −4/5.
• M rises to ₹40: parallel outward shift, intercepts 10 and 8.

• p₂ falls to ₹4: pivots out on the vertical axis (new intercept 5). Slope becomes −1.

• Doubling income and both prices leaves the budget set unchanged. 2p₁x₁ + 2p₂x₂ = 2M reduces to the original line.

• Bundle (6, 8) at ₹6 and ₹8 uses the whole income, so M = 36 + 64 = ₹100.

5. Consumer's optimum and derivation of the demand curve

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Where the optimum lies

• The rational consumer's problem: reach the highest IC possible within her budget set.
• On the budget line, not below it. With monotonic preferences, any point below the line has a point on the line with more of one good and no less of the other. Points above the line cannot be bought.
• At the tangency point. The optimum (x₁, x₂) is where the budget line just touches an IC. Any other point on the line lies on a lower IC. Higher ICs cannot be reached (Fig 2.12).
• This point is the consumer equilibrium:
• MRS = p₁/p₂ (slope of IC = slope of budget line).
• What she is willing to swap equals what the market lets her swap.
• Cardinal version: MUx/Px = MUy/Py.

Why a point with MRS ≠ price ratio is not optimal

• Suppose MRS = 2 and both goods have the same price, so the price ratio is 1.
• She would give up 2 mangoes for one more banana. The market asks for only 1.
• So she buys the extra banana with 1 mango and ends up with more of both goods than the "2-mango" trade would leave her. That is a better bundle.
• The same logic works when MRS < price ratio.
• Corner solution (footnote 9): sometimes the optimum is at an axis, with all income spent on one good.

Demand

• Demand is the quantity a consumer is willing to buy and able to afford, given the prices of goods, her income, and her tastes and preferences.
• Demand function: X = f(P). It relates her optimal quantity to the good's own price, with everything else held constant.
• NCERT box on functions:
• In maths, the independent variable (x) goes on the horizontal axis.

• Economics reverses this: price (independent) goes on the vertical axis and quantity (dependent) on the horizontal axis.

• The demand curve is the graph of the demand function. It shows the quantity demanded at each price, with other prices, income and preferences held constant. It generally slopes down.

Deriving the demand curve from ICs (Fig 2.14)

• Start at price P′₁. She is in equilibrium at C, buying X′₁. Plot (X′₁, P′₁).
• Lower the price to P̄₁, keeping P′₂ and M fixed. The budget line swings outward (flatter) and the new tangency is at D on a higher IC. She buys X̄₁ > X′₁. Plot this point.
• Lower the price again to P̂₁ and she buys X̂₁. Plot this point too.
• Joining the points gives a negatively sloped demand curve.

Demand schedule and individual demand (Class 9, The Price Puzzle)

• Individual demand: the quantity one consumer wants to buy at different prices, other factors held constant.
• Demand schedule: a table of quantities demanded at different prices. Srivalli's mangoes:

Price per kg	Srivalli's demand
₹150	1 kg (point A)
₹100	2 kg (point B)
₹50	3 kg (point C)

• Joining A, B and C gives the downward-sloping demand curve DD′.

Linear demand curve

• Linear demand curve: d(p) = a − bp for 0 ≤ p ≤ a/b, and d(p) = 0 for p > a/b.
• At p = 0 demand is a. At p = a/b demand is 0.
• Demand falls by b units for each ₹1 rise in price.
• NCERT error: Class 12 calls a the "vertical intercept" and −b the "slope". With price on the vertical axis:
• a is the quantity (horizontal) intercept.
• The price (vertical) intercept is a/b.
• The slope of the drawn curve is −1/b.
• −b is Δq/Δp, the slope only if quantity were on the vertical axis.

6. Law of demand, substitution and income effects, normal/inferior/Giffen goods and exceptions

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Law of demand

• Law of demand: other things equal, price and quantity demanded are inversely related. When price rises, demand falls. When price falls, demand rises.
• In Class 9, mangoes cost ₹150 early in the season, so people buy little. As prices fall, they buy more.

Why the demand curve slopes down

• Diminishing marginal utility (cardinal reason): each extra unit is worth less, so she buys more only at a lower price.
• Substitution effect: when bananas get cheaper relative to mangoes, she substitutes bananas for mangoes at the same level of satisfaction.

• For a price fall, this always raises the quantity of the cheaper good.

• Income effect: a lower price raises her purchasing power (real income).

• For a normal good, this raises demand further (for bananas, and mangoes too).

