Revise Introduction to the circular flow of income for Cambridge 9708 (4.2)

The circular flow of income is a fundamental model illustrating how money moves through an economy. In its simplest form, the two-sector model includes only households and firms. Households own all factors of production (land, labour, capital, enterprise) and supply these to firms. In return, firms pay households factor incomes (rent, wages, interest, profit). This represents the 'money flow'. Simultaneously, firms use these factors to produce goods and services, which are then sold to households. This is the 'real flow' of goods. Households use their income to purchase these goods and services, creating expenditure which becomes the revenue for firms. In this basic model, it is assumed that all income received by households is spent, creating a continuous, closed loop where Income = Output = Expenditure.

In a more realistic model, not all income earned by households is spent on domestically produced goods and services. Withdrawals, or leakages, are parts of income that are not passed on in the circular flow, thereby reducing the demand for domestic products. There are three key withdrawals: Savings (S), Taxation (T), and Imports (M). Savings represent income that is set aside in financial institutions rather than spent. Taxation is income paid to the government. Spending on imports involves money flowing to firms in other countries. Each of these actions diverts spending power away from the domestic firms within the circular flow, reducing the volume of the flow.

Injections are additions to the circular flow of income that do not originate from the current income of households. They represent spending that increases the demand for domestically produced goods and services. The three main injections are: Investment (I), Government Spending (G), and Exports (X). Investment is spending by firms on capital goods like machinery and buildings. Government spending includes expenditure on public services, infrastructure, and welfare payments. Exports represent spending by foreigners on the country's goods and services, bringing money into the economy from abroad. Each of these injections adds to the revenue of firms and increases the volume of the circular flow.

An economy is in a state of macroeconomic equilibrium when the rate of injections into the circular flow is equal to the rate of withdrawals from it. The condition for equilibrium is therefore: Injections = Withdrawals, or I + G + X = S + T + M. When this condition holds, the level of national income is stable. However, if injections and withdrawals are not equal, the economy is in disequilibrium. If total injections are greater than total withdrawals (J > W), national income will rise as there is a net increase in spending. Conversely, if total withdrawals are greater than total injections (W > J), national income will fall as there is a net leakage of spending from the economy.

In the simplest two-sector model:

1. Households supply factor services (labour, land, capital) to firms.
2. Firms pay factor incomes (wages, rent, interest, profit) to households.
3. Households spend consumption (C) on goods and services from firms.

Adding government introduces G and T. Adding international trade introduces X and M.

Equilibrium: J = W

Injections: J = I + G + X

Withdrawals: W = S + T + M

Multiplier: k = $\frac{1}{1 - MPC}$ = $\frac{1}{MPS + MPT + MPM}$

When a firm invests $10m, workers receive income and spend part of it (MPC). That spending becomes income for others, who also spend — the initial injection ripples through the economy.

The higher the MPC, the larger the multiplier because less leaks into saving at each round.