Demand

Topic: Consumer Behaviour, Demand and Elasticity · NCERT: Class 7, Ch 12 "Understanding Markets"; Class 9, Ch 9 "The Price Puzzle: What Drives the Market"; Class 12, Ch 2 "Theory of Consumer Behaviour"

Meaning

Demand is the quantity of a good that a consumer is willing to buy and able to pay for at a given price. This quantity depends on the price of the good, the prices of other goods, the consumer's income and their tastes and preferences. The demand function links the quantity a consumer chooses to the good's own price, while everything else stays the same: X = f(P). In its simple straight-line form it is written d(p) = a − bp.

Demand matters because it is one half of every market. It tells us how people react when prices change. It also explains how subsidies, cash transfers and taxes change what households buy and how well off they feel.

Explanation

What counts as demand

• Willingness alone is not demand. It must also be backed by the ability to pay.
• Wanting a car without the money to buy it is not demand.

• Class 7 NCERT: needs are essentials such as food and clothing. Wants are extra things we would like. Either one becomes demand only when the person can pay for it.

• Four things decide demand:

• the price of the good itself,
• prices of other goods,
• the consumer's income,

• the consumer's tastes and preferences.

• Demand function X = f(P): here we look only at the good's own price. The other three factors are held fixed. This is called ceteris paribus ("other things being equal").

• Demand schedule: a table of the quantities demanded at different prices.
• Demand curve: the graph of that schedule. It shows the quantity demanded at each price, with other prices, income and preferences held constant.
• It generally slopes down from left to right. When price falls, quantity demanded rises [3].

• Axis trap: in maths, the independent variable goes on the horizontal axis. Economics reverses this. Price goes on the vertical axis and quantity on the horizontal axis [3]. This habit comes from Alfred Marshall's diagrams.

• Individual demand: the quantity one consumer wants to buy at different prices, with other factors held constant.

• Worked example (Class 9, 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′.

Where the demand curve comes from: consumer's optimum

• Rational consumer: a person who picks the bundle (a combination of two goods) they like best among all the bundles they can afford.
• Budget line: all the bundles that cost exactly the consumer's income, p₁x₁ + p₂x₂ = M. Its slope is −p₁/p₂.
• Indifference curve (IC): a curve joining bundles that give equal satisfaction. It slopes down and is convex to the origin (bends inwards towards the zero point of the graph) [2]. A higher IC means more satisfaction.
• The optimum lies on the budget line.
• With monotonic preferences (the consumer always prefers more of a good), a point below the line wastes income, so it is never the best choice.

• Points above the line cost more than the income, so the consumer cannot buy them.

• The optimum is where the budget line just touches (is tangent to) the highest reachable IC [2]. This point is called consumer equilibrium.

• Ordinal condition: MRS = p₁/p₂. MRS (marginal rate of substitution) is how much of good 2 the consumer is willing to give up for one more unit of good 1. The price ratio is how much of good 2 the market makes them give up.
• MRS = MU₁/MU₂ [2]. MU (marginal utility) is the extra satisfaction from one more unit of a good.

• Cardinal condition (when satisfaction is measured in numbers): MUx/Px = MUy/Py. The last rupee spent on each good must give the same extra satisfaction.

• Corner solution: sometimes the best point lies on an axis, which means all income goes to one good. At that point the tangency rule need not hold.

Deriving the demand curve (price falls, everything else fixed)

• Price of good 1 falls while p₂ and income M stay the same.
• The budget line swings outward around its good-2 intercept and becomes flatter.
• The new tangency lies on a higher IC.

• The consumer buys more of good 1.

• Plot each (quantity, price) pair and join the points. The result is a downward-sloping demand curve.

• Every point on the demand curve is a best choice at that price.
• The line joining the tangency points in the IC diagram is the price-consumption curve. The demand curve shows the same information on a price–quantity graph.
• Worked example: M = ₹120 and p₂ = ₹10. The consumer always spends half their income on good 1, so x₁ = 60/p₁.

p₁	Optimal x₁
₹20	3
₹12	5
₹10	6

• As the price falls, the quantity rises. So the curve slopes down.

Linear demand curve

• d(p) = a − bp for 0 ≤ p ≤ a/b. d(p) = 0 for p > a/b. Both a and b are positive.
• a = quantity bought when the good is free. This is the quantity (horizontal) intercept.
• a/b = choke price (the price at which the consumer stops buying). This is the price (vertical) intercept.

• The slope of the curve as drawn (price on the vertical axis) is −1/b.

• NCERT trap: Class 12 NCERT calls a the "vertical intercept" and −b the "slope". With price on the vertical axis, both labels are wrong.

