Demand

What Is Demand?

In economics, demand does not simply mean wanting something. It means being both willing and able to buy it. A teenager who wants a new games console but has no money is not contributing to demand in the economic sense — there is no “effective demand” unless the desire is backed by the purchasing power to act on it. Economists focus on this distinction because markets respond to spending, not wishes.

Demand is always for a good or service at a particular price, over a particular time period. When economists talk about “the demand for petrol”, they mean the total quantity all consumers are willing and able to purchase at each possible price, holding all other factors constant. This set of price–quantity relationships is captured in a demand schedule and drawn as a demand curve.

The Law of Demand

The central rule of demand is the law of demand: as the price of a good rises, the quantity demanded falls. As the price falls, the quantity demanded rises. The relationship between price and quantity demanded is inverse — they move in opposite directions.

Think about the petrol market. When fuel prices spike to 170p a litre, many drivers combine journeys, take public transport more often, or walk short distances they would normally drive. Fewer litres are bought. When prices fall to 130p, the same drivers fill up more readily and make trips they previously avoided. The quantity demanded rises. This behaviour is consistent and predictable across virtually every market.

There are two reasons why demand behaves this way. First, as a good becomes more expensive it also becomes expensive relative to alternatives, so consumers switch to substitutes — this is the substitution effect. Second, a price rise reduces the real purchasing power of a consumer’s income, so they can afford to buy less overall — this is the income effect. Both effects push quantity demanded in the same direction: downwards when price rises.

Movement Along the Demand Curve

A change in the price of the good itself causes a movement along the demand curve. If petrol rises from 140p to 160p per litre, consumers move up the demand curve to a lower quantity demanded. If it falls to 120p, they move down to a higher quantity. The curve itself does not move — only the position of the consumer on it changes.

It is important to use precise language here. A rise in price causes a contraction of demand — less is demanded. A fall in price causes an extension of demand — more is demanded. Use these terms rather than simply saying demand “goes up” or “goes down” when the price changes.

Shifts in the Demand Curve

A shift in the demand curve happens when a factor other than price changes. The whole curve moves — at every possible price, consumers now want a different quantity. A rightward shift means more is demanded at every price; a leftward shift means less is demanded at every price. The factors that cause shifts are sometimes called the conditions of demand.

To picture this: imagine petrol demand before and after a 20% rise in household incomes. At every price point, drivers are now willing to buy more fuel — they take more car journeys, use the car for shorter trips, and are less sensitive to cost. The entire demand curve shifts to the right. Price has not changed; the conditions of demand have changed.

Factors That Shift Demand

Income. For most goods (called normal goods), a rise in income shifts the demand curve to the right. People buy more holidays, restaurant meals and new clothes when they earn more. For a small category of goods (called inferior goods), demand actually falls as income rises — for example, demand for budget own-brand foods tends to fall when incomes improve, as consumers trade up to branded alternatives.

Tastes and fashion. Consumer preferences change over time. The rise of plant-based diets shifted the demand curve for meat alternatives sharply to the right through the 2010s. Meanwhile, demand for physical DVDs shifted to the left as streaming services became dominant. Advertising and social media trends can accelerate these changes in taste rapidly.

Prices of related goods. Two goods can be related as substitutes or complements. If the price of bus fares rises sharply, demand for cars may increase — consumers switch from one mode of transport to the other. If the price of car insurance rises steeply, demand for cars themselves may fall, because the total cost of motoring has risen even though the car’s own price is unchanged.

Population. A larger population, or a change in its age structure, shifts demand. An ageing population, for instance, increases demand for healthcare, mobility aids and retirement housing. A rising number of young adults increases demand for rented accommodation and entry-level smartphones.

Advertising. Effective advertising changes consumer preferences, shifting the demand curve to the right for the advertised product. When a supermarket runs a major promotional campaign for its own-brand products, demand at every price point may rise.

A Worked Example

Consider the market for electric vehicles (EVs). Suppose manufacturers cut the price of a popular EV model from £35,000 to £28,000. The fall in the good’s own price causes an extension of demand — a movement down the demand curve to a higher quantity. This is a movement along the existing curve; the curve itself does not move. At the same time, a high-profile advertising campaign promotes the environmental benefits of EVs, changing consumer tastes in favour of them. This causes a rightward shift of the entire demand curve. Both changes increase the quantity of EVs demanded, but they work through different mechanisms and would be shown differently on a demand diagram: the price cut is a movement along the curve; the advertising campaign is a shift of the curve.