Rational Decisions and Demand
~40 min · WEC11 · 1.3.2
WEC11 · 1.3.2 · 40 min
The says a consumer chooses to maximise utility — and is also the exact reason their own slopes downward. The six spec-named reasons a real consumer might not maximise utility aren't exceptions to rational behaviour so much as rational behaviour once the cost of deciding gets counted too.
Key terms in this lesson
Before you read on
Two or three questions on exactly what this lesson teaches. Being wrong here is fine — it's the fastest way to find out what to pay attention to next.
Two questions the rationality assumption is built to answer
Spec 1.3.2 opens with a single assumption that every model on this paper quietly relies on: consumers aim to maximise (their own satisfaction) and firms aim to maximise profit — an irrational consumer, by the same mark scheme's own definition, is simply one who fails to maximise utility, not some separate, exotic category needing its own theory. It's a starting point, not a claim that every real decision is perfectly optimised — and the spec spends its very next sub-point listing six specific, named ways real consumers depart from it. Both halves matter for marks: assume rationality unless a question gives you a reason not to, and know the six named departures precisely enough to explain, not just label, each one.
The is where the rationality assumption goes to work. It plots the quantity of a good demanded against its price, holding every other determinant constant — and it slopes downward for a reason that isn't a drawing convention: . As a consumer buys more of something in a given period, each further unit typically adds less extra satisfaction than the one before, which means a rational consumer is only willing to pay progressively less for each additional unit. Derive that link once and the curve's shape stops being something to memorise.
The single most exam-costly distinction on this topic is versus of that same curve. A change in the good's OWN price moves the consumer to a new point on the same, unchanged curve — quantity demanded changes, demand itself doesn't. That isn't a labelling convention to memorise: a demand curve IS the full set of (price, quantity) pairs for a good, so a change in the good's own price is simply reading a point the curve already contained — no new information has entered the model, so the curve itself has no reason to move. A change in anything else moves the whole curve to a new position, precisely because it changes something the curve was NOT already plotting: the price of substitutes or complements, real income, tastes, the size and age distribution of the population, or advertising (spec 1.3.2.2d) — each one alters how much is demanded at a GIVEN price, which only a new curve can represent. Confuse these two and every diagram built on top of them — tax incidence, welfare loss, market equilibrium — inherits the error.
The six spec-named reasons a consumer might not maximise utility — herding, habitual behaviour, inertia, poor computational skills, the need to feel valued, and framing and bias — are a closed, memorisable list, and examiner reports confirm candidates can usually name all six. What separates a Level 2 answer from a Level 3 one is whether each name comes with an actual mechanism attached, which is what the next section derives rather than asserts.
The six named reasons aren't the only content a real 'evaluate possible reasons why consumers did not switch' essay credits, either. The mark scheme for the actual past-paper question this lesson is built from — Oct 2024 Section D Q13, on UK consumers who didn't switch mortgage, mobile phone and broadband supplier — also credits three genuinely structural reasons that need no behavioural mechanism at all: information failure, specifically asymmetric information, where the provider simply knows more than the consumer about whether a better deal exists elsewhere; the sheer complexity of comparing bills across providers, which can defeat even a fully rational, well-motivated consumer; and consumers who are locked into a fixed-term contract and are genuinely unable to switch yet, whatever they would otherwise prefer. None of these needs a psychological explanation — the barrier is structural, not behavioural — which is exactly why a complete answer to this question type draws on both categories rather than treating the six named reasons as the whole story. (Asymmetric information gets its own full diagram-and-mechanism treatment later, under market failure — spec 1.3.5.4 — this is only the context-specific point this exact essay itself credits, not a substitute for that lesson.)