Classifying goods by income response

• Normal good: demand moves with income. This covers most goods.
• Inferior good: demand moves against income. Example: low-quality food such as coarse cereals.

• A good can be normal at low incomes and inferior above a threshold. A very poor household buys more coarse cereal as income rises. Beyond a point it switches to better cereals.

• Giffen good: an inferior good where the negative income effect is stronger than the substitution effect, so demand rises with price (upward-sloping demand).

• For an inferior good, the two effects pull in opposite directions:
  • Substitution effect stronger: the law of demand still holds.
  • Income effect stronger: it is a Giffen good.
• All Giffen goods are inferior; not all inferior goods are Giffen.
• Evidence:
  • Robert Giffen/Marshall's bread and potato story. The potato case during the Irish famine (1845-49) is often cited but historically disputed.
  • Jensen and Miller (2008) found rice in Hunan (China) behaved as a Giffen good for very poor households receiving a subsidy. Wheat in Gansu showed weaker evidence.

Exceptions and anomalies beyond NCERT

• Veblen good: a luxury or status good whose demand rises with price because a high price signals prestige. Examples: luxury cars, designer labels, jewellery.
• Linked to conspicuous consumption (Thorstein Veblen, The Theory of the Leisure Class, 1899): spending to display wealth and status, not for the good's usefulness.

• Giffen vs Veblen: Giffen is an income-effect case among the poor with inferior goods. Veblen is a prestige/preference case among the rich with luxury goods.

• Bandwagon effect (Harvey Leibenstein, 1950): demand rises because many others already buy the good. Examples: fashion trends, new smartphones. Class 9 mentions long launch-day queues and pre-bookings even at higher prices.

• Snob effect (Leibenstein): some buyers cut demand as a good becomes common, because they want exclusivity. It is the opposite of the bandwagon effect.
• Demonstration effect (James Duesenberry, 1949; Ragnar Nurkse, 1953, applied it across countries): people copy the consumption of richer groups or richer countries. This raises consumption and lowers savings, which holds back capital formation in developing economies.
• Panic buying / price expectations: if buyers expect prices to rise further, they may buy more as prices rise. Examples: onion hoarding, COVID-19 sanitiser rush (Class 9). This is really a shift caused by expectations (section 7), not a true exception.
• Some texts also list necessities bought in fixed amounts and ignorance of quality ("higher price = better").

7. Determinants of demand: shifts vs movements

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Determinants of demand: besides its own price, demand depends on the following.

1. Prices of related goods

• Related goods: goods whose demand is linked, so a change in one's price or availability affects demand for the other.
• Substitute goods replace each other. Examples: tea/coffee, mango/banana, AC/cooler.
• Demand moves in the same direction as the substitute's price.

• If coffee costs more, tea demand rises and tea's demand curve shifts right.

• Complementary goods are used together. Examples: tea/sugar, shoes/socks, pen/ink (Class 12); smartphone/earphones, cars/petrol, printers/cartridges, movie tickets/cinema popcorn (Class 9).

• Demand moves opposite to the complement's price.
• If sugar costs more, tea demand falls and its curve shifts left.
• Costlier movie tickets mean fewer cinema visits, so popcorn demand falls.

2. Consumer income

• Consumer income: the money income a household has.
• With prices unchanged, a rise in income:
• shifts a normal good's curve right;

• shifts an inferior good's curve left.

• Trap: Class 9 asks whether "an increase in income always leads to a rise in demand". Refute it. Inferior goods such as coarse grains or cheap cloth see demand fall.

3. Tastes and preferences

• Ice-cream demand shifts right in summer.
• News that cold drinks may harm health shifts their demand left.
• Srivalli will not switch to oranges even if they are cheaper.

4. Population size and composition

• Population size and composition: the size and age structure of the population shape both the level and type of demand.
• Size: India, the most populous nation, draws growth from domestic demand.
• Composition:
• more children → more sports shoes;
• more working adults → more formal shoes;

• more elderly → more orthopaedic/comfort shoes.

• The number of buyers also drives market demand (section 8).

5. Seasonality

• Crowded bookshops when the academic session starts.
• Sweets at festivals.
• Woollens in winter.
• These are driven by weather, festivals and habits, not price.

6. Future price expectations

• Future price expectations: if people expect a price fall, they postpone buying and present demand falls. People delay durables before Diwali or New Year sales.
• If they expect a rise, they advance purchases and present demand rises.
• Sellers react the same way (potato wholesalers holding stock).