• Worked example 1: d(p) = 10 − 2p.
• Quantity intercept = 10. Choke price = 10/2 = ₹5. Slope as drawn = −1/2.

• At ₹3, demand = 10 − 6 = 4 units. At ₹6, demand = 0.

• Worked example 2 (Srivalli): q = 4 − (1/50)p, so a = 4 and b = 1/50.

• Check: at ₹150, q = 1. At ₹100, q = 2. At ₹50, q = 3.
• Choke price = 4 ÷ (1/50) = ₹200. Slope of DD′ = −50, so the price must fall by ₹50 for her to buy 1 kg more.

In India

• Ability to pay is measured through MPCE. MPCE (monthly per capita consumption expenditure) is how much each person in a household spends on consumption in a month.
• Survey: Household Consumption Expenditure Survey (HCES), run by MoSPI/NSO [4][5].
• Rural MPCE: ₹1,430 (2011-12) → ₹3,773 (2022-23) → ₹4,122 (2023-24) [4].

• Urban MPCE: ₹2,630 (2011-12) → ₹6,459 (2022-23) → ₹6,996 (2023-24) [4].

• The pattern of demand is changing.

• Food makes up about 47% of rural MPCE (2023-24) [5].
• Non-food items make up about 53% (rural) and 60% (urban) of MPCE (2023-24) [5].

• As incomes rise, the share spent on food falls. This is Engel's law.

• Consumption has become more equal. The Gini coefficient measures inequality: 0 means perfect equality and 1 means perfect inequality.

• Rural: 0.266 (2022-23) → 0.237 (2023-24) [4].

• Urban: 0.314 (2022-23) → 0.284 (2023-24) [4].

• Everyday example: cheap or free food grain through PDS lowers the price a household faces. Its budget line swings outward, and it can reach a higher IC.

Don't confuse with

• Want / desire: a wish to have something. It becomes demand only when the person is able to pay.
• Quantity demanded: the amount bought at one price, which is one point on the curve. A change in the good's own price moves you along the demand curve. A change in income, tastes or other prices shifts the whole curve.
• Demand schedule vs demand curve: the schedule is a table and the curve is its graph. Both hold other factors constant.
• Price-consumption curve: joins the tangency points in the IC diagram (good 1 against good 2). The demand curve shows the same choices on a price–quantity graph.

Prelims Hooks

• Demand = willingness + ability to pay. Desire alone is not demand.
• Demand function X = f(P), with other prices, income and tastes held constant (ceteris paribus). On the graph, price goes on the vertical axis and quantity on the horizontal axis, which is the reverse of the maths convention [3].
• Consumer equilibrium: MRS = p₁/p₂ (ordinal approach) and MUx/Px = MUy/Py (cardinal approach). MRS = MU₁/MU₂ [2].
• If the price of good 1 falls while p₂ and M stay fixed, the budget line becomes flatter and swings outward. Each tangency point gives one point on the downward-sloping demand curve.
• Linear demand d = a − bp: quantity intercept = a, price intercept (choke price) = a/b, slope as drawn = −1/b. NCERT's "vertical intercept a" is wrong.
• HCES 2023-24 (MoSPI): MPCE is ₹4,122 rural and ₹6,996 urban. The consumption Gini is 0.237 rural and 0.284 urban [4].

Mains Points

• In-kind subsidy vs cash transfer (GS-III: subsidies, PDS, DBT):
• A price subsidy, such as cheap food grain, swings the budget line outward and makes it flatter.
• A cash transfer shifts the budget line out in parallel.

• Comparing the two tangency points shows why economists debate which one gives more welfare for the same fiscal cost.

• Distorted prices waste welfare:

• Taxes, subsidies and price controls push the market price ratio away from true costs.
• Consumers then set MRS equal to a wrong price ratio.

• This supports rationalising GST rates and cutting untargeted subsidies.

• Changing demand patterns and policy (GS-III: growth, inclusive development):

• MPCE is rising and the food share is falling. The non-food share is about 53% in rural areas and about 60% in urban areas (2023-24) [5].
• This matters for CPI weights, crop diversification towards milk, vegetables and processed food, and demand-led manufacturing.
• Limit: the model assumes rational, fully informed consumers. Behavioural economics explains why "nudges" (e.g. Swachh Bharat) sometimes work better than price signals alone.

Related concepts

• Consumer equilibrium
• Demand function
• Demand curve
• Demand schedule
• Individual demand
• Linear demand curve

Read more

• Consumer's optimum and derivation of the demand curve