Mechanism
The real cost 'rational' leaves out, and the six departures it explains
Full utility-maximisation assumes a consumer can gather perfect information about every option and compare them without cost. Neither is true: researching alternatives takes real time, and comparing them accurately takes real cognitive effort — both of which are themselves scarce resources with an opportunity cost. A genuinely rational agent, correctly accounting for the cost of DECIDING and not just the cost of the good itself, will often pick a cheap-to-run decision rule over an expensive, perfectly-optimising one — and that single reallocation generates all six spec-named departures as specific cases, not six unrelated exceptions. Herding lets a consumer copy a choice someone else has already paid the research cost to make, substituting a nearly-free signal for an expensive one. Habitual behaviour skips paying the decision cost a second time by repeating what worked last period, which is a good approximation exactly as long as circumstances haven't moved much since — and, separately, that same loyalty can be independently rational on its own merits, not just as a cost-saving shortcut, whenever the provider is still genuinely delivering the service quality that first justified the choice, exactly the mark scheme's own point that this loyalty 'is rational as it comes from good levels of customer service' [Oct 2024 MS, Q13]. Inertia is the same logic applied specifically to the ACT of switching: research plus paperwork plus the risk of a worse outcome is a real, felt cost, and where it's perceived to exceed the saving on offer, staying put is the cheaper option even though it isn't the utility-maximising one in narrow financial terms. Poor computational skills is the honest limit case — genuinely bounded capacity to compare options accurately, so the chosen option can differ from the true maximum even when the consumer is trying their best. The need to feel valued works differently: it isn't that the consumer miscalculates, it's that a seller can attach a real emotional payoff (being remembered, treated as a loyal customer) to staying, which competes directly against the financial saving of leaving — the consumer is still maximising SOMETHING, just not the narrow financial deal a mark scheme is asking them to evaluate. And framing and bias is the case that doesn't fit the decision-cost story at all: it's not about the cost of gathering information, it's about presentation changing the choice even when the substantive information is identical and already in hand — which is exactly why it gets its own beyond-spec explanation below rather than folding into the others.
Worked, in full
From diminishing marginal utility to a downward-sloping demand curve, step by step
- 01
Utility is measured here in utils — an artificial, invented unit with no real-world equivalent; only the RELATIVE size of one total against another carries meaning, not the number itself. Assume a rational consumer whose marginal utility of money is constant at 1 util per £1 — a standard simplifying assumption that lets a utility value be read directly as a maximum acceptable price in pounds. A market stallholder sells glasses of lemonade on a hot day; a customer's total utility (TU) from successive glasses is 20, 36, 48, 56, 60, 60 utils for glasses 1 through 6.
Earns: K — the simplifying assumption named explicitly, not silently built into the numbers.
- 02
Marginal utility (MU) of each glass is the CHANGE in TU: glass 1 MU=20 (20−0), glass 2 MU=16 (36−20), glass 3 MU=12 (48−36), glass 4 MU=8 (56−48), glass 5 MU=4 (60−56), glass 6 MU=0 (60−60). Every glass from the second onward adds less than the one before it — glass 2's MU of 16 is already below glass 1's MU of 20, so diminishing marginal utility sets in at the second glass, the earliest point the concept can apply at all (there is no glass before the first to compare it against).
Earns: An1 — MU derived by subtraction from the TU column, not presented as a separate, unconnected table.
- 03
A rational utility-maximiser buys the nth glass only if its MU is worth at least its price: paying more than a glass's MU wastes utility that the same money would have kept if spent elsewhere, and refusing a glass whose MU exceeds its price leaves utility on the table. So the maximum price worth paying for the nth glass is exactly MU(n) — £20 for the 1st, £16 for the 2nd, £12 for the 3rd, £8 for the 4th, £4 for the 5th, £0 for the 6th.
Earns: An2 — the price–utility link derived from a buy/don't-buy comparison, not stated as a rule to memorise.
- 04
Read as a (quantity, price) schedule, this IS the individual demand curve: (1,£20), (2,£16), (3,£12), (4,£8), (5,£4), (6,£0). It slopes downward because a monotonically falling MU column forces a monotonically falling maximum-price column, unit for unit — exactly the spec's own point (1.3.2.2c) about diminishing marginal utility's 'significance for the shape of the individual demand curve,' not a separate empirical claim about what demand curves tend to look like.
Earns: Eval — the curve's shape identified as a forced arithmetic consequence, not a drawing convention.
Source — Examiner report, Jun 2024
"The utility is rising but at a slower rate."
x-axis: Quantity demanded per period, Q · y-axis: Price, £
- D1
- The original demand curve — downward sloping, derived from diminishing marginal utility above.
- D2
- D1 shifted rightward by a non-price determinant (a rise in real income for a normal good, a fall in the price of a complement, or a successful advertising campaign) — more is now demanded at EVERY price, not just one.
