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

Detail

1. The law of demand

• Law of demand: if other things stay the same, price and quantity demanded are inversely related. They move in opposite directions.
• Price rises → quantity demanded falls.

• Price falls → quantity demanded rises.

• "Other things equal" (ceteris paribus) means that income, the prices of related goods, tastes and expectations are held fixed. Only the good's own price changes.

• Demand function: q = d(p). It shows how much of a good the consumer wants to buy at each price.
• Linear demand curve: d(p) = a − bp, where a > 0 and b > 0. The minus sign carries the law of demand.

• Worked example: d(p) = 100 − 2p.

  • At p = ₹20, q = 100 − 40 = 60 units.
  • At p = ₹30, q = 100 − 60 = 40 units.
  • The price rose by ₹10, so demand fell by 20 units. The curve slopes down.

• Class 9 example: early in the season, mangoes cost ₹150 a kg, so people buy only a few. As the season goes on and prices fall, they buy more.

• Movement vs shift:
• A change in the good's own price moves the buyer along the same demand curve.
• A change in anything else (income, expectations, tastes) shifts the whole curve.
• This difference matters for the "exceptions" in section 6.

2. Why the demand curve slopes down

(a) Diminishing marginal utility (cardinal approach)

• Utility means the satisfaction a person gets from a good.
• Marginal utility (MU) means the extra satisfaction from one more unit.
• Law of diminishing marginal utility: each extra unit gives less satisfaction than the one before.
• A consumer is in balance (consumer's equilibrium) when MU in money terms = price. With many goods, the rule is MUx / Px = MUy / Py = MU of money.
• Worked example:
• The 1st mango is worth ₹50 to her, the 2nd ₹40 and the 3rd ₹30.
• At a price of ₹40 she buys 2 mangoes.
• She buys the 3rd mango only if the price falls to ₹30.
• So a lower price is needed to sell more units. This gives a downward-sloping demand curve.

(b) Substitution effect

• Substitution effect: the change in quantity that happens only because one good becomes cheaper or dearer compared with other goods. The consumer's level of satisfaction is kept the same.
• Example: bananas become cheaper relative to mangoes, so she buys more bananas and fewer mangoes, but her satisfaction stays at the same level.
• Direction rule: when a good's price falls, the substitution effect always raises the quantity of that good. This holds for normal, inferior and Giffen goods alike.

(c) Income effect

• Income effect: the change in quantity demanded caused by a change in real income (purchasing power) when a price changes. Real income is what money income can actually buy.
• A price change alters what a consumer can afford. This changes her demand for that good and for other goods. It is a separate effect from switching to cheaper substitutes [2].
• Worked example of the purchasing-power gain:
• She earns ₹1,200 a month and buys 20 kg of bananas at ₹40 a kg (₹800).
• The price falls to ₹30 a kg. Buying the same 20 kg now costs ₹600.

• ₹200 is freed. Her money income has not changed, but her real income has gone up by ₹200.

• For a normal good, this extra real income raises demand further, both for bananas and for other goods such as mangoes.

(d) Total price effect = substitution effect + income effect

Type of good	Substitution effect (price ↓)	Income effect (price ↓)	Total effect	Law of demand?
Normal	+ 4 kg	+ 2 kg	+ 6 kg	Holds (strongly)
Inferior (not Giffen)	+ 4 kg	− 1 kg	+ 3 kg	Holds (weakly)
Giffen	+ 2 kg	− 5 kg	− 3 kg	Violated

• The numbers are an illustration. What matters is the sign and the relative size of each effect.

3. Classifying goods by how demand responds to income

• Normal good: demand moves in the same direction as income. When income rises, demand rises. Most goods are normal goods.
• Inferior good: demand moves in the opposite direction to income. When income rises, demand falls.

• Example: low-quality food such as coarse cereals.

• Normal at low income, inferior above a threshold:

• A very poor household buys more coarse cereal as its income rises from a very low level. Here it is a normal good.

• Beyond a certain income, the household switches to better cereals such as rice or wheat. Coarse cereal now becomes an inferior good.

• Indian data: the income–consumption link

• Average MPCE (monthly per capita consumption expenditure, which means average household spending per person per month) was ₹4,122 in rural areas and ₹6,996 in urban areas (HCES 2023-24). This excludes items received free through welfare schemes [4].
• Food's share of spending in 2023-24 was 47.04% in rural areas and 39.68% in urban areas [4].
• Between 2011-12 and 2022-23, the food share fell from about 53% to 46% in rural areas and from about 43% to 39% in urban areas [5].
• As incomes rose, households moved from starchy cereals towards higher-value, more nutritious foods [5][6].
• In short, cereals behave like a necessity, and some low-quality cereals behave like inferior goods. Demand for them grows slowly or falls as income rises.

4. Giffen goods

• Giffen good: an inferior good whose negative income effect is stronger than its substitution effect. As a result, demand rises when price rises, and the demand curve slopes upward.
• For an inferior good, the two effects pull in opposite directions:
• Substitution effect stronger → the law of demand still holds. This is an ordinary inferior good.

• Income effect stronger → it is a Giffen good.

• How it works (price rise):

• A very poor family spends most of its budget on a cheap staple, such as bread or potatoes.
• The staple's price rises, so the family's real income falls sharply.
• The family can no longer afford the costlier food, such as meat.