Movement vs shift

	Movement along the demand curve	Shift in demand curve
Cause	Change in the good's own price only	Change in income, related prices, tastes, population, season, expectations
Name	Extension (price falls → more bought) / Contraction (price rises → less bought) of quantity demanded	Increase (shift right) / Decrease (shift left) in demand
Graph	Same curve, new point	New curve

• Movement along the demand curve: quantity demanded changes only because the good's own price changed.
• Shift in demand curve: the curve moves right or left because a factor other than own price changed.

Purchasing power

• Purchasing power is the ability to pay for goods. Class 9 defines demand as desire backed by purchasing power.
• A need with no money behind it, such as housing for the very poor, is not market demand. This is why markets can leave basic needs unmet.
• Class 9's margin definition ("what one unit of currency can buy at a particular time") is the macro sense of the term (the value of money). The micro sense is the buyer's ability to pay.

Exercise drill (Class 9): petrol price doubles

• Diesel cars: demand rises. Substitute fuel and substitute car.
• Electric vehicles: demand rises. Substitute.
• Car accessories: demand falls. Complement to car use.
• Public transport: demand rises. Substitute for private driving.

8. Market demand

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Definition

• Market demand: the total quantity demanded by all consumers in the market at each price.
• It is found by horizontal summation: at each price, add the individual quantities (along the quantity axis, the horizontal axis). Prices are not added.

Class 9 table (Table 9.1)

Price	Srivalli	Alex	Israt	Market demand
₹150	1 kg	2 kg	3 kg	6 kg
₹100	2 kg	4 kg	6 kg	12 kg
₹50	3 kg	6 kg	9 kg	18 kg

Adding linear demands (Class 12)

• d₁(p) = 10 − p (zero above ₹10) and d₂(p) = 15 − p (zero above ₹15).
• Market demand:
• 25 − 2p for p ≤ 10 (both buy);
• 15 − p for 10 < p ≤ 15 (only consumer 2 buys);

• 0 for p > 15.

• Result: a kinked market demand curve, with the kink at p = 10, where q = 5.

• Rule: add only the consumers who buy at that price.

Identical consumers

• With n identical consumers, market demand = n × individual demand.
• Class 12 exercise: 20 consumers each with d(p) = 10 − 3p. Market demand = 200 − 60p for p ≤ 10/3, and 0 above.

Other drills

• Class 12 table exercise: add d₁ and d₂ at each price. At p = 1: 9 + 24 = 33. At p = 6: 4 + 12 = 16.
• Unequal cut-off prices (d₁ = 20 − p up to ₹20; d₂ = 30 − 2p up to ₹15):
• 50 − 3p for p ≤ 15;
• 20 − p for 15 < p ≤ 20;
• 0 above 20.

Number of buyers

• The number of buyers is itself a determinant of market demand. More buyers (population growth, new markets, online reach) shift the market curve right even if each person's demand is unchanged.

NCERT error (Class 9, "Don't miss out" box)

• Class 9 says the market curve is flatter and so "more responsive". That mixes up slope and elasticity.
• A flatter curve shows a bigger absolute change in quantity (12 kg vs 2 kg).

• Elasticity is about percentage changes.

• Here market demand is exactly 6 × Srivalli's at every price. Every percentage change in quantity is identical.

• ₹150 → ₹50: Srivalli goes from 1 to 3 kg (+200%) and the market from 6 to 18 kg (+200%).

• So the elasticities are the same.

• The market curve is flatter only because the quantity scale is bigger.

9. Price elasticity of demand: measurement, determinants and expenditure

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Price elasticity of demand

• Price elasticity of demand (eD) measures how strongly quantity demanded responds to a change in the good's own price.
• eD = % change in quantity demanded ÷ % change in price = (ΔQ/Q) × (P/ΔP)
• It is a pure number (unit-free), so it can be compared across goods measured in kg, litres or rupees.
• It is negative (law of demand) but is quoted in absolute value.

Example 2.2 (Class 12)

• Bananas: ₹5 → ₹7 (P₁ = 5), quantity 15 → 12 (Q₁ = 15).
• %ΔQ = (12−15)/15 × 100 = −20%. %ΔP = (7−5)/5 × 100 = +40%.
• |eD| = 20/40 = 0.5, so demand is inelastic.
• Exercise 22: ₹4 → ₹5 and 25 → 20 units gives −20%/25% = 0.8.

