Nobody sets the price of coffee. No committee meets to decide it. Yet a price exists, it changes, and it somehow rations a limited supply among everyone who wants some. Understanding how that happens without anyone arranging it is the single most useful thing economics teaches, and it starts with two curves.
Demand
The quantity of a good consumers are willing and able to buy at a given price over a given period.
Supply
The quantity of a good producers are willing and able to sell at a given price over a given period.
A single consumer's demand for a good is individual demand. Adding up the quantities demanded by every consumer at each price gives market demand, so the market demand curve is the sum of all the individual demand curves. Market supply is built in the same way from the supply of every firm. A movement along a demand curve caused by a change in the good's own price is called an extension of demand when quantity demanded rises and a contraction when it falls; the same words are used for supply.
The demand curve slopes downward because as price falls, buyers who were previously priced out enter the market, and existing buyers find the good more attractive relative to substitutes. The supply curve slopes upward because higher prices make production more profitable, so firms devote more resources to that good and higher-cost producers find it worth entering.
The phrase 'willing and able' is doing real work and examiners look for it. Wanting a car is not demand. Wanting a car and having the money to buy one is. Candidates who write only 'how much people want to buy' rarely secure the second mark.
Economists call demand backed by the ability to pay effective demand, and at AS it is the only kind that counts. Notional demand, the wish to buy without the means, has no effect on price because it never reaches the market. The distinction matters when income changes: a rise in income turns some notional demand into effective demand, which is why demand curves shift right when incomes rise.
Shifts versus movements
This distinction separates strong answers from weak ones, and it comes down to a single question: what changed? If the price of the good itself changed, you move along the existing curve, and that is called a change in quantity demanded or quantity supplied. If anything else changed, the whole curve shifts, and that is called a change in demand or supply. Income, tastes, the price of substitutes and complements, and population all shift demand. Production costs, technology, taxes, subsidies, weather and the number of firms all shift supply.
Each cause of a shift works for a reason worth stating. Demand shifts right when incomes rise, for normal goods, because buyers can afford more at every price; for inferior goods, such as the cheapest own-brand foods, it shifts left, as buyers switch to better alternatives. A rise in the price of a substitute shifts demand right, and a rise in the price of a complement shifts it left. Advertising and changes in fashion or tastes shift demand in the direction the change favours. A larger population, or a change in its age structure, changes how many buyers there are for particular goods. Expectations matter too: if buyers expect prices to rise next month they buy more now.
Supply shifts for the same kind of reasons on the producer's side. A rise in the cost of raw materials, wages or energy makes each unit dearer to produce, so supply shifts left. Better technology lowers costs and shifts supply right. An indirect tax is like an extra cost and shifts supply left; a subsidy works the other way. More firms entering the market shift supply right. Weather and disease shift the supply of farm goods, sometimes sharply. And goods produced together, such as beef and leather, are in joint supply, so a rise in the output of one raises the supply of the other.
The most common diagram error in this topic is shifting a curve when the question describes a price change caused by the other curve moving. If the price of coffee rises because a frost destroyed the harvest, supply shifts left and you move along the demand curve. Demand itself does not shift. Ask yourself what caused the price change before you draw anything.
The government raises income tax, reducing household disposable income. What happens in the market for restaurant meals, a normal good?
Show the effect on the market for wheat of a government subsidy paid to farmers.
Supply shifts right. Mark both the original and new equilibrium, label the axes, and show the direction of change in price and quantity.
Equilibrium price
The price at which quantity demanded equals quantity supplied, so there is no tendency to change.
Away from equilibrium the market corrects itself. Above the equilibrium price, supply exceeds demand and the resulting surplus pushes sellers to cut prices. Below it, demand exceeds supply and the shortage lets sellers raise prices. Neither adjustment requires anyone to plan it, which is the central insight of the price mechanism.
Finding equilibrium and disequilibrium from a schedule
The weekly market for a snack is shown below. Price $1.00: quantity demanded 900, quantity supplied 300. Price $1.50: quantity demanded 700, quantity supplied 500. Price $2.00: quantity demanded 600, quantity supplied 600. Price $2.50: quantity demanded 500, quantity supplied 700. Find the equilibrium, then describe the market at $1.50 and at $2.50.
