Determination of Income and Employment
How the short-run equilibrium level of national income and output is determined by aggregate (effective) demand in Keynes's model, with the price level and interest rate held fixed.
Prelims repeatedly tests the precise definitions and relations here — MPC, MPS, APC, APS, MPC+MPS=1, ex ante vs ex post, autonomous vs induced consumption, and the multiplier. For GS-III (Indian Economy), this is the theoretical backbone of demand management, counter-cyclical fiscal policy and why recessions/unemployment arise from deficient demand.
Understand the chapter
The Keynesian Frame and Ceteris Paribus
Macroeconomics builds models to explain what determines national income, the price level and the interest rate, and why economies face recessions, inflation or unemployment. Since all variables cannot be solved at once, others are held constant (ceteris paribus). This chapter determines National Income assuming a fixed price of final goods and a constant rate of interest, using the theory of John Maynard Keynes.
- Ceteris paribus = other things remaining equal.
- Model is demand-led and based on Keynes.
- Price of final goods and the interest rate are held fixed throughout.
- With no indirect taxes/subsidies, Y (GDP) equals National Income, used interchangeably.
Ex Ante vs Ex Post
The same terms — consumption, investment, output — carry two meanings. Ex post is the actual, accounting value realised in the economy in a year; ex ante is the planned value. Income determination works with ex ante (planned) magnitudes of aggregate demand's components.
- Ex ante = planned/intended; ex post = actual/realised.
- Example: planned investment Rs 100, but Rs 30 sold from stock makes ex post investment only Rs 70.
- The accounting identity holds for ex post values always; the equilibrium condition holds for ex ante values only at equilibrium.
The Consumption Function
Household income is the chief determinant of consumption. The consumption function C = C-bar + cY splits demand into autonomous consumption (C-bar, occurring even at zero income) and induced consumption (cY, varying with income). The slope c is the Marginal Propensity to Consume.
- Autonomous consumption (C-bar): independent of income, the subsistence/intercept level.
- Induced consumption (cY): the part that rises with income.
- MPC (c) = change in C / change in Y; lies between 0 and 1 inclusive.
- Imagenia example: C = 100 + 0.8Y gives autonomous consumption 100 and MPC 0.8.
Saving, MPS and the Average Propensities
Saving is the part of income not consumed: S = Y - C. The Marginal Propensity to Save measures the change in saving per unit change in income and equals 1 - c, so MPC and MPS always sum to 1. Average propensities express consumption or saving per unit of total income.
- MPS (s) = change in S / change in Y = 1 - c; therefore MPC + MPS = 1.
- APC = C/Y; APS = S/Y.
- Every extra rupee of income is either consumed or saved, so c + s = 1.
Investment as Autonomous Expenditure
Investment is the addition to the stock of physical capital (machines, buildings, roads) plus changes in the inventory of finished goods. Investment goods are final goods, not intermediate goods, since they yield services over many years rather than being used up. Although investment really depends on the interest rate, the model assumes it is autonomous: I = I-bar.
- Two parts: fixed capital investment + inventory (stock) investment.
- Investment goods are final goods, not used up like raw materials.
- I-bar is exogenous/autonomous — same at every income level (a horizontal line).
- Inventory investment can be positive (stock rises) or negative (stock depletes).
Two-Sector Equilibrium and Inventories
Without government, aggregate demand AD = C + I, giving the equilibrium condition Y = A + cY, where A = C-bar + I-bar is total autonomous expenditure. Ex ante supply equals ex ante demand only in equilibrium. When planned output exceeds planned demand, the gap appears as unintended (unplanned) inventory accumulation, keeping the ex post identity intact.
- A = C-bar + I-bar; C-bar is fairly stable while I-bar fluctuates periodically.
- Planned inventory investment (deliberate) vs unplanned (when sales differ from plan).
- Excess of planned output over planned demand = unintended inventory build-up.
- Standard result: equilibrium income Y = A / (1 - c); the multiplier = 1/(1 - c) = 1/MPS.
Short-Run Equilibrium: Fixed Price and the 45-degree Line
Analysis proceeds in two stages — first with the price level fixed, then allowing it to vary. Price is held fixed because, with unused resources, the law of diminishing returns does not operate, so extra output is produced at constant marginal cost. Aggregate supply is drawn as a 45-degree line and equilibrium income is where the ex ante AD function cuts it. A government can be added with autonomous T and G.
- Two reasons for fixed price: unused resources (no diminishing returns) and a simplifying assumption.
- AD function = vertical sum of consumption and investment; parallel to C, with the same slope c.
- 45-degree line: every point has equal vertical and horizontal coordinates, so whatever the GDP, that much is supplied.
- With government: disposable income Yd = Y - T; Y = C-bar + I-bar + G + c(Y - T); T and G are autonomous.
Key terms
- Ceteris paribus
- Other things remaining equal — holding all other variables constant while determining one variable.