Degrees of elasticity

Type	Value	Meaning	Typical case
Perfectly inelastic	0	Quantity does not change at all	Vertical curve
Inelastic	< 1	%ΔQ < %ΔP	Essentials (food, salt, medicine)
Unitary elastic	= 1	%ΔQ = %ΔP	Rectangular hyperbola
Elastic	> 1	%ΔQ > %ΔP	Luxuries
Perfectly elastic	∞	Any price above P̄ drops quantity to zero	Horizontal curve

• Elastic demand: |eD| > 1.
• Inelastic demand: |eD| < 1.
• Unitary elastic demand: |eD| = 1, and expenditure does not change.
• One good can be elastic, unitary or inelastic at different prices.

Elasticity along a linear demand curve

• For q = a − bp, Δq/Δp = −b, so eD = −bp/q = −bp/(a − bp). This formula is the elasticity along a linear demand curve.
• At p = 0 (on the quantity axis): eD = 0.
• At the midpoint p = a/2b: |eD| = 1.
• Between 0 and a/2b: < 1. Above a/2b: > 1.

• At q = 0 (on the price axis): eD = ∞.

• A straight line has a constant slope but not a constant elasticity.

• Exercise 23: D(p) = 10 − 3p at p = 5/3 gives q = 5, so eD = −3 × (5/3)/5 = −1, unitary. This is the midpoint.

Geometric measure of elasticity of demand

• At any point D on a straight-line demand curve that meets the quantity axis at A and the price axis at B:
• eD = lower segment ÷ upper segment = DA/DB (proof by similar triangles).
• At A: 0/DB = 0. At B: DA/0 = ∞. At the midpoint: 1.

Constant-elasticity curves (Fig 2.20)

• Vertical demand curve: perfectly inelastic demand, |eD| = 0 at every price.
• Horizontal demand curve: perfectly elastic demand, |eD| = ∞. This is the demand curve facing a perfectly competitive firm.
• Rectangular hyperbola: a curve xy = c, where the product of the two variables is constant.
• As demand, pq = constant, so |eD| = 1 at every point.
• Every rectangle under the curve has the same area, so expenditure is the same at every price.
• The same shape describes the average fixed cost (AFC) curve.

Beyond NCERT: arc (midpoint) formula

• NCERT uses initial values, so a rise and a fall between the same two points give different answers.

• Example 2.2 reversed (₹7 → ₹5, 12 → 15): +25%/−28.6% = 0.875, not 0.5.

• The midpoint formula fixes this:

• eD = [ΔQ/((Q₁+Q₂)/2)] ÷ [ΔP/((P₁+P₂)/2)]
• Example 2.2: (3/13.5)/(2/6) = 0.67 in both directions.

Determinants of price elasticity of demand

• NCERT gives two:
• Nature of the good: necessities (food) are inelastic; luxuries are elastic.

• Close substitutes: food as a whole is inelastic, but one variety of pulses is elastic because buyers switch to another variety.

• Beyond NCERT:

• Share of budget: salt is inelastic because it is a tiny share; cars are elastic.
• Time horizon: long-run demand is more elastic as people find substitutes (e.g. switching to EVs).
• Habit or addiction: tobacco and alcohol are inelastic.
• Breadth of definition: "food" is less elastic than "basmati rice of brand X".
• Number of uses: goods with many uses (electricity, milk) are more elastic.

Elasticity and expenditure

• Expenditure on a good = P × Q.
• Elasticity and expenditure:
• Elastic: price and expenditure move in opposite directions.
• Inelastic: they move in the same direction.
• Unitary: expenditure does not change.

Table 2.5

Row	Price change	Quantity change	Expenditure	Type
1	+10%	−8%	Rises	Inelastic
2	+10%	−12%	Falls	Elastic
3	+10%	−10%	No change	Unitary
4	−10%	+15%	Rises	Elastic
5	−10%	+7%	Falls	Inelastic
6	−10%	+10%	No change	Unitary

• Algebra: ΔE = qΔp + pΔq + ΔpΔq. For small changes, ΔE ≈ Δp · q(1 + eD).
• eD < −1 (elastic): ΔE has the opposite sign to Δp.
• eD > −1 (inelastic): ΔE has the same sign as Δp.
• eD = −1 (unitary): ΔE = 0.

