AQA A-Level Economics · Revision notes

4.1.2.1 Consumer Behaviour

Much of microeconomics rests on a simple model of how consumers and firms make choices. This topic sets out the assumptions behind that model, the role of economic incentives, and the utility theory that explains consumer choice: total and marginal utility, diminishing marginal utility, utility maximisation and the importance of the margin.

What "rational" means in economics

In everyday speech, rational means sensible. In economics it has a narrower meaning: an economic agent is rational if it weighs up the costs and benefits of the options available and chooses the one that leaves it best off, given its own objectives.

Three ideas sit inside that definition:

• Agents have clear objectives. A consumer knows what they value; a firm knows it wants profit.
• Agents have the information they need. They know the prices, the qualities of the goods on offer and the consequences of each choice.
• Agents can process that information. They can compare options accurately and pick the best one.

Watch out: Rationality says nothing about whether a choice is wise or moral. A consumer who spends their income on sweets rather than savings can be rational, provided that is what gives them the most satisfaction.

Economic incentives

Economic incentives are the rewards and penalties that influence these choices. A higher wage encourages people to work more hours, a lower price encourages consumers to buy more, and a tax makes an activity less attractive. Because rational agents respond to costs and benefits, changing incentives is one of the main ways firms and governments influence behaviour.

Price rewards behaviour. A higher price is an incentive for producers to supply more, because the profit from each unit rises. It is simultaneously an incentive for consumers to buy less, and to look for substitutes.

Consumers aim to maximise utility

Utility is the satisfaction or benefit a consumer gets from consuming a good or service. It cannot be measured directly, but economists treat it as something consumers try to get as much of as possible.

A rational consumer has limited income and faces prices for everything they buy. They spend their income on the combination of goods that gives the greatest total utility.

Key idea: The decision happens at the margin. Before buying one more unit of a good, the consumer compares the extra utility it would bring with its price. If the extra satisfaction is worth more to them than the money, they buy it.

If not, they spend the money elsewhere.

This idea links directly to diminishing marginal utility: each extra unit of the same good tends to bring less additional satisfaction than the one before. That is why a rational consumer buys more of a good only if its price falls, which gives the demand curve its downward slope.

Total utility, marginal utility and diminishing marginal utility

Total utility is the overall satisfaction a consumer gets from consuming a given quantity of a good. While marginal utility is positive, each extra unit adds to total utility, but once marginal utility is diminishing it adds less each time.

Marginal utility is the extra satisfaction a consumer gets from consuming one more unit of a good. The law of diminishing marginal utility states that, as a consumer consumes more of a good, the marginal utility from each additional unit falls.

The first slice of pizza when hungry brings a lot of satisfaction. The fourth brings much less.

Diminishing marginal utility and the shape of the demand curve

This explains the downward slope of the demand curve:

• A rational consumer buys one more unit only if the satisfaction it brings is worth at least its price.
• Because each additional unit brings less satisfaction than the last, the consumer will only buy more if the price falls.
• So a lower price is needed to persuade consumers to buy a larger quantity.

Two other effects reinforce the downward slope. When price falls, consumers' real income rises, so they can afford more (the income effect). The good also becomes cheaper relative to substitutes, so consumers switch towards it (the substitution effect).

Firms aim to maximise profits

Profit is total revenue minus total cost. The model assumes that firms choose the price and output that make the gap between the two as large as possible.

The reasoning is that firms are owned by shareholders who want the highest possible return on the money they have put in. Firms that fail to maximise profit risk being taken over or driven out of the market by rivals that do.

Like consumers, firms decide at the margin. A firm increases output as long as producing one more unit adds more to revenue than it adds to cost. The point where the extra revenue and the extra cost are equal is where profit is greatest.

Other rational agents

The same idea of rational choice can be applied to other economic agents:

• Workers aim to maximise their income, choosing between jobs and deciding how many hours to work by weighing the pay on offer against what they give up.
• The government aims to maximise social and economic welfare, weighing the costs and benefits of its decisions for society as a whole rather than for any one group.

Why the assumptions matter

The two assumptions make the behaviour of consumers and firms predictable, and that predictability is what allows economists to build models:

• The demand curve assumes consumers respond to price in a consistent, utility-maximising way.
• The supply curve assumes firms respond to price in a consistent, profit-maximising way.
• The price mechanism relies on both groups reacting rationally to price signals.

Without these assumptions, it would be hard to say anything general about how markets work.

Limits of the assumptions

The assumptions are a simplification, and the specification returns to them later:

• Consumers do not always behave rationally. They copy others, act out of habit and struggle with complex calculations. These ideas are covered in 4.1.2.3 Aspects of behavioural economic theory.
• Firms do not always maximise profit. Managers may pursue revenue, sales or growth instead, or settle for a satisfactory profit. These alternatives are covered in 4.1.5.2 The objectives of firms.

A good way to think about the model is as a starting point. It explains a great deal of behaviour, and the cases where it fails are interesting precisely because they differ from what the model predicts.

Key terms

Common mistakes

• Treating rational as meaning sensible or good. In economics it means choosing the best option for one's own objectives, whatever those are.
• Forgetting the margin. Rational choices are about whether one more unit is worth it, not whether the good is worth buying at all.
• Stating the assumptions as facts. They are assumptions of a model, and later topics show where they break down.
• Mixing up the two assumptions. Consumers maximise utility; firms maximise profit.
• Describing the curve without explaining it. The downward slope needs a reason: diminishing marginal utility, supported by the income and substitution effects.

How this has been examined

In the AQA papers indexed on this site, this topic has appeared in one format:

• 1-mark multiple choice — asked 7 times across 6 series (2017 to 2024)

Where this goes next

Utility maximisation and diminishing marginal utility shape the demand curve in 4.1.3.1 The determinants of the demand for goods and services, and profit maximisation shapes the supply curve in 4.1.3.3 The determinants of the supply of goods and services. The role of information in decision making is covered in 4.1.2.2 Imperfect information, and the reasons consumers may not behave rationally in 4.1.2.3 Aspects of behavioural economic theory. Alternatives to profit maximisation are covered in 4.1.5.2 The objectives of firms, and the marginal reasoning behind profit maximisation is developed in 4.1.4.6 Marginal, average and total revenue and 4.1.4.4 Costs of production.