- Ex ante
- The planned or intended value of consumption, investment or output.
- Ex post
- The actual, realised (accounting) value of a variable measured within the economy in a year.
- Autonomous consumption (C-bar)
- Consumption that takes place even when income is zero; independent of income.
- Induced consumption (cY)
- The part of consumption that depends on and varies with the level of income.
- Marginal Propensity to Consume (MPC)
- Change in consumption per unit change in income (c = change in C/change in Y); lies between 0 and 1.
- Marginal Propensity to Save (MPS)
- Change in saving per unit change in income (s = 1 - c).
- Autonomous investment (I-bar)
- Investment fixed exogenously, the same at every level of income.
- Inventory investment
- Change in the stock of unsold finished goods; can be planned or unplanned, positive or negative.
- Aggregate (effective) demand
- Total planned/ex ante expenditure on final goods = C + I (+ G in a three-sector model).
Must-know facts exam-ready
- The model is based on John Maynard Keynes and assumes a fixed price of final goods and a constant rate of interest.
- Consumption function: C = C-bar + cY (autonomous consumption + induced consumption).
- MPC (c) lies between 0 and 1 inclusive; MPC + MPS = 1.
- MPS (s) = 1 - c; APC = C/Y; APS = S/Y.
- Saving S = Y - C — the part of income not consumed.
- Investment = addition to physical capital + change in inventory; investment goods are final goods, not intermediate goods.
- Autonomous investment I = I-bar, independent of income.
- Two-sector equilibrium: Y = A + cY, where A = C-bar + I-bar is total autonomous expenditure.
- Ex ante = planned, ex post = actual; the accounting identity Y = C + I always holds ex post.
- Excess of planned output over planned demand causes unintended (unplanned) inventory accumulation.
- With government: disposable income Yd = Y - T; Y = C-bar + I-bar + G + c(Y - T); T and G are autonomous.
- Aggregate supply is shown by a 45-degree line; equilibrium is where ex ante AD meets it (multiplier = 1/(1 - c)).
Memory tricks remember it for good
Traps to avoid
- Equilibrium condition Y = A + cY holds only ex ante (at equilibrium); the identity Y = C + I holds always but only for ex post values — do not equate the two.
- MPC vs APC (and MPS vs APS): MPC/MPS are marginal (slope, change-per-unit), APC/APS are average (total per unit of income).
- Autonomous consumption (C-bar, the intercept, exists at zero income) is not the same as induced consumption (cY); total consumption is not all autonomous.
- Investment goods (machines) are FINAL goods, not intermediate goods like raw materials.
- Unplanned inventory accumulation happens when demand falls short of output — it is unintended, not a deliberate planned-inventory decision.
- MPC lies between 0 and 1 inclusive (it can equal 0 or 1), not strictly between them.
Exam focus
🧠 Prelims angles
- Definitions and relations: MPC, MPS, APC, APS and MPC + MPS = 1 (numerical/matching items).
- Ex ante vs ex post and autonomous vs induced consumption (assertion-reason or match-the-pairs).
- Components of aggregate demand (C + I + G) and which variables are autonomous (I-bar, G, T).
- Aggregate supply as the 45-degree line and the equilibrium condition AD = AS.
- Numericals: given C = C-bar + cY, compute equilibrium income or the multiplier 1/(1 - c).
- Investment defined as physical capital + inventory change, with investment goods as final goods.
✍️ Mains angles GS-III
- Keynesian demand management as a remedy for recession and unemployment.Use AD = C + I + G; show how autonomous spending (especially G) raises equilibrium income through the multiplier — link to counter-cyclical fiscal policy.
- Why does effective (aggregate) demand, not supply, drive output in the short run?Contrast with the classical full-employment view; stress unused resources, fixed prices and the central role of planned expenditure.
- Inventories as a signal of macroeconomic disequilibrium.Show that unplanned inventory accumulation or depletion reveals a demand-output mismatch and pushes output back toward equilibrium.
Last-minute revision tick as you recall
- Chapter = Keynesian short-run determination of income; price and interest held fixed.
- Ex ante = planned; ex post = actual.
- C = C-bar + cY: autonomous + induced; MPC = c, between 0 and 1.
- MPS = 1 - c; MPC + MPS = 1; APC = C/Y, APS = S/Y.
- Investment autonomous (I = I-bar) = physical capital + inventory; investment goods are final goods.
- Two-sector equilibrium: Y = A + cY, with A = C-bar + I-bar.
- Output exceeding demand causes unintended inventory accumulation.
- Government: Yd = Y - T; Y = C-bar + I-bar + G + c(Y - T); T and G autonomous.
- Aggregate supply = 45-degree line; equilibrium where AD meets it; multiplier = 1/(1 - c).
Distilled from NCERT Class 12 · Introductory Macroeconomics for UPSC. Always cross-check facts with the original NCERT.