IB Economics SLTopic 1 — Introduction to EconomicsPaper 1 & 2Core idea~9 min read
The Circular Flow of Income
Your wages come from a firm. You spend them at other firms. Those firms pay wages to other people, who spend them too. Follow that loop all the way round and you have the second model of this topic — and the foundation of everything in macroeconomics.
📚 What you need to know
The circular flow of income is a model showing how money moves around an economy.
In the simple version, households supply factors of production to firms and receive income; firms supply goods and services and receive spending.
The fuller version adds the government, the financial sector and foreign trade.
Injections add money to the flow: investment (I), government spending (G) and exports (X).
Leakages (withdrawals) take money out: savings (S), taxation (T) and imports (M).
If injections exceed leakages the flow grows; if leakages exceed injections real GDP falls.
The model is a direct illustration of interdependence, one of the nine central concepts.
The simple model: households and firms
Start with just two groups. Households own the wealth in an economy — specifically, they own the factors of production. Firms need those factors in order to produce anything.
So households supply land, labour and capital to firms, and are paid for it: rent, wages, interest and profit. That income is then spent on the goods and services firms produce, which becomes sales revenue for those firms, which pays for the next round of factors. Round it goes.
Two loops, not one. The grey loop is the real economy of things and effort; the green loop is the money paid for them.
The left-hand side is the factor market: households sell their resources and firms buy them. The right-hand side is the product market: firms sell output and households buy it. Every arrow has a matching arrow going the other way, which is why the model is called circular.
This is where the four factor incomes from the previous page earn their keep. Rent, wages, interest and profit are not four random words — they are the green arrow on the left of this diagram, and together they add up to the national income of the whole economy. That connection is the whole basis of national income accounting later in the course.
The fuller model: injections and leakages
The simple model has only two actors, which is not how any real economy works. The fuller version adds three more: the government, the financial sector (banks) and the foreign sector (trade). Each of them either adds money to the flow or takes money out of it.
Each of the three extra sectors appears twice: once on the left putting money in, once on the right taking money out.
Sector
Injection (money in)
Leakage (money out)
Government
Government spending (G) on schools, roads, healthcare and public sector wages
Taxation (T) taken from both households and firms
Financial sector
Investment (I) by firms, funded through banks
Savings (S) by households, which is income not spent
Foreign sector
Exports (X), because foreign money flows in to buy domestic output
Imports (M), because domestic money flows out to foreign producers
The rule that matters
injections > leakages → the flow grows | leakages > injections → the flow shrinks
Injections represent new income entering the economy. Anything that changes government spending, investment, consumption or net exports will change the relative size of the circular flow. A rise in interest rates, for example, encourages households to save more — a leakage — while also discouraging consumption and making investment more expensive. Three effects, all pulling the flow the same way.
Do not just count the arrows — weigh them. What matters is the net effect and the proportions. If a government launches a very large spending programme, that single injection may easily outweigh the combined leakage from savings and imports. Three leakages against three injections tells you nothing on its own; the sizes are the answer.
This model is interdependence in one picture. Households, firms, the government, banks and the foreign sector are all linked, so a change in any one of them travels to all the others. That is why a downturn in one large economy is felt by its trading partners within months — and why interdependence is one of the nine central concepts.
Worked examples
WORKED EXAMPLE
In one year an economy records: government spending 120bn, investment 90bn, exports 140bn, savings 80bn, taxation 150bn, imports 110bn. Calculate the net effect on the circular flow and explain what it means. [4]
Step 1: total the injectionsG + I + X = 120 + 90 + 140 = 350bnStep 2: total the leakagesS + T + M = 80 + 150 + 110 = 340bnStep 3: find the net position350 − 340 = +10bn net injectionThe circular flow of income grows by 10bnStep 4: say what that means
More money is entering the flow than leaving it, so incomes and output rise. This is consistent with economic growth over the period.
Group the three injections and three leakages before subtracting. Doing it pair by pair is where errors creep in.
WORKED EXAMPLE
Explain the likely effect of a rise in interest rates on the circular flow of income. [4]
Effect one: savings
Higher interest rates reward saving, so households save more.
savings up → leakage upEffect two: consumption
Money saved is money not spent, so consumption within the flow falls.
Effect three: investment
Borrowing costs more, so firms invest less.
investment down → injection downPut it together
Leakages rise while injections fall, so the circular flow of income shrinks and real GDP is likely to fall.
Leakages up, injections down, flow contractsNaming which arrow each effect moves is what turns a list into an explanation.
💡 Exam tip
Learn the six letters in pairs: I with S, G with T, X with M. Each sector gives you one injection and one leakage.
Say which direction each arrow moves. “Savings rise, so leakages rise” scores; “savings rise” on its own does not.
Two loops, not one. Describe the real flow of factors and goods as well as the money flow.
Weigh the sizes. Evaluation marks come from discussing net effect and proportionality, not from listing arrows.
Connect it to interdependence by name. It is one of the nine concepts and this model is the clearest example of it.
Do not confuse this with the circular economy, which is an entirely different idea about reusing materials.
⚠️ Common mix-up
Calling savings an injection. Saved income has left the flow. It only returns as an injection when a bank lends it out as investment.
Putting imports on the wrong side. Money paid for imports leaves the domestic economy, so imports are a leakage.
Assuming three injections beat three leakages. The count is irrelevant; only the total sizes matter.
Confusing the circular flow model with the circular economy. The first is about money moving; the second is about materials being reused.
Forgetting the real flow. The model shows factors and goods moving as well as money, and questions often ask about both.
Thinking taxation is simply lost. It leaves as a leakage and comes back as government spending. What matters is whether the two match.
Up next: Positive and Normative Economics — the difference between what is and what ought to be, and why economists disagree so much even when they share the same data.
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