Complements and Substitutes: Understanding Market Connections Revision Notes | GCSE AQA Economics

1. Why Markets Do Not Work in Isolation

When the price of petrol rises sharply, sales of large fuel-hungry cars tend to fall — even though no one changed the price of cars. That is the puzzle this lesson unlocks: markets are connected, and a shift in one can ripple through others in predictable ways.

AQA GCSE Economics requires you to explain how changes in demand, supply, and price in one market affect other related markets. The two key relationships you need to master are substitutes and complements.

2. Substitute Goods: When One Good Replaces Another

Think about butter and margarine. Both spread on toast and serve the same basic purpose. If the price of butter rises, some consumers will switch to margarine — demand for margarine shifts to the right, pushing its equilibrium price up and quantity sold up too.

The same logic applies across many markets: Pepsi and Coca-Cola, train travel and coach travel, beef and chicken. Whenever two goods compete for the same consumer spending, they are substitutes.

Notice the chain of causation: price of butter rises → butter becomes relatively more expensive → consumers switch to margarine → demand for margarine increases → equilibrium price and quantity of margarine both rise. That four-step chain is exactly what an AQA examiner wants to see.

3. Complementary Goods: When Two Goods Are Consumed Together

Printers and ink cartridges are a classic example. You cannot use a printer without cartridges, and cartridges are worthless without a printer. If the price of printers rises sharply, fewer printers are sold — and because fewer people own printers, demand for ink cartridges falls too.

Other strong examples include: cars and petrol, games consoles and video games, smartphones and mobile data plans. The defining feature is joint demand — the goods are used together. When demand for one complement rises, demand for the other also rises (they move in the same direction). However, when the price of one complement rises, demand for the other falls — the price rise reduces consumption of both goods jointly.

The direction of the effect is the opposite of what happens with substitutes. With complements, a price rise in one market causes demand to fall in the related market. With substitutes, a price rise in one market causes demand to rise in the related market. Keeping that contrast clear is essential for exam questions.

4. How Supply Changes in One Market Affect Related Markets

So far the examples have started with a price change. But the trigger can also be a supply-side event — and the cross-market effects follow the same logic.

Suppose a drought reduces the supply of wheat. Supply shifts left in the wheat market, pushing the price of wheat up. Bread is made from wheat, so the cost of producing bread rises — supply of bread shifts left too, raising bread prices. Now consider a substitute for bread: rice. As bread becomes more expensive, consumers switch to rice, so demand for rice shifts right and rice prices rise. One supply shock in the wheat market has rippled through two related markets.

Worked example: Tracing a Supply Shock Through Related Markets

A poor harvest reduces the supply of coffee beans worldwide. Trace the effects on (a) the coffee market, (b) the tea market (a substitute for coffee), and (c) the coffee cup market (a complement to coffee).

1. Supply of coffee beans falls → supply curve in the coffee market shifts left — A poor harvest is a supply-side event. Fewer beans are available at every price level, so the supply curve shifts to the left.
2. Supply shifts left → equilibrium price of coffee rises (P1 to P2) and equilibrium quantity falls (Q1 to Q2) — With less coffee available and demand unchanged, the price is bid up and fewer cups are sold.
3. Price of coffee rises → coffee becomes relatively more expensive → consumers switch to tea → demand for tea shifts right → equilibrium price and quantity of tea both rise — Tea and coffee are substitutes. A higher coffee price makes tea comparatively better value, increasing its demand.
4. Quantity of coffee consumed falls → fewer people buying coffee → demand for coffee cups shifts left → equilibrium price and quantity of coffee cups both fall — Coffee cups and coffee are complements — they are used together. Less coffee consumption means fewer cups are needed.

Answer: A supply shock in the coffee market raises coffee prices and reduces quantity. This simultaneously increases demand (and price) in the substitute market (tea) and decreases demand (and price) in the complementary market (coffee cups).

5. Demand Changes in One Market and Their Cross-Market Effects

The trigger does not have to be a price change or a supply shock. A shift in consumer tastes, income, or advertising can change demand in one market and send ripples into related markets.

Consider the rise in popularity of cycling in the UK. Demand for bicycles increased — the demand curve for bicycles shifted right, pushing up equilibrium price and quantity. Because cycling helmets are a complement to bicycles, demand for helmets also shifted right. Meanwhile, demand for bus passes (a substitute for cycling as a form of commuting) shifted left as more people chose to cycle instead.

6. Putting It Together: A Real-World Market Connection

Earlier we traced a supply shock through the coffee market. Now consider a demand-side example that brings together everything covered so far.

In 2023, the UK government announced plans to expand EV charging infrastructure. Consumer confidence in electric vehicles rose, increasing demand for EVs. The demand curve for EVs shifted right, raising equilibrium price and quantity. Petrol-powered cars are substitutes for EVs; as more drivers switched to electric, demand for petrol-powered cars fell, which in turn reduced demand for petrol — a complement to petrol-powered cars. Meanwhile, demand for home charging units (a complement to EVs) rose sharply, pushing up prices in that market too.

The chart illustrates the direction of demand changes across four connected markets. Positive values indicate rising demand; negative values indicate falling demand. Notice how the substitute market moves in the opposite direction to the primary market, while the complementary market moves in the same direction.

7. Exam Technique: Getting Full Marks on Market Connection Questions

AQA questions on this topic typically ask you to 'explain' or 'analyse' the effect of a change in one market on another. The marks are awarded for the chain of reasoning, not for stating the conclusion alone.

A weak answer says: 'If the price of petrol rises, demand for electric vehicles will increase.' A strong answer says: 'If the price of petrol rises, petrol-powered cars become more expensive to run. Since petrol-powered cars and electric vehicles are substitutes, consumers switch towards electric vehicles. Demand for electric vehicles increases, shifting the demand curve to the right. This leads to a higher equilibrium price and a greater equilibrium quantity of electric vehicles sold.' The second answer earns the higher marks because it shows causation at every step.

When a question asks you to use a diagram, remember to label both axes (Price on the vertical axis, Quantity on the horizontal axis), label both curves (D, D1 or S, S1), mark the original and new equilibrium points (E1, E2), and show the direction of the shift with an arrow. Missing labels cost marks.