Most of the economics you have learned so far quietly assumes one thing: people always pick what is best for them. Behavioural economics asks a fair question — is that actually true? Once you look at how people really shop, save and choose, the answer is “not quite”, and that gap is what this page is about.
📚 What you need to know
Rational choice theory says a consumer weighs up the options and picks the one that gives the most utility (satisfaction) for the money.
It stands on three assumptions: consumers are rational, they maximise utility, and they have perfect information.
Behavioural economics tests those assumptions against real behaviour, using ideas from psychology.
There are five limitations you must be able to name: biases, bounded rationality, bounded self-control, bounded selfishness, and imperfect information.
“Bounded” means limited, not absent. People do try to choose well — they just run out of time, brainpower and willpower.
In the exam, a limitation is only half a mark-scheme. The example is the other half, so always attach one.
What economists mean by “rational”
In everyday English, calling someone irrational is an insult. In economics it is not a judgement at all — it is a modelling assumption. A rational agent is simply one who can look at the options in front of them, work out the net benefit of each, and choose the biggest one.
That is where the demand curve comes from. If a price falls and nothing else changes, the good now gives more satisfaction per pound, so a rational buyer buys more. The whole downward-sloping demand curve is really just rational choice drawn out.
Draw this in your head before you criticise the model. Every criticism in this topic is an attack on one of the three pillars.
Notice the assumptions are about ability, not morality. Economists are not saying people should be selfish calculators; they are saying the maths works out neatly if we pretend they are.
Why the model is still worth having
Students often finish this topic thinking rational choice theory has been proved wrong and can be thrown away. That is not what examiners want. A model is a simplification, and the test of a simplification is whether it predicts well enough to be useful.
Rational choice theory predicts a huge amount correctly. Petrol prices rise, people drive less. Wages rise, more people apply. Behavioural economics does not replace this — it patches the places where the prediction misses, mostly small, repeated, low-value decisions where nobody bothers to calculate.
Use this as an evaluation line. “Rational choice theory remains a good approximation for large, planned purchases such as a car or a mortgage, but a poor one for small, habitual purchases such as a chocolate bar at a till.” That single sentence is worth more than listing five biases.
The five limits of rational consumer choice
Behavioural economics groups the failures into five headings. Learn the headings first, then hang one example on each.
Three of the five start with the word “bounded”. That is your memory hook: brain, willpower, selfishness — all limited.
1. Biases
A bias is a mental shortcut. The brain has to make thousands of small decisions a day, so it uses rules that are fast rather than correct. Most of the time the shortcut is fine. Sometimes it steers you to a worse option.
Bias
What it does
Example you can use
Rule of thumb
You repeat what you chose last time instead of comparing again
Ordering the same coffee every morning, even when a cheaper loyalty deal exists
Anchoring
The first number you see drags every later judgement towards it
A jacket shown as “was £120, now £70” feels cheap, even if it was never really worth £120
Framing
The same fact feels different depending on how it is worded
Yoghurt labelled “90% fat free” sells better than the identical pot labelled “10% fat”
Availability
Whatever comes to mind easily feels more likely than it is
Refusing to fly after a crash is on the news, then driving instead — the far riskier option
Anchoring and framing are the two that examiners love, because they are the two that firms deliberately use on you. Learn those properly and skim the rest.
2. Bounded rationality
Herbert Simon’s idea: people want to decide well, but they have limited thinking capacity, limited information and limited time. So they stop searching once they find something good enough rather than carrying on to find the best.
Too much choice makes it worse. Faced with forty pension options full of jargon, many people pick nothing at all — which is the one choice guaranteed to be wrong.
3. Bounded self-control
Willpower is a resource that runs down. You can know exactly what is in your long-term interest and still not do it, because the reward is now and the cost is later. That is why gym memberships go unused in February and why supermarkets put sweets at the till and not by the door — you are at your weakest at the end of the shop.
4. Bounded selfishness
Classical theory says you act in your own interest. In reality people give blood, tip in restaurants they will never visit again, and donate to strangers. This is altruism: acting to help others with no expectation of return. It does not fit a model where utility depends only on what you personally consume.
5. Imperfect information
Perfect information almost never exists. Information costs money and time to gather, some of it is protected by patents, and often there is simply too much of it to process. Worse, it is frequently asymmetric — one side knows more than the other. The seller of a second-hand car knows about the rattle; the buyer does not.
Link this forward. Asymmetric information is a genuine cause of market failure later in the course. Mentioning that connection in a longer answer shows the examiner you can see across the syllabus.
Worked examples
WORKED EXAMPLE 1
Explain, using an example, how anchoring can lead a consumer to a choice that does not maximise utility. [4]
Step 1: define the term
Anchoring is when a consumer leans too heavily on the first piece of information they see, and judges everything after it against that starting number.
Step 2: give a concrete exampleA retailer prints “usual price £200” next to a sale price of £120. The £200 becomes the anchor.
Step 3: link it back to utility
The buyer judges the deal against £200 instead of against the actual benefit of the coat or the price of rival coats, so they may pay more than the item is worth to them.
Utility is not maximised because the comparison used was the wrong oneNotice the shape: define, example, consequence. Four marks, three moves.
WORKED EXAMPLE 2
A supermarket places chocolate next to every checkout. State and explain which limitation of rational consumer choice this is designed to exploit. [2]
Step 1: name the limitationBounded self-control.Step 2: explain the mechanism
Shoppers reach the till tired and already carrying a full basket, so their willpower to resist an unplanned treat is at its lowest. The purchase is driven by impulse in the moment, not by comparing benefit against cost.
Bounded self-control — 1 mark for naming it, 1 for the explanationDo not write “bad decision making”. Use the syllabus term.
💡 Exam tip
Name the term exactly. “Bounded self-control” scores; “people are weak” does not.
One example per limitation, memorised and short. You will not have time to invent one under pressure.
For evaluate questions, argue both ways: the model is a poor description of individuals but a decent predictor of markets as a whole.
Say whose utility is affected. Examiners want to see the consumer, the firm or society named.
Keep bounded rationality and imperfect information apart in your head: one is about the brain, the other is about the world.
This is HL only material, so it sits in Paper 1 and Paper 3 rather than the SL data response.
⚠️ Common mix-up
Thinking “irrational” means stupid. In economics it just means the choice does not fit the model’s prediction.
Confusing bounded rationality with bounded self-control. Bounded rationality is “I could not work it out”; bounded self-control is “I worked it out and did it anyway”.
Treating altruism as proof the model is useless. It is one limitation, not a demolition.
Mixing up anchoring and framing. Anchoring is about a number you saw first. Framing is about the wording of the same fact.
Writing a list with no example. Half the marks in this topic live in the example.
Saying firms have perfect information. Firms face exactly the same limits — that matters later for business objectives.
Up next: Behavioural Economics and Nudges — if people do not choose perfectly, how do governments and firms deliberately steer those choices? That is choice architecture, and it is the natural follow-on from everything above.
Want this explained one-to-one?
Book a free session with an experienced IB Economics tutor and get your trickiest topics made simple.
