National Income & the Circular Flow
National Income & the Circular Flow
The circular flow of income model illustrates how money flows between households and firms, and how injections and withdrawals determine the level of national income.
The Basic Circular Flow (Two-Sector Model)
In the simplest economy with only households and firms:
- Households supply factors of production (labour, land, capital, enterprise) to firms
- Firms pay factor incomes (wages, rent, interest, profit) to households
- Households spend their income on goods and services produced by firms
- Firms receive revenue from household spending
This creates a continuous loop. In this simple model, all income is spent and national income is constant.
Injections and Withdrawals
The real economy has three additional sectors: government, financial, and international.
Withdrawals (Leakages) — W
Money that leaves the circular flow and is not passed on as spending:
| Withdrawal | Description |
|---|---|
| Savings (S) | Income not spent, deposited in financial institutions |
| Taxation (T) | Income taken by government (income tax, VAT, corporation tax) |
| Imports (M) | Spending on foreign-produced goods and services |
Injections — J
Money that enters the circular flow from outside:
| Injection | Description |
|---|---|
| Investment (I) | Spending by firms on capital goods (funded by savings channelled through financial institutions) |
| Government spending (G) | Public expenditure on goods, services, and infrastructure |
| Exports (X) | Foreign spending on domestically produced goods and services |
Equilibrium in the Circular Flow
National income is in equilibrium when total injections equal total withdrawals:
I + G + X = S + T + M
Or equivalently: J = W
Disequilibrium
If J > W: more money enters than leaves → national income rises (expansion)
- Firms experience rising demand → increase output → hire more workers → incomes rise → further spending (multiplier effect)
If J < W: more money leaves than enters → national income falls (contraction)
- Firms experience falling demand → reduce output → lay off workers → incomes fall → further decline (negative multiplier)
The economy adjusts until a new equilibrium is reached where J = W at a different level of national income.
The Multiplier Process
The multiplier explains how an initial injection leads to a larger final change in national income, because each round of spending generates further income and spending.
How the Multiplier Works
Suppose the government injects £100m in new spending:
| Round | New Spending | Withdrawal (MPW = 0.4) |
|---|---|---|
| 1 | £100m | £40m |
| 2 | £60m | £24m |
| 3 | £36m | £14.4m |
| 4 | £21.6m | £8.64m |
| ... | ... | ... |
| Total | £250m | £100m |
Multiplier = 1 / MPW = 1 / 0.4 = 2.5
Final change in national income = £100m × 2.5 = £250m
The Marginal Propensity to Withdraw (MPW)
MPW = MPS + MPT + MPM
Where:
- MPS = marginal propensity to save
- MPT = marginal propensity to tax (effective rate)
- MPM = marginal propensity to import
The larger the MPW, the smaller the multiplier (more leaks out at each round).
The Marginal Propensity to Consume (MPC)
MPC + MPW = 1
Therefore: Multiplier = 1 / (1 - MPC)
The UK has a relatively high MPW due to high taxes, a high import propensity (open economy), and moderate savings — so the multiplier is estimated at 1.0 to 1.5 (much lower than theoretical examples suggest).
Keynesian vs Classical Views
Keynesian View
Keynes (1936, General Theory) argued:
- The economy can settle at an equilibrium below full employment (deflationary gap)
- Savings do not automatically translate into investment (the "paradox of thrift": if everyone saves more, aggregate demand falls, incomes fall, and total savings may actually decrease)
- Government should use fiscal policy to manage AD and close the output gap
- The multiplier justifies government spending during recessions — the final increase in income exceeds the initial spending
- Animal spirits drive investment — business confidence is volatile and not simply determined by interest rates
Classical View
Classical and monetarist economists argue:
- Say's Law: "supply creates its own demand" — producing goods generates enough income to buy them
- Flexible wages and prices ensure markets clear, including the labour market (no involuntary unemployment in the long run)
- Savings are channelled into investment through the interest rate mechanism
- Government intervention is unnecessary and may cause crowding out (government borrowing raises interest rates, reducing private investment)
- The multiplier is much smaller than Keynesians suggest, especially near full employment
National Income Accounting Identities
GDP by Expenditure
GDP = C + I + G + (X - M)
Relationship Between Key Aggregates
| Aggregate | Formula |
|---|---|
| GDP | Output within borders |
| GNI (Gross National Income) | GDP + net property income from abroad |
| NNI (Net National Income) | GNI - capital depreciation |
| Disposable income | Income after taxes and transfers |
The AD/Circular Flow Connection
The circular flow is the foundation of the AD model:
- AD = C + I + G + (X - M) comes directly from the expenditure approach
- Changes in injections/withdrawals shift the AD curve
- The multiplier determines the magnitude of the shift
Evaluation
Strengths of the Circular Flow Model
- Provides a clear, intuitive framework for understanding macroeconomic relationships
- Shows the interconnection between sectors
- Illustrates the multiplier mechanism
- Underpins national income accounting (GDP measurement)
Limitations
- Simplification: real economies are far more complex (multiple sectors, international linkages, financial markets)
- Assumes a closed loop: in reality, financial flows are complex and not all savings become investment
- Does not show time lags in the multiplier process
- Does not distinguish between types of spending (consumption of imports vs domestic goods)
- The multiplier value is uncertain and debated
- Does not capture supply-side factors (productivity, technology)
- Assumes stable relationships between income and spending (MPC), which may change during crises
The Paradox of Thrift (Keynes)
- If all households increase saving simultaneously, aggregate demand falls
- Firms reduce output, incomes fall, and the total level of saving may decrease
- What is rational for an individual (saving more) is irrational for the economy as a whole — a fallacy of composition
- This justifies government intervention to maintain spending during recessions
Exam Technique
- Draw the circular flow diagram with all five sectors (households, firms, government, financial, international)
- Show injections entering and withdrawals leaving the flow
- Use numerical examples to demonstrate the multiplier process
- In essays, contrast Keynesian and classical views on whether the economy self-corrects
- Evaluate the multiplier: is it large or small? What determines its size? Does crowding out limit its effectiveness?
- Apply to UK context: the UK's high import propensity and tax rates reduce the multiplier's power