The functions of price
At AS Level you are expected to explain not just how price is determined but what it does. Price performs three jobs at once. It rations, by allocating a limited quantity to those willing to pay most. It signals, by transmitting information about relative scarcity to producers and consumers who have no other way of knowing what others want. And it incentivises, by rewarding firms that move resources into markets where prices have risen. A rising price is simultaneously a rationing device, a message and a reward.
How a market absorbs a supply shock
- A poor harvest reduces the quantity producers can supply at every price, shifting supply left.
- At the original price, quantity demanded now exceeds quantity supplied, creating a shortage.
- Unsatisfied buyers bid against one another, so price rises.
- The higher price rations the reduced supply, since buyers who value the good least withdraw first, and this is a movement along the demand curve.
- The higher price simultaneously signals scarcity to producers elsewhere.
- It also incentivises them, since production is now more profitable, so resources are drawn into the market.
- A new equilibrium is reached at a higher price and lower quantity, and in the longer run the entry of new supply moderates the price rise.
Notice the chain names all three functions of price as it runs. Weaving the theory through the mechanism, rather than listing it separately, is what earns the analysis marks.
Practise this chain →Markets are also linked to one another. Complements are in joint demand, so a fall in the price of one raises demand for the other. Substitutes are in alternative demand, so a fall in the price of one reduces demand for the other. Derived demand arises where a good is wanted not for itself but for what it helps produce, as with steel and cars, or labour and the output it makes. Joint supply occurs where producing one good automatically produces another, as with beef and leather.
Consumer and producer surplus
Consumer surplus is the gap between what buyers would have been willing to pay and what they actually paid, shown as the area under the demand curve and above the market price. Producer surplus is the gap between the price producers receive and the minimum they would have accepted, shown as the area above the supply curve and below the price. Together they measure the total gain from the market existing at all, which is why economists use them to judge whether an intervention has made society better or worse off.
Draw a market in equilibrium and shade consumer surplus and producer surplus. Then shift demand right and show how each area changes.
Both areas grow, but by how much depends on the elasticity of supply. Say which assumption you are making about the slope of the supply curve.
Surplus questions are almost always really about elasticity. How much surplus changes when price moves depends on how steep the curves are. If you are asked to analyse the effect of a shift on consumer surplus, state the elasticity assumption you are making and your answer immediately gains depth.
Applied context
Ride-hailing surge pricing is the price mechanism operating in real time and in public view. When demand spikes after a concert, the algorithm raises fares, which rations the available cars to those who value the journey most and simultaneously signals to drivers elsewhere in the city that it is worth heading over. Supply responds within minutes. It is also the clearest illustration of why the mechanism is politically contentious: efficient rationing by willingness to pay looks a great deal like profiteering to the person paying four times the usual fare.
This example lets you make the efficiency case and the equity objection from a single illustration, which is exactly what an evaluation question rewards.
Where the demand curve actually comes from
At AS Level the downward-sloping demand curve is asserted. At A Level you are expected to derive it. Marginal utility is the additional satisfaction gained from consuming one more unit of a good, and the law of diminishing marginal utility states that it falls as consumption rises. Since a rational consumer will only pay an amount reflecting the utility gained, and each successive unit yields less, the consumer will buy an extra unit only at a lower price. The demand curve slopes downward because marginal utility diminishes.
Total utility
The total satisfaction a consumer gains from consuming a given quantity of a good. It is the sum of the marginal utilities of each unit consumed.
Calculating marginal utility from total utility
A student's total utility from cups of coffee in a day is 20 utils for 1 cup, 36 for 2, 48 for 3, 56 for 4, 60 for 5 and 60 for 6. Calculate the marginal utility of each cup, and explain what the figures show.
MU₁ ÷ P₁ = MU₂ ÷ P₂ = ... = MUₙ ÷ PₙA consumer maximises total utility when the marginal utility per unit of currency is equal across all goods. If one ratio is higher, reallocating spending toward that good raises total utility.
Marginal utility theory has clear limitations, and evaluation questions expect them. Utility cannot be measured, so utils are a teaching device rather than a real unit. Consumers do not calculate marginal utility per unit of currency for every good; they rely on habit, rules of thumb and brand loyalty. Many decisions are made with incomplete information, or under the influence of advertising and what other people buy. Some goods are bought in whole units, such as a car or a holiday, so spending cannot be adjusted by tiny amounts. And behavioural economics finds that people are often inconsistent, valuing a loss more than an equal gain and putting off decisions they know would benefit them. The theory explains the direction of consumer choice well; it does not describe how consumers actually think.