- A — starting point on D1
- The consumer's original price and quantity demanded, before either change below is applied.
- B — movement along D1
- A movement caused only by a change in the good's own price: price falls from 70 to 40, quantity demanded rises from 20 to 50 — but this stays on the SAME curve, because D1 already encodes every (price, quantity) pair; demand itself does not change.
- C — shift to D2, same price as A
- A shift to an entirely new curve at the SAME price as A (70): quantity demanded is 30 on D2 versus 20 on D1 at that price, caused by anything other than the good's own price — demand itself has changed.
Common error: Drawing a shift for a price-driven change, a movement along the curve for a non-price-driven change, or getting the DIRECTION of a correctly-identified shift wrong.
Correct: Price change → movement along the same curve. Any other determinant → the curve itself moves — and check whether the good is normal or inferior before choosing which way a real-income shift points.
examiner-report · Oct 2023 · Q7
In your own words
In one sentence: why does a rise in the price of a good move a consumer along their OWN demand curve, while a rise in real income shifts the ENTIRE curve to a new position?
Complete it yourself
Complete the chain — why herding over a bank choice is a rational response, not just imitation
- 01
Many retail banking customers hold their savings account at the same bank as their parents, and have never seriously compared it against a competitor.
- 02
This is herding — spec 1.3.2.1(b)'s term for a purchase decision shaped by the influence of other people's behaviour rather than by independent research.
- 03
Independently researching and comparing every bank's interest rates, fees and service quality is itself costly — it takes real time, and comparing providers accurately requires a level of financial literacy not everyone has spare hours to build from scratch.
Named traps
- inertia-vs-habitual-behaviour
- The single most repeated confusion on this topic, confirmed independently across multiple series. Directly verified: "The concept of inertia is often confused with habitual behaviour. It is important that the difference between habitual behaviour and inertia is understood. Key is that inertia is where the consumer feels the effort to make the change is too great and they decide not to switch." [Oct 2023 ER, Q8] Reinforced independently a series earlier: "The topic of irrational consumer behaviour and in particular inertia was commonly confused with many unable to identify that is occurred when consumers felt the effort to switch was too great." [Oct 2022 ER, Paper Summary] The fix: habitual behaviour is about REPETITION without reconsidering; inertia is specifically about the EFFORT of switching outweighing the perceived gain. A candidate who has actively weighed and rejected switching is describing inertia, not habit.
- naming-without-mechanism
- Confirmed directly, and this is the exact failure mode this lesson's derive-don't-assert approach is built to prevent: "Many could identify reasons why consumers do not switch including habitual behaviour, inertia, poor computational skills, influence of others behaviour (herding) and need to feel valued. Many could then offer some chain of reasoning as to how this results in decisions that do not maximise utility. However, many struggled to offer a developed chain." [Oct 2024 ER, Section D Q13] The same paper's Section A confirms the same pattern one level down, at the level of individual words being pattern-matched rather than understood: "Most could correctly identify that consumers exhibit habitual behaviour but the words computation and feeling valued persuaded some to opt for the responses. But it is a weakness of computation and it is current providers making them feel valued that causes them not to switch." [Oct 2020 ER, Q4] Naming all six reasons is a Level 1–2 skill; a developed mechanism for the specific one that actually fits the stem is what separates Level 3 from Level 2.
- diminishing-marginal-utility-precise-onset
- Confirmed across four separate series, always the same underlying error: identifying diminishing marginal utility one step too late — at the point marginal utility hits zero (where total utility peaks) or turns negative (where total utility itself starts falling), rather than at the exact unit where MU first falls while still positive. "It was common for candidates to identify that the movement to 5 glasses saw diminishing marginal utility when in fact this indicates where decreasing marginal returns occurs." [Jan 2022 ER, Q6] "Many defined diminishing marginal utility inaccurately and in fact were defining decreasing marginal utility." [Oct 2019 ER, Q11] "The topic of diminishing marginal utility was challenging for many with many identifying where decreasing marginal utility occurs rather than where diminishing marginal utility starts." [Jan 2021 ER, Paper Summary] Most precisely stated: "Many identified it as where total utility fell but it is where marginal utility falls. The utility is rising but at a slower rate." [Jun 2024 ER, Q8] The exact rule: find every marginal utility value, then find the first one that is LOWER than the value before it — that unit, not the peak of total utility and not the point total utility starts falling, is where diminishing marginal utility sets in.
