IB Economics HLTopic 1 — Introduction to EconomicsPaper 1 & 2Core skill~10 min read
Building and Using Economic Models
A model is a deliberately simple picture of something complicated. You have already met one — the PPC. Here we look at how models are built, the assumption that makes them possible at all, and the big one you will use for the rest of the course: the circular flow of income.
📚 What you need to know
A model is a simplified version of reality used to explain and predict.
Models rely on assumptions, especially rational behaviour and ceteris paribus.
Ceteris paribus means “all other things staying the same” — it lets you isolate one cause.
The circular flow of income shows money moving between households and firms.
Households supply factors of production and receive income; firms supply goods and services and receive spending.
Injections (investment, government spending, exports) add money in. Leakages (savings, taxes, imports) take money out.
If injections exceed leakages the flow grows; if leakages exceed injections it shrinks.
What a good model does
A model has one job: to strip a situation down until the important relationship is visible. To build one you decide which variables you will study, which you will ignore, and how you think the ones you kept are connected.
🧩 How an economic model gets built
Ask a question. Why did rents rise in this city?
Form a hypothesis. Rents rose because more people moved in while housing supply stayed fixed.
State the assumptions. Nothing else changed: incomes, interest rates, building rules held constant.
Gather evidence. Population figures, building permits, rent data.
Test and refine. If the evidence contradicts the hypothesis, the model is refuted and must change.
Notice step 5. A statement that could be proved wrong by evidence is doing real work. If no possible evidence could ever contradict your claim, it is not a scientific model — it is an opinion in disguise.
Economists cannot repeat an experiment in a lab, so the same hypothesis can give different results in different countries, different decades, or different cultures. That is not sloppy work. It is what happens when your subject is people.
Ceteris paribus
Dozens of things affect the demand for coffee at once: its price, the price of tea, incomes, the weather, health stories in the news, the time of year. If they all move together you can never say which one did what.
So economists freeze all of them but one. That freeze is ceteris paribus, Latin for “all other things being equal”. Change the price, hold everything else still, and now you can see the effect of price alone.
Crossed out does not mean these things stop mattering. It means we are pretending they hold still so one effect can be seen clearly — which is exactly the weakness to raise in evaluation.
The circular flow of income
Now the big one. The circular flow shows how money moves round an economy. In its simplest form there are only two groups: households (that is you) and firms.
Households own the factors of production — their labour, their land, their savings. They sell those to firms and get paid: wages, rent, interest and profit. Then they take that income and spend it buying goods and services back from the same firms. Round and round.
The green loops are money. The dashed loops are the real things moving the other way — hours of work going one direction, haircuts and phones coming back.
Why this matters later: because the flow is a circle, one person’s spending is another person’s income. Cut spending and you cut somebody’s wage — which cuts their spending too. That chain reaction is the whole basis of macroeconomic policy.
Leakages and injections
The two-sector model is too tidy. In reality money does not just circle forever: some of it drains out, and new money is added in.
Three pairs to memorise: savings and investment, taxes and government spending, imports and exports. One of each pair takes money out, the other puts it back in.
Sector
Injection (money in)
Leakage (money out)
Financial sector
Investment (I) — firms buy new equipment
Savings (S) — households put income in the bank
Government
Government spending (G) — schools, roads, wages
Taxes (T) — income taken out of the flow
Foreign sector
Exports (X) — foreign money comes in
Imports (M) — domestic money goes abroad
The rule to remember
Injections > Leakages → the economy grows
Leakages > Injections → the economy shrinks
Examiners like the word interdependence here. Households, firms, banks, the government and the rest of the world are all wired into the same loop, so a shock to any one of them travels round to the others. That is a ready-made link to one of the nine central concepts.
Judging a model
Because every model is simplified, criticising one is easy and worth marks — provided you are specific. Weak criticism says “it is unrealistic”. Strong criticism names the assumption and says what changes without it.
Assumption
Why it may fail
What changes as a result
Consumers act rationally
People buy on impulse, habit and emotion
Demand may not respond to price the way the model predicts
Firms maximise profit
Some chase market share, ethics or reputation
Output and pricing decisions differ from the theory
Everyone is well informed
Sellers usually know more than buyers
Buyers make poor choices and markets misallocate resources
Ceteris paribus holds
In reality many things change at once
The predicted effect may be swamped by something else
Worked examples
WORKED EXAMPLE 1
Classify each as an injection or a leakage: (a) a family buys a Korean television; (b) a firm builds a new warehouse; (c) income tax is deducted from a wage; (d) a winery sells 2 000 bottles to Canada. [4]
Ask: is money entering the domestic flow or leaving it?
(a) Money goes abroad → import → leakage (M)
(b) Firms spending on capital → injection (I)
(c) Income removed by the government → leakage (T)
(d) Foreign money comes in → export → injection (X)leakage, injection, leakage, injectionInvestment means buying capital goods, not buying shares. That trips up half the class.
WORKED EXAMPLE 2
Using the circular flow model, explain the likely effect of a rise in interest rates on the size of the flow. [4]
Step 1: what interest rates do to savings
Saving now pays more, so households save a larger share of income → leakages (S) riseStep 2: what they do to borrowing
Borrowing costs more, so firms delay new equipment → injections (I) fallStep 3: compare the twoLeakages > injectionsStep 4: conclude
The circular flow shrinks, so national income and output are likely to fall.
Flow shrinks: higher leakages, lower injectionsFinish with a ceteris paribus line: this holds only if government spending and exports do not change.
WORKED EXAMPLE 3
“The demand for train tickets will fall if fares rise.” Explain why the assumption of ceteris paribus is needed for this claim. [3]
Step 1: state what is being isolated
The claim links only one cause (fare) to one effect (quantity demanded).
Step 2: name what must be held still
Petrol prices, incomes, service quality, the weather, fuel taxes.
Step 3: say what happens without it
If petrol prices rose at the same time, demand for tickets could rise despite the fare increase — and the prediction would look wrong even though the theory is fine.
Ceteris paribus isolates one cause from the noiseGive a concrete “other thing” that could change. Naming it is what earns the third mark.
💡 Exam tip
Learn the six letters: S, T, M leak out; I, G, X flow in. Alphabetical pairs help: S with I, T with G, M with X.
Use “ceteris paribus” at least once per essay. It shows you understand the limits of your own argument.
Define a model in one line before you criticise it, so the examiner knows you understand it first.
Name the assumption you are attacking. Vague criticism scores nothing.
Link the circular flow to interdependence — that is one of the nine central concepts.
For any policy question, trace the effect round the loop: whose income changes, and what do they do next?
⚠ Common mix-up
Calling buying shares “investment”. In economics, investment means firms buying capital goods.
Thinking savings are bad. Savings leak out, but banks lend them on as investment, which injects back in.
Treating ceteris paribus as a claim that nothing else changes. It is a temporary assumption, not a description.
Mixing up the two loops in the circular flow. Money goes one way, factors and goods go the other.
Saying a model is wrong because it is simple. Simplicity is the point; the question is whether the answer is still useful.
Forgetting the government and foreign sectors and then wondering why an economy can grow at all.
Up next: Positive and Normative Statements — how to tell a claim about facts from a claim about values, and why examiners test it every year.
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