• It cuts that food and buys even more of the staple to fill the gap in calories.

• Conditions usually needed:

• The good is strongly inferior.
• It takes up a large share of the budget.
• There are few close substitutes.

• The buyers are very poor.

• Key rule: all Giffen goods are inferior goods, but not all inferior goods are Giffen goods.

• Evidence:
• Robert Giffen / Alfred Marshall: the story of poor British labourers buying more bread when bread prices rose.
• Potatoes in the Irish famine (1845–49) are often cited as an example, but historians dispute this case.
• Jensen and Miller (2008): rice in Hunan (China) behaved as a Giffen good for very poor households that received a price subsidy. When the subsidy made rice cheaper, these households bought less rice. Wheat in Gansu showed weaker evidence.

5. Beyond NCERT: exceptions and anomalies

(a) Veblen good

• Veblen good: a luxury or status good whose demand rises with price, because a high price signals prestige.

• Examples: luxury cars, designer labels, jewellery.

• Conspicuous consumption: a term coined by Thorstein Veblen in The Theory of the Leisure Class (1899) [3].

• It means buying goods of higher quality, or in larger amounts, than is practically needed. The aim is to show off wealth and social status [3].
• Example: any car gets you to your destination. A luxury car also draws attention to how rich the driver seems [3].

(b) Giffen vs Veblen: a common exam trap

Point	Giffen	Veblen
Type of good	Inferior staple	Luxury or status good
Who buys it	Very poor	Rich
Cause of the upward slope	Income effect (real income falls)	Prestige / preferences (a high price itself is attractive)
Fits standard theory?	Yes (Slutsky/Hicks decomposition)	No. Tastes depend on price itself

(c) Bandwagon effect

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

(d) Snob effect

• Snob effect (Leibenstein): some buyers cut their demand as a good becomes common, because they want to own something exclusive.
• It is the opposite of the bandwagon effect.

(e) Demonstration effect

• Demonstration effect (James Duesenberry, 1949): people copy the consumption of richer groups.
• Ragnar Nurkse (1953) applied it across countries. People in poor countries copy consumption patterns of rich countries.
• What follows:
• Consumption rises and savings fall.
• Less money is left for investment.
• Capital formation (building factories, machines and infrastructure) slows down in developing economies.

(f) Panic buying and price expectations

• If buyers expect prices to rise further, they may buy more even while prices are rising.

• Examples: onion hoarding, the rush for sanitiser during COVID-19 (Class 9).

• This is not a true exception. The change in expectations shifts the demand curve (section 7). It is not a movement along the curve.

(g) Other exceptions listed in some textbooks

• Necessities bought in fixed amounts: for example salt or life-saving medicines. Quantity barely changes with price.
• Ignorance of quality: some buyers think "higher price = better quality".

Prelims Hooks

• Law of demand: price and quantity demanded are inversely related ceteris paribus. In the linear form d(p) = a − bp, b > 0 shows the downward slope.
• For a price fall, the substitution effect always raises the quantity of the cheaper good, whatever the type of good.
• Inferior good means the income effect is negative. Giffen good means the negative income effect is greater than the substitution effect.
• Trap: "All inferior goods are Giffen goods" is false. "All Giffen goods are inferior goods" is true.
• Giffen is an income-effect case (poor people, staple goods). Veblen is a prestige case (rich people, luxury goods). Both have upward-sloping demand.
• The Theory of the Leisure Class (1899) by Thorstein Veblen introduced "conspicuous consumption" [3].
• Bandwagon and snob effects: Harvey Leibenstein (1950). Demonstration effect: Duesenberry (1949), later applied across countries by Nurkse (1953).
• The Irish potato famine (1845–49) Giffen case is historically disputed. The Jensen–Miller (2008) evidence is for rice in Hunan.
• Panic buying caused by expected price rises is a shift in demand, not a true exception to the law.
• HCES 2023-24: average MPCE was ₹4,122 (rural) and ₹6,996 (urban). Food's share was 47.04% (rural) and 39.68% (urban) [4].

Mains Points

• Food subsidies and the Giffen risk: for very poor households, cutting the price of a staple can lead them to buy less of it and more of other foods (Jensen–Miller 2008).
• This suggests that PDS-type price subsidies can end up supporting dietary variety rather than more grain intake.

• Nutrition policy should therefore look at the whole diet, not only the amount of cereal consumed.

• Food's falling share of spending and changing diets: the food share fell from about 53% to 46% (rural) and about 43% to 39% (urban) between 2011-12 and 2022-23 [5].

• Households are moving away from cereals, some of which behave as inferior goods.
• This supports crop diversification and more production of pulses, dairy and horticulture.

• It also warns against over-procurement of rice and wheat.

• Promoting millets vs the inferior-good tag: coarse cereals lose demand as incomes rise.

• Rebranding them as nutritious and higher-value foods (for example, the International Year of Millets push) tries to shift tastes, so that they stop behaving like inferior goods.

• Demonstration effect and savings: when people copy the consumption of richer groups, household savings fall and capital formation slows (Nurkse).

• This is useful for GS-III answers on India's falling household financial savings and on growth driven by consumer credit.