Demand curve

Topic: Consumer Behaviour, Demand and Elasticity · NCERT: Class 9, Ch 9 "The Price Puzzle: What Drives the Market"; Class 12, Ch 2 "Theory of Consumer Behaviour"; Class 12, Ch 5 "Market Equilibrium"

Meaning

A demand curve is the graph of the demand function. It shows how much of a good a consumer will buy at each price, while other prices, her income and her tastes stay the same. It generally slopes down from left to right, because price and quantity demanded move in opposite directions [3].

• Formula (demand function): X = f(P), with everything else held constant (ceteris paribus, "other things being equal").
• Linear form: d(p) = a − bp for 0 ≤ p ≤ a/b, and d(p) = 0 for p > a/b (a > 0, b > 0).

It matters because it links consumer choice to market prices. Every price, tax, subsidy or transfer decision depends on how buyers react when the price changes.

Explanation

How the curve is built: from best choices to a curve

• Demand means the quantity a consumer is willing to buy and able to pay for. Wanting a car without the money to buy it is not demand.
• Demand schedule: a table of quantities demanded at different prices. The demand curve is the graph of that table.
• Axis convention (a common exam trap):
• In maths, the independent variable goes on the horizontal axis.
• Economics reverses this. Price goes on the vertical axis and quantity goes on the horizontal axis [3].

• The habit comes from Alfred Marshall's diagrams.

• Each point on the curve is a consumer's optimum, meaning her best affordable choice:

• She picks the bundle where her budget line (all bundles that cost exactly her income, p₁x₁ + p₂x₂ = M) touches the highest indifference curve she can reach. An indifference curve joins bundles that give her equal satisfaction [2].
• At that point, MRS = p₁/p₂. MRS (marginal rate of substitution) is how much of good 2 she is willing to give up for one more unit of good 1.
• So the demand curve is a list of best choices at different prices.

Deriving it step by step (NCERT Class 12, Fig 2.14)

• Step 1: Price of good 1 is P′₁. Price of good 2 and income M are fixed. Her optimum is at point C and she buys X′₁. Plot (X′₁, P′₁).
• Step 2: the price falls to P̄₁.
• The budget line swings outward around the point where it meets the good-2 axis. It becomes flatter, because its slope −p₁/p₂ gets smaller when p₁ falls.

• The new optimum is at D, on a higher indifference curve. She buys X̄₁ > X′₁. Plot (X̄₁, P̄₁).

• Step 3: The price falls again to P̂₁. She buys X̂₁. Plot this point too.

• Step 4: Join the points to get a negatively sloped demand curve.
• The line joining C, D and the other optimum points in the indifference-curve diagram is the price-consumption curve. The demand curve shows the same information redrawn with price and quantity on the axes.
• Worked example: M = ₹120, p₂ = ₹10. She always spends half her income on good 1, so x₁ = 60/p₁.

p₁	Optimal x₁	Point on demand curve
₹20	3	(3, 20)
₹12	5	(5, 12)
₹10	6	(6, 10)

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

The linear demand curve: reading it correctly

• d(p) = a − bp
• At p = 0, demand = a. This is the most she would take if the good were free, so a is the quantity (horizontal) intercept.
• At p = a/b, demand = 0. This is the choke price (the price at which she stops buying), so a/b is the price (vertical) intercept.
• Demand falls by b units for every ₹1 rise in price.

• Slope of the curve as drawn = −1/b (change in price ÷ change in quantity).

• NCERT error (Class 12): NCERT calls a the "vertical intercept" and −b the "slope". That is wrong when price is on the vertical axis. −b is Δq/Δp, which would be the slope only if quantity were on the vertical axis.

• Worked example: d(p) = 10 − 2p.
• Quantity intercept = 10. Price intercept = 10/2 = ₹5.
• Slope as drawn = −1/2.
• At p = ₹3: d = 10 − 6 = 4 units. At p = ₹6: d = 0, because ₹6 is above the choke price of ₹5.

Movement along the curve vs shift of the curve

• A change in the good's own price → movement along the same curve (a change in quantity demanded).
• A change in any factor held constant → the whole curve shifts (a change in demand):
• Income rises → she can buy more at every price → for a normal good, the curve shifts right.
• Price of a substitute rises (e.g. tea, when coffee gets costlier) → the curve shifts right.
• Price of a complement rises (e.g. petrol for a car) → the curve shifts left.

