The interaction of markets

What you'll learn

• How demand and supply interact to determine market price and quantity.
• What ceteris paribus means and why economists use it.
• How to explain equilibrium, surpluses and shortages using a diagram.
• How a change in one market can affect related markets, and how to evaluate the size of that effect.

Starting point: what is a market?

A market is any arrangement where buyers and sellers interact to exchange a good or service. It does not have to be a physical place: the market for train tickets, coffee, housing rentals, or online streaming subscriptions all count.

Demand is the quantity consumers are willing and able to buy at different prices over a period of time. Supply is the quantity producers are willing and able to sell at different prices over a period of time.

In most markets:

• the demand curve slopes downwards because, as price falls, consumers are usually willing and able to buy more;
• the supply curve slopes upwards because, as price rises, producers have a stronger incentive to supply more.

Ceteris paribus

This matters because real markets are messy. For example, petrol sales might change because of petrol prices, incomes, weather, commuting patterns, electric vehicle ownership, or public transport fares. Ceteris paribus lets you focus on one cause at a time.

The interaction of demand and supply

Neither demand nor supply alone determines the market outcome. The market price and quantity are determined by the point where buyers’ plans and sellers’ plans are compatible.

At equilibrium:

$$Q_d=Q_s$$

where $Q_d$ means quantity demanded and $Q_s$ means quantity supplied.

The diagram below shows demand and supply meeting at equilibrium. It also shows what happens if the price is set above or below equilibrium.

At price Pe, the market clears: consumers want to buy exactly the quantity firms want to sell, Qe.

At price P1, price is above equilibrium. Quantity supplied is greater than quantity demanded, creating a surplus, also called excess supply. Firms may have unsold stock, so they have an incentive to cut prices.

At price P2, price is below equilibrium. Quantity demanded is greater than quantity supplied, creating a shortage, also called excess demand. Some consumers cannot buy the good at that price, so there is upward pressure on price if the market is allowed to adjust.

Market disequilibrium

Disequilibrium occurs when quantity demanded does not equal quantity supplied at the current price.

There are two main cases:

• Excess supply: firms want to sell more than consumers want to buy.
• Excess demand: consumers want to buy more than firms want to sell.

Shifts versus movements along curves

A movement along a demand or supply curve happens when the good’s own price changes.

A shift of a curve happens when a non-price factor changes. For demand, this could be income, tastes, advertising, population, or the price of related goods. For supply, it could be costs of production, technology, indirect taxes, subsidies, weather, regulation, or productivity.

For example, if the price of coffee rises, there is a movement along the demand curve for coffee. But if consumer incomes rise and coffee is a normal good, the whole demand curve for coffee may shift right.

Related markets

Markets do not exist in isolation. A change in one market can affect another market if the goods are related.

A substitute is a good that can be used instead of another good, such as tea and coffee. If coffee becomes more expensive, demand for tea may rise.

A complement is a good used together with another good, such as games consoles and video games. If more consoles are bought, demand for games may increase.

Derived demand means demand for a good or factor arises from demand for another product. For example, demand for lithium batteries is partly derived from demand for electric cars.

The diagram below shows how an increase in demand in one market can transmit into a related market.

In the electric car market, demand shifts right from D1 to D2, increasing both price and quantity. Because electric cars use lithium batteries, firms need more batteries, so demand in the lithium battery market also shifts right.

Evaluating the impact on related markets

For evaluation, avoid writing as if the effect is automatic and equally large in every case. The impact depends on several factors.

1. How closely related are the goods?

The closer the relationship, the stronger the spillover effect. A rise in the price of butter may noticeably increase demand for margarine because they are close substitutes. But a rise in the price of butter is unlikely to have much effect on demand for rice.

Economists often use cross-price elasticity of demand to measure this link. Cross-price elasticity of demand shows how responsive demand for one good is to a change in the price of another good. A positive value suggests substitutes; a negative value suggests complements.

2. How elastic are demand and supply?

If supply in the related market is price inelastic, a demand increase causes a larger rise in price and a smaller rise in quantity. This is common where production capacity is limited, such as housing, energy, or key minerals.

If supply is more price elastic, firms can increase output more easily, so the quantity effect may be larger and the price effect smaller.

3. Short run versus long run

In the short run, markets may struggle to adjust. For example, after global supply-chain shocks, shortages of semiconductors limited car production because chip supply could not quickly expand.

In the long run, firms can invest, train workers, develop new suppliers, or redesign products. This means the same initial shock may have a smaller effect over time.

4. The size and duration of the original shock

A temporary change in demand may have little effect on related markets if firms expect it to fade. A permanent shift, such as the green transition increasing demand for renewable energy infrastructure, is more likely to trigger investment and wider market adjustments.

5. Government intervention and market power

Government policy can strengthen or weaken market links. Subsidies for electric vehicles, taxes on petrol and regulation of emissions all affect related markets. Firms with market power may also absorb cost changes in profit margins rather than passing them fully to consumers.

Bringing it together

The full chain of analysis is:

1. Identify the original market and the initial change.
2. Decide whether demand or supply shifts, and show the new equilibrium.
3. Identify the related market link: substitute, complement, input, or derived demand.
4. Show the likely shift in the related market.
5. Evaluate how large, fast and long-lasting the effect is likely to be.

This is a useful synoptic skill. For example, an increase in global energy prices affects household bills, business costs, transport markets, food prices and inflation. One market shock can spread across the economy.