- real-income-shift-direction
- Confirmed directly: "Many showed demand increasing incorrectly. With falling real income for a normal good the demand would shift leftwards." [Oct 2023 ER, Q7] Getting the shift itself right (real income is a shift factor, not a movement) isn't enough — the direction depends on whether the good in question is normal (demand moves the SAME way as income) or inferior (demand moves the OPPOSITE way). A question that doesn't explicitly say which type of good is involved is testing whether you check before assuming.
- dmu-affects-demand-not-supply
- Confirmed directly: "A common error was to identify that the supply curve will slope upwards, this is incorrect as diminishing marginal utility benefits consumers and affects demand and not supply." [Oct 2022 ER, Q1] Diminishing marginal utility is a consumer-side, demand-side concept from first principles — it has no mechanism that reaches supply at all. If an answer's chain of reasoning ends up touching the supply curve, the chain has gone wrong somewhere before that point, not the diagram.
The conditional move
Complete: "A rise in real income shifts a good's demand curve to the right only if ___."
Complete: "Inertia is a credible explanation for a consumer staying with an uncompetitive deal only if ___."
Complete: "Poor computational skills is a credible explanation for a consumer staying with a worse deal only if ___."
Complete: "The need to feel valued is a credible reason for a consumer to stay with their current provider only if ___."
Complete: "Non-switching only needs one of the six named departures from utility-maximisation (herding, habit, inertia, poor computation, the need to feel valued, framing) to explain it if ___."
Complete: "Information failure is a credible reason for a consumer not switching supplier only if ___."
Beyond the spec
The spec names 'framing and bias' as a single closing bullet with no explanation of the mechanism behind it — a student can define the term but rarely defend WHY presentation changes behaviour when the substantive choice hasn't. No examiner-report commentary on this specific spec point turned up in any of the 15 WEC11 examiner reports checked this session (a genuine absence, not a gap in research) — which makes the underlying theory the only way to actually understand it rather than just recite it.
Daniel Kahneman and Amos Tversky's prospect theory (1979, Econometrica, 'Prospect Theory: An Analysis of Decision under Risk') is the real mechanism behind the spec's single line on framing and bias. Its central finding: people evaluate outcomes as gains or losses relative to a reference point, not against an absolute scale — and losses are felt roughly twice as intensely as equivalent gains, a property called loss aversion. That's exactly why 'you'll lose £120 a year by not switching' and 'you could save £120 a year by switching' describe the identical financial fact yet produce measurably different switching rates: the first framing places the £120 in the loss domain, where its psychological weight is roughly doubled. That dependence on presentation is not unlimited, though: competition authorities and sector regulators can and do intervene against providers who mislead by framing options in a better light than the substance actually justifies — the real mark scheme's own evaluative check on how much framing and bias alone can explain persistent non-switching ('competition authorities will challenge providers who mislead by framing the options in a better light' [Oct 2024 MS, Q13]), the same 'the effect is conditional, not absolute' treatment already given above to herding and inertia. Kahneman won the 2002 Nobel Memorial Prize in Economic Sciences for this work (Tversky had died in 1996 and was ineligible). A second figure worth knowing for the same reason: Richard Thaler's work on the status quo bias and 'default effects' (Nobel 2017) is the more precise theoretical account of what the spec calls inertia specifically — his finding that simply changing which option is pre-selected as the default, without changing the options themselves or the effort needed to switch, measurably changes what people choose. That's the theoretical basis for why regulators increasingly focus on making switching the automatic default action rather than just publicising the size of the saving.
Retrieval — with feedback on every choice
A student's total utility from successive rounds of coffee during a revision session is: round 1 = 12 utils, round 2 = 25 utils, round 3 = 36 utils, round 4 = 44 utils. At which round does diminishing marginal utility first occur?
The price of long-haul flights falls. As a direct result, with every other determinant of demand held constant, the quantity of long-haul flights demanded rises. What has happened to demand for long-haul flights itself?
