Theory of Demand

Why Demand Matters in the CDS Paper

The CDS General Studies and OTA economics portion regularly asks one or two questions on basic microeconomics, and demand is the single most repeated theme. Understanding it also unlocks supply, market price, and elasticity, so a few hours here pay off across many marks.

In everyday language ‘demand’ means a wish to have something. In economics it means much more. Demand is the quantity of a good a consumer is both willing and able to buy at a given price during a given time. A wish without purchasing power and a price tag is not demand — it is merely a desire. A poor student may want a car, but unless he has the money and is ready to spend it on a car at its market price, his want creates no demand for cars.

Notice the three boundaries built into the definition. Demand is always tied to a specific price, because the same person buys different amounts at different prices. It is tied to a specific period of time — demand per day, per week or per year — otherwise the figure is meaningless. And it relates to a particular market or place. Whenever you read a CDS statement about demand, mentally attach these three tags: which price, what time, which market.

Demand Schedule and Demand Curve

A demand schedule is a simple table listing how many units a buyer will purchase at each price. When we plot that table on a graph — price on the vertical axis, quantity on the horizontal axis — we get the demand curve.

• Individual demand: the demand of a single consumer.
• Market demand: the sum of all individual demands for the good at each price.

The typical demand curve slopes downward from left to right. As price falls, quantity demanded rises, so the curve runs ‘downhill’. This negative price–quantity relationship is the visual signature of demand. To build a market demand curve, you take the demand schedules of every buyer and add up the quantities they each want at each price. Because all individual curves slope downward, their horizontal sum — the market curve — also slopes downward, and it is usually flatter than any single buyer’s curve.

A small worked picture helps. Suppose at ₹10 a buyer takes 2 units, at ₹8 takes 4 units, and at ₹6 takes 6 units. Plotting the three points (10,2), (8,4) and (6,6) and joining them gives a straight, downward-sloping demand line. Each fall in price has coaxed out two more units of purchase — the law of demand in action.

The Law of Demand

The law of demand states that, other things remaining constant, the quantity demanded of a good rises when its price falls and falls when its price rises. The relationship is inverse (negative).

The phrase ceteris paribus (‘other things being equal’) is vital. The law only holds if income, tastes, prices of related goods, and expectations stay fixed. If those change, the whole curve can move and the simple rule no longer describes what we see. For instance, if your salary doubles in the same month that the price of restaurant meals rises, you might eat out more often even though meals cost more — but that is the income change talking, not a failure of the law of demand. The law speaks only about the effect of the good’s own price, with everything else held still.

This is why economists insist on stating assumptions clearly. The law of demand is a conditional statement: it predicts behaviour accurately only inside the laboratory of ‘all else constant’. Real markets rarely hold everything else constant, so observed buying can look messy — yet the underlying tendency captured by the law remains the single most reliable rule in microeconomics.

Why the Demand Curve Slopes Downward

Three forces pull the curve down as price falls:

1. Law of diminishing marginal utility: each extra unit gives less satisfaction, so a buyer pays less for more units — tempting them to buy more only when price drops.
2. Income effect: a price cut leaves the buyer with more real purchasing power, so they can afford additional units.
3. Substitution effect: when one good becomes cheaper, buyers switch to it from costlier substitutes, raising its quantity demanded.

Determinants of Demand

Price is not the only thing that affects how much people buy. The main non-price determinants are:

• Income of the consumer: higher income usually raises demand for normal goods and lowers it for inferior goods.
• Prices of related goods: for substitutes (tea and coffee) a rise in one good’s price raises demand for the other; for complements (car and petrol) a rise in one lowers demand for the other.
• Tastes and preferences: fashion, advertising and habit.
• Expectations: if buyers expect prices to rise, they buy more now.
• Population and distribution of income: a larger market, or income spread more evenly among many buyers, means larger demand.
• Government policy: taxes, subsidies and rationing can raise or lower how much people are able to buy.

Among these, the relationship between income and demand deserves special attention because it splits goods into types. For a normal good, demand rises as income rises — think branded clothing or eating out. For an inferior good, demand falls as income rises because the buyer switches to a better substitute — a household may buy less coarse grain once it can afford finer cereals. CDS questions often hinge on correctly labelling a good as normal or inferior.

Movement Along vs Shift of the Curve

This distinction is a favourite trap in objective papers, so be precise:

• Movement along the curve (change in quantity demanded): caused only by a change in the good’s own price. An expansion is a downward movement (price falls, more bought); a contraction is an upward movement (price rises, less bought).
• Shift of the curve (change in demand): caused by any non-price factor — income, tastes, related prices, expectations. A rightward shift means an increase in demand at every price; a leftward shift means a decrease.

Price Elasticity of Demand

Price elasticity of demand (E_d) measures how strongly quantity demanded responds to a change in price.

Reading the value:

• E_d > 1: elastic — quantity reacts strongly (luxuries).
• E_d < 1: inelastic — quantity barely moves (necessities like salt).
• E_d = 1: unitary elastic.
• E_d = 0: perfectly inelastic (vertical curve).
• E_d = ∞: perfectly elastic (horizontal curve).

Income elasticity and cross elasticity follow the same idea, replacing ‘price’ with income or the price of a related good. Income elasticity is positive for normal goods and negative for inferior goods, so its sign alone tells you the type of good. Cross elasticity is positive for substitutes (a costlier coffee raises tea demand) and negative for complements (costlier petrol lowers car demand). Knowing the sign rules lets you answer many objective questions without any calculation at all.

Why does elasticity matter beyond the exam? It guides real decisions. A government taxing an inelastic good like salt or fuel earns steady revenue because buyers cannot cut back much. A seller of an elastic luxury must be careful: a price rise can lose so many customers that total revenue actually falls. This link between elasticity and total revenue is itself a favourite CDS one-liner — for an elastic good, price and total revenue move in opposite directions.

Worked Example: Calculating Elasticity

Exceptions to the Law of Demand

A few goods break the normal downward rule — their demand can rise even as price rises. CDS examiners enjoy testing these names:

• Giffen goods: very inferior staples (a classic example is a cheap coarse grain). When their price rises, poor consumers cannot afford costlier substitutes and end up buying even more of the staple.
• Veblen goods (conspicuous consumption): luxury items like diamonds bought for status — a higher price can attract more buyers.
• Speculative demand: when buyers expect prices to keep rising (shares, gold), a price rise can trigger more buying.
• Necessities and emergencies: goods like life-saving medicine show little response to price.

Previous-Year Style Practice

Another common form asks you to identify a Giffen good or to mark whether a fall in a good’s own price causes a ‘shift’ or a ‘movement’. Practise spotting these keywords.

Quick Revision

Revise this list the night before the exam and attempt five mixed objective questions to confirm your speed.