Indifference analysis reaches the same conclusion without needing utility to be measurable. An indifference curve joins combinations of two goods giving equal satisfaction, and it is convex because the marginal rate of substitution diminishes: the more of a good you hold, the less of the other you will surrender for another unit of it. A budget line shows all combinations affordable at given prices and income, with a slope equal to the ratio of the prices. The consumer optimises where the budget line is tangential to the highest attainable indifference curve, so that the marginal rate of substitution equals the price ratio.
The budget line moves in two different ways, and the diagram must show which. A change in income, with prices unchanged, shifts the whole budget line parallel: outward if income rises, inward if it falls, because the consumer can afford more or less of both goods but the price ratio, and so the slope, is the same. A change in the price of one good pivots the budget line around the intercept of the other good, because the most that can be bought of the unchanged good stays the same while the slope changes. A parallel shift produces only an income effect; a pivot produces both a substitution effect and an income effect.
Decomposing a price fall into substitution and income effects
- The price of good X falls, so the budget line pivots outward along the X axis while the intercept on the Y axis is unchanged.
- The consumer moves to a new tangency on a higher indifference curve, so total consumption of X rises.
- That total change can be split into two parts.
- The substitution effect is the change in consumption caused purely by X becoming relatively cheaper than Y, holding real income constant. It is always positive for the good whose price has fallen.
- The income effect is the change caused by the consumer's real purchasing power having risen, since the same money income now buys more.
- For a normal good the income effect reinforces the substitution effect, so demand rises unambiguously.
- For an inferior good the income effect works against the substitution effect, and if it is strong enough to dominate, the good is a Giffen good and the demand curve slopes upward over that range.
Constructing the hypothetical budget line used to isolate the substitution effect is where most candidates lose marks. Draw it parallel to the new budget line and tangential to the original indifference curve.
Practise this chain →Draw an indifference map with a budget line. Show the effect of a fall in the price of good X, then decompose the total change into the substitution and income effects.
You need three budget lines: the original, the new one after the price fall, and the hypothetical compensating line parallel to the new one and tangential to the original indifference curve.
This foundation supports much of the rest of the course. Consumer and producer surplus become the measuring tools for welfare loss when externalities or monopoly distort the market. The elasticity of demand determines how far a price change moves quantity and therefore the incidence of any tax. And the assumption of rational utility maximisation is precisely what behavioural economics challenges, so a strong evaluation of this model often draws on bounded rationality and the role of heuristics.
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Evaluate the usefulness of indifference curve analysis in explaining consumer behaviour. (13 marks)
- Define and frameDefine indifference curves and budget lines, and state the consumer optimum condition. Frame usefulness against two tests: does it explain observed behaviour, and does it predict responses to price changes?
- Analysis: the strengthsIt requires only ordinal preferences, not measurable utility, which is a real advance on marginal utility theory. It derives the demand curve rather than assuming it, and it separates income from substitution effects, which explains inferior and Giffen goods that simple demand analysis cannot.
- Analysis: the assumptionsSet out what the model requires: consistent and transitive preferences, full information, and a consumer who optimises. Note that the tangency condition presupposes the consumer can identify the highest attainable curve.
- EvaluationWeigh rather than list. Behavioural evidence shows preferences are unstable and reference-dependent, so the indifference map shifts with framing. Consumers use heuristics rather than optimising. The two-good restriction limits realism. Against that, the model remains a powerful predictive tool at market level even where individual behaviour deviates, since errors may cancel out in aggregate.
- JudgementReach a conditional conclusion. The model is most useful for explaining directional responses to price changes and for isolating income and substitution effects. It is least useful for predicting individual choices under uncertainty or where framing dominates. State which use you are judging.
AO3 evaluation is 30% of Papers 2 and 4. A judgement that specifies the conditions under which the model holds will always outscore one that concludes the model is simply unrealistic.
How far can the demand and supply model be trusted?
- The model assumes buyers and sellers have good information and act rationally. Behavioural evidence shows both assumptions fail regularly.
- It assumes prices adjust quickly. In practice wages and many contract prices are sticky, so shortages and surpluses can persist for long periods.
- Rationing by price allocates to those with the most money, not the greatest need, which is an equity objection rather than an efficiency one.
- The analysis holds other markets constant, yet in reality a shift in one market moves prices in linked markets simultaneously.
- The speed and size of adjustment depend on elasticity, so any conclusion about the magnitude of a price change should state the elasticity assumption behind it.