The circular flow of income, injections and withdrawals, and the determination of national income equilibrium.

What this dot point is asking

WJEC wants you to model how income circulates between households and firms, to identify injections and withdrawals, and to explain how national income reaches equilibrium and how the multiplier magnifies a change in injections.

The answer

The circular flow of income

In the simplest two-sector model, all household income is spent on firms' output and all firm revenue is paid back to households, so the flow continues unchanged. The real economy is more complex because money leaks out of and is added to this loop. The model is the foundation of macroeconomics because it shows that one agent's spending is another's income, so a change anywhere ripples around the whole economy.

Injections and withdrawals

When injections exceed withdrawals, more money is being added to the flow than is leaking out, so national income rises until the larger income generates enough extra withdrawals to restore balance. When withdrawals exceed injections, income falls. This is why a rise in exports or government spending tends to raise national income, while a rise in saving or taxation tends to lower it. The condition $I + G + X = S + T + M$ is the macroeconomic equilibrium of the circular flow.

The multiplier

The multiplier is the ratio of the final change in national income ($\Delta Y$) to the initial change in injections ($\Delta J$). An initial injection becomes income for some households, who spend a fraction of it (the marginal propensity to consume, MPC), which becomes income for others, and so on. Because each round adds further spending, the total rise in income exceeds the initial injection. The size of the multiplier depends inversely on the marginal propensity to withdraw (MPW, the fraction of extra income saved, taxed or spent on imports): the more that leaks out at each round, the smaller the multiplier. A fall in injections produces a negative multiplier, deepening a downturn.

Examples in context

Example 1. Fiscal stimulus and the UK multiplier. When the UK government raised spending sharply during the financial crisis and the pandemic, the multiplier was central to the debate: each pound of spending was expected to raise national income by more than a pound as it was re-spent. Estimates of the multiplier vary, and are larger in a recession when spare capacity exists and households spend rather than save extra income, which is why the timing and target of stimulus matter.

Example 2. Imports as a leakage in an open economy. The UK has a high marginal propensity to import, so a substantial share of any extra spending leaks abroad rather than circulating domestically. This weakens the multiplier compared with a more closed economy: a stimulus that boosts demand for imported goods raises foreign income as well as domestic income, illustrating why the openness of an economy reduces the domestic impact of an injection.

Try this

Q1. List the three injections and three withdrawals in the circular flow. [3 marks]

• Cue. Injections: investment (I), government spending (G), exports (X). Withdrawals: saving (S), taxation (T), imports (M).

Q2. If the marginal propensity to withdraw is 0.25, calculate the multiplier. [2 marks]

• Cue. Multiplier equals 1 divided by the marginal propensity to withdraw, so $1 / 0.25 = 4$.