Exercise traps

• eD = −0.2 with a 5% price rise: quantity falls 1%.
• eD = −0.2 with a 10% price rise: quantity falls 2%, so expenditure rises (by about 1.1 × 0.98 − 1 ≈ 7.8%).
• A 4% price cut raises expenditure by 2%: the price fell and spending rose, so demand is elastic. Quantity rose by about 6.25%, so |eD| ≈ 1.56.

10. Income and cross elasticity, Engel's law and the policy uses of elasticity

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Income elasticity

• Income elasticity of demand: eY = % change in quantity demanded ÷ % change in income.

eY	Type of good	Examples
< 0	Inferior	Coarse grains for richer households, cheap cloth
0 to 1	Necessity (normal)	Foodgrains, salt, basic clothing
> 1	Luxury (normal)	Jewellery, foreign travel, cars

• Necessity good: an essential like foodgrains. Demand rises less than in proportion to income (0 < eY < 1) and is price inelastic.
• Luxury good: demand rises more than in proportion to income (eY > 1) and is usually price elastic.
• These map to Class 12's statements that necessities are price-inelastic and luxuries price-elastic, and to its normal/inferior classification.

Cross elasticity

• Cross elasticity of demand: eXY = % change in quantity of X ÷ % change in price of Y.
• Positive for substitutes (coffee price up → tea demand up).
• Negative for complements (sugar price up → tea demand down).

• About zero for unrelated goods.

• This is the numerical form of section 7's related-goods rule.

Engel's law

• Engel's law (Ernst Engel, 1857): as household income rises, the share of spending on food falls, even though the absolute amount spent on food rises.
• India, HCES (share of food in monthly per capita consumption expenditure, MPCE):

Year	Rural food share	Urban food share
2011-12	~52.9%	~42.6%
2022-23	~46.4%	~39.2%
2023-24	~47% / ~39.7% (verify current)

• Within food:
• The cereal share is shrinking.
• Beverages, refreshments and processed food are rising.
• Protein items (milk, eggs, fish, meat) are rising.

• Mark: verify current.

• Why it matters: CPI weights must be revised with each new base year.

• Food and beverages weigh 45.86% in the 2012-base CPI (combined).
• A new CPI series with a newer base built on HCES data is expected to give food a lower weight (verify current).
• How the index is built belongs to the inflation-price-indices topic.

Policy uses of elasticity

1. Food prices are volatile

• Food demand is price inelastic. Short-run farm supply is also inelastic, because crops take a season to respond.
• So a small supply shock (rain, heat, pests) needs a large price rise to cut demand enough. This is why tomato, onion, potato (TOP) and pulses prices spike.
• A bumper harvest needs a large price fall before buyers absorb the extra. With inelastic demand, farm revenue falls (P × Q falls).
• This is Gregory King's law (17th century England).

• It is also the answer to Class 7's question on tomato farmers throwing away their produce.

• Responses (details in agri-marketing-msp-pds):

• buffer stocks, cold storage and processing;
• Operation Greens (2018, TOP, later widened);
• the Price Stabilisation Fund.

2. Tax incidence and revenue

• Inelastic goods (petroleum, tobacco, alcohol) give stable revenue: sales hardly fall when taxed.
• Most of the burden falls on consumers. This is often regressive, because poorer households spend a larger share of income on fuel.
• Class 9 asks why tobacco and alcohol carry high taxes. The aim is healthier choices, but inelastic demand means consumption falls only a little.
• Incidence mechanics are in the taxation topic.

3. Pricing strategy

• Festival and online sales cut prices on elastic goods (electronics, apparel) to raise revenue.
• Airlines and hotels charge more to inelastic buyers (last-minute, New Year's Eve: the Goa hotel charging ₹25,000 vs ₹1,500 in Class 9) and less to elastic buyers.

4. Limits of the "rational consumer"

• Economic Survey 2018-19, ch. 2 applied behavioural economics ("nudge"): people use habits, social norms and defaults, not only price and income.
• Examples: Swachh Bharat, Beti Bachao Beti Padhao, and nudges for tax compliance.

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Exam angles

Prelims — high-yield facts and traps

• Giffen vs Veblen vs inferior:
• All Giffen goods are inferior; not all inferior goods are Giffen.
• Giffen: income effect > substitution effect.
• Veblen: demand rises with price because of prestige, not income effect.

• "Every inferior good has an upward-sloping demand curve" — FALSE.

• Pairings:

• Conspicuous consumption: Veblen, Theory of the Leisure Class, 1899.
• Bandwagon and snob effects: Leibenstein, 1950.
• Demonstration effect: Duesenberry (Nurkse for international).
• Revealed preference: Samuelson, 1938.
• Consumer surplus: Marshall.
• Engel's law: Ernst Engel, 1857.