A broadband provider finds that far more customers accept a new contract when it is advertised as 'Save $120 a year' than when the mathematically identical contract is advertised as '$10 a month less than you might be overpaying now.' Which spec-named reason for not maximising utility does this best illustrate?
In 2022, UK consumers who did not switch broadband, mobile phone and mortgage provider at the end of a deal collectively overpaid by roughly £1.3 billion — an estimated 630,000 mortgage consumers, 1.5 million mobile phone consumers and 7 million broadband consumers — even though comparing and switching providers typically takes well under an hour online and costs nothing.
Using the concept of inertia, explain why so many consumers fail to switch even when the financial saving is well-documented and switching itself is fast and free.
Same question, every level
Discuss the extent to which the rationality assumption provides a realistic basis for modelling consumer demand. (VERIDIAN-original question, written in the pattern of WEC11's Section D 'Discuss'/'Evaluate' essay format — 20 marks total, split 12 for Knowledge, Application and Analysis and 8 for Evaluation, confirmed directly against the Pearson IAL Economics Unit 1 Exemplar Materials (Feb 2020), Question 14 — not a reproduction of any single past-paper question.)
20 marks available
Consumers are supposed to make the best choice for themselves, but sometimes they don't. For example, some people just copy what their friends do, or they don't want to switch bank because it's a hassle. So the assumption isn't always true.
No named mechanism, no correctly-used spec terms (herding and inertia are described but not named), no diagram, no definition of the rationality assumption itself — floor band on both the KAA and Evaluation scales.
- Rationality assumption: consumers maximise utility, firms maximise profit — default unless a question signals otherwise.
- Six reasons consumers may not maximise utility: herding, habitual behaviour, inertia, poor computational skills, need to feel valued, framing and bias — all but framing are a response to decision cost.
- Movement along D = price change only. Shift of D = subs/complements, real income, tastes, population, advertising.
- DMU = first unit where MU falls (TU still rising, slower) — not where TU peaks (MU=0) or falls (MU<0).
- Real income ↑ shifts demand right only for a normal good — left for an inferior good.
- Non-switching isn't only explained by the six reasons: information failure, bill complexity and fixed-term contract lock-in are separate, non-behavioural reasons the real mark scheme also credits — and market-wide price rises can make not switching itself the rational choice.
- Section D 'evaluate reasons' essays cap KAA at Level 3 if only one reason is developed, however well-explained — a top-band answer draws on multiple reasons (verified: Oct 2024 MS Q13, 'N.B. Award a maximum of level 3 if only one reason is given').
- Section D 'evaluate reasons' essays also cap KAA at Level 3 with no tie back to the specific scenario/services named — the same context-anchoring gate Section C enforces via extract-sourced examples (verified: Oct 2024 MS Q13, 'N.B. Award a maximum of level 3 if there is no reference to these services').
Not affiliated with or endorsed by Pearson Edexcel. Every quotation and figure attributed to a mark scheme or examiner report in this lesson was independently verified against the primary Pearson document this session — several beyond this course's earlier research pass's own read set — not carried over from prior course material.
A student's total utility from successive rounds of coffee during a revision session is: round 1 = 12 utils, round 2 = 25 utils, round 3 = 36 utils, round 4 = 44 utils. At which round does diminishing marginal utility first occur?
- ARound 1
Diminishing marginal utility describes a FALL in marginal utility compared to the unit before it — there is no previous round to compare round 1 against, so the concept cannot apply here at all.
- BRound 2
Marginal utility is actually still RISING here — round 1's MU is 12 (12−0) and round 2's MU is 13 (25−12), a rise, not a fall. Diminishing marginal utility hasn't set in yet.
- Round 3
Correct. MU is 12 for round 1, 13 for round 2 (still rising), then 11 for round 3 (36−25) — the first fall in the sequence. Work out every marginal utility first, then find where the sequence turns from rising to falling.
- DRound 4
MU has already fallen once by this point (13→11) and falls again here (11→8, since 44−36=8) — diminishing marginal utility is already well underway, not just beginning.
Traps tested: Dmu needs no comparison point · Misreads rising mu as falling · Identifies established not onset
The price of long-haul flights falls. As a direct result, with every other determinant of demand held constant, the quantity of long-haul flights demanded rises. What has happened to demand for long-haul flights itself?