• Tastes change in favour of the good → the curve shifts right.

• "Generally" downward sloping: in rare textbook cases, such as a Giffen good, the curve can slope upward. The downward slope is the normal case, not a law without exceptions.

In India

• Srivalli's mangoes (NCERT Class 9, The Price Puzzle):

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 curve DD′.
• It is a straight line: q = 4 − (1/50)p, so a = 4 and b = 1/50.
• Price intercept = a/b = ₹200. At ₹200 she buys no mangoes.

• Slope as drawn = −50. The price must fall by ₹50 for her to buy 1 kg more.

• Ability to pay, measured: the Household Consumption Expenditure Survey (HCES), run by MoSPI/NSO [4][5]:

• Average MPCE (monthly per capita consumption expenditure, i.e. how much each person spends on consumption in a month):
  • Rural: ₹1,430 (2011-12) → ₹3,773 (2022-23) → ₹4,122 (2023-24) [4].
  • Urban: ₹2,630 (2011-12) → ₹6,459 (2022-23) → ₹6,996 (2023-24) [4].
• Rising incomes shift demand curves to the right, especially for non-food goods.
• Food share is about 47% of rural MPCE (2023-24). Non-food items make up about 53% (rural) and 60% (urban) of MPCE (2023-24) [5]. As incomes rise, food takes a smaller share of the budget. This is Engel's law.

• The consumption Gini (0 = perfect equality, 1 = perfect inequality) fell from 0.266 to 0.237 (rural) and from 0.314 to 0.284 (urban) between 2022-23 and 2023-24 [4].

• Subsidised food grains under PDS act like a price cut. For the household, the budget line swings outward and it moves down its demand curve to buy more grain.

Don't confuse with

• Demand schedule: this is a table of prices and quantities. The demand curve is its graph. Both keep other factors constant.
• Change in quantity demanded vs change in demand: a change in the good's own price is a movement along the curve. A change in income, other prices or tastes shifts the whole curve.
• Price-consumption curve: this joins the optimum points in the indifference-curve diagram (good 1 against good 2). The demand curve shows the same choices in price–quantity space.
• Market demand curve: this is the horizontal sum of all individual demand curves (add up the quantities at each price). An individual demand curve is for one consumer.

Prelims Hooks

• The demand curve is the graph of X = f(P), with other prices, income and tastes held constant. It generally slopes down [3].
• Price goes on the vertical axis and quantity on the horizontal axis. This reverses the maths convention and comes from Marshall [3].
• Linear demand d = a − bp: quantity intercept = a, price (choke) intercept = a/b, slope as drawn = −1/b. Trap: NCERT's "vertical intercept a" is wrong.
• Every point on the demand curve is a consumer optimum, where MRS = p₁/p₂. Also, MRS = MU₁/MU₂, where MU (marginal utility) is the extra satisfaction from one more unit [2].
• A fall in p₁ (p₂ and M fixed) makes the budget line flatter, and it swings outward around the good-2 intercept. Joining the new optimum points traces out the demand curve.
• Demand = willingness plus ability to pay. HCES 2023-24 (MoSPI) puts average MPCE at ₹4,122 rural and ₹6,996 urban [4].

Mains Points

• Subsidy vs cash transfer (GS-III: subsidies, PDS, DBT):
• A price subsidy (cheap or free grain) swings the budget line outward. The household moves down its demand curve and buys more of that good.
• A cash transfer shifts the budget line out in parallel. The household can spend the money on whatever it values most.

• For the same fiscal cost, comparing the two optimum points shows why economists debate in-kind vs cash transfers.

• Distorted prices waste welfare:

• Taxes, subsidies and price controls move market prices away from true costs.
• Consumers then choose points on their demand curves based on the wrong signals.

• This supports rationalising GST rates and cutting untargeted subsidies.

• Shifting demand curves and the changing Indian consumer:

• Rising MPCE and a non-food share of about 53% (rural) and 60% (urban) in 2023-24 show demand moving towards non-food goods [5].
• This matters for CPI weights, farm diversification (milk, vegetables, processed food) and demand-led manufacturing.
• Limit of the model: it assumes rational, fully informed buyers. Habits and framing (the focus of behavioural economics) explain why "nudges" sometimes work better than price signals alone.

Related concepts

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

Read more

• Consumer's optimum and derivation of the demand curve