• DMU and equi-marginal utility: Gossen's first and second laws.

• Elasticity values:

• Vertical curve = 0; horizontal = ∞; rectangular hyperbola (pq = constant) = 1 everywhere.

• "A straight-line demand curve has constant elasticity" — FALSE: 0 at the quantity axis, 1 at the midpoint, ∞ at the price axis.

• Geometric measure = lower segment / upper segment. Elasticity is a pure number.

• "A flatter demand curve is always more elastic" — FALSE. Slope ≠ elasticity. Class 9's market-demand example has the same elasticity as Srivalli's curve.
• Expenditure logic:
• Inelastic: a price rise raises total spending (eD = −0.2 with a 10% rise → spending up).
• Elastic: a price cut raises spending.

• Unitary: spending unchanged.

• Sign conventions:

• Cross elasticity > 0 for substitutes, < 0 for complements, ≈ 0 for unrelated goods.

• Income elasticity < 0 inferior; 0-1 necessity; > 1 luxury.

• Budget line:

• Slope −p₁/p₂; intercepts M/p₁ and M/p₂.
• An income change is a parallel shift; a price change is a pivot.

• Doubling M and both prices leaves the budget set unchanged.

• Equilibrium: MRS = p₁/p₂ (ordinal); MUx/Px = MUy/Py (cardinal).

• Indifference curves:
• Slope down, convex (diminishing MRS), never intersect, higher = better.

• Perfect substitutes → straight line; perfect complements → L-shape.

• Terms: movement = extension/contraction (own price only); shift = increase/decrease (other determinants).

• Utility:
• TU is maximum when MU = 0; MU can be negative.

• "TU is maximum when MU is maximum" — FALSE.

• Definitions:

• Engel's law is about the share of food, not the absolute amount. "Food spending falls as income rises" — FALSE.
• Consumer surplus = willingness to pay − price paid.

• The diamond-water paradox is resolved by marginal, not total, utility.

• "An income rise always raises demand" — FALSE (inferior goods).

• NCERT linear-demand trap: in d(p) = a − bp with price on the vertical axis, a is the quantity intercept and a/b the price intercept.

Mains — GS-III themes

1. Inelastic food demand and supply in India: - Why TOP vegetables and pulses see sharp price swings. - Why bumper harvests hurt farmers (King's law; tomato/onion dumping). - Role of buffer stocks, cold chains, food processing, Operation Greens and the Price Stabilisation Fund.

2. Elasticity and taxation: - Why fuel, tobacco and alcohol are heavily taxed. - Incidence on consumers and regressivity. - Revenue stability vs health (behavioural) goals. - GST rate rationalisation and the demand response.

3. Engel's law and India's changing consumption basket (HCES 2022-23/2023-24): - Falling cereal share; rising protein, beverages and processed food. - The narrowing rural-urban gap. - Implications for CPI weights, nutrition policy and crop diversification. - The Economic Survey 2023-24 argument for exploring an inflation target that excludes food.

4. Inferior goods and preference shifts: - Millets became "inferior" as incomes rose and PDS rice and wheat spread. - International Year of Millets 2023 / Shree Anna as an attempt to shift the demand curve right through tastes and awareness.

5. Critique of the rational-consumer model: - Behavioural economics and nudges (Economic Survey 2018-19). - Demonstration effect and consumerism vs household savings in a developing economy. - Veblen and bandwagon effects in "premiumisation".

Current-affairs hooks

• MoSPI HCES releases: MPCE, food share, rural-urban and top-bottom fractile gaps. The revised CPI base year built on them (verify current).
• Monthly CPI prints and food-inflation spikes (TOP vegetables, pulses, edible oils). RBI MPC statements and Monetary Policy Reports on food-price shocks under the 4% ± 2% inflation target.
• Economic Survey chapters on inflation and consumption demand. The Union Budget 2025-26 income-tax relief (no tax on income up to ₹12 lakh under the new regime) framed as a consumption-demand boost.
• GST rationalisation (2025): two main slabs of 5% and 18% plus a higher rate on sin and luxury goods (verify current). Fuel excise changes as live examples of elasticity and incidence.
• Festival-season sales, luxury-goods demand, and the "premiumisation" vs weak mass consumption debate in Economic Survey and RBI Bulletin articles.