- ADemand has increased — the whole curve has shifted right
This describes what a non-price determinant (a rise in real income, for instance) would cause. A change driven by the good's own price is a movement along the existing curve, not a shift of it.
- There has been a movement along the same demand curve to a new point
Correct. 'Every other determinant held constant' plus 'the good's own price changed' is exactly the definition of a movement along a fixed demand curve — quantity demanded changes, demand itself does not.
- CThe demand curve has shifted left
A price fall raising quantity demanded is entirely consistent with the existing downward-sloping curve — nothing here gives the curve itself a reason to move, in either direction.
- DIt cannot be determined without knowing the price elasticity of demand
PED tells you HOW MUCH quantity demanded changes, not WHETHER this is a movement or a shift — that's fixed entirely by what caused the change, which the question already states.
Traps tested: Conflates quantity demanded with demand · Direction reversed · Overclaims uncertainty
A broadband provider finds that far more customers accept a new contract when it is advertised as 'Save $120 a year' than when the mathematically identical contract is advertised as '$10 a month less than you might be overpaying now.' Which spec-named reason for not maximising utility does this best illustrate?
- AHerding
Herding is about copying other people's observed choices — nothing here involves customers responding to what anyone else has done, only to how the SAME deal is worded.
- BPoor computational skills
Both versions state the identical saving in different units ($120/year vs. $10/month) — a computational error would mean getting the maths wrong, not responding differently to two correctly-stated, equivalent figures.
- Framing and bias
Correct. The substantive deal is identical in both cases — only its presentation changed — and that alone measurably changed how many customers accepted it. That's exactly what framing and bias names: choice responding to how options are presented, not just what they substantively offer.
- DInertia
Inertia explains why a customer stays with their EXISTING provider rather than switching — this scenario is about which of two adverts for the SAME new contract wins more takers, not about staying versus switching at all.
Traps tested: Wrong concept entirely · Confuses framing with computation
In 2022, UK consumers who did not switch broadband, mobile phone and mortgage provider at the end of a deal collectively overpaid by roughly £1.3 billion — an estimated 630,000 mortgage consumers, 1.5 million mobile phone consumers and 7 million broadband consumers — even though comparing and switching providers typically takes well under an hour online and costs nothing.
Using the concept of inertia, explain why so many consumers fail to switch even when the financial saving is well-documented and switching itself is fast and free.
- Inertia predicts that consumers weigh the perceived time and effort of switching against the perceived financial gain, not the objective size of either — where the perceived effort is larger than the perceived saving, even a fast, free, well-documented switching process isn't enough to overcome it, which is why regulators increasingly focus on making switching the automatic default rather than just publicising the size of the saving
Correct — names the actual mechanism (perceived effort vs. perceived gain, not their objective size), explains why 'fast and free' alone doesn't fix it, and draws out the real policy implication rather than stopping at the label.
- BConsumers don't switch because switching costs them more than staying, so not switching is the rational choice
This restates the stem's own numbers back as a conclusion — it never explains WHY the perceived cost of switching outweighs a saving the stimulus already says is fast, free and well-documented, which is the actual question being asked.
- CConsumers don't switch because they are copying the behaviour of friends and family who also haven't switched
That mechanism is herding, not inertia — the stimulus gives no information at all about what other people are doing, only about each consumer's individual cost-benefit comparison.
- DConsumers don't switch because they are unable to calculate which deal would actually save them money
That's poor computational skills, a different spec-named reason — and the stimulus specifically says the saving is 'well-documented,' which argues against an inability to know the numbers being the barrier here.
Traps tested: Restates stem as analysis · Confuses inertia and herding · Confuses inertia and poor computation
Practice this for real
This site teaches the mechanism; the exam is sat on Pearson's own real questions. Go find and attempt these yourself — nothing here substitutes for actually sitting a timed paper.
- Examiner report
- Jun 2024 · Q8 — cited directly in this lesson
- Examiner report
- Oct 2023 · Q7 — cited directly in this lesson
Select International Advanced Level → Economics → any series, then look for WEC11.
Up next
Price, Income and Cross-Elasticities of Demand
Price elasticity of demand isn't a fixed property of a good — it changes continuously along a single straight-line demand curve, and whether a price cut raises or destroys total revenue depends entirely on which part of that curve a firm is standing on.
40 min