Determination of Income & Employment

Chapters 1-3 introduced national income, money and inflation. Chapter 4 ties them together and answers the big question of Keynesian macroeconomics: what determines the level of output, income and employment in an economy with idle resources?

Before Keynes, classical economists believed the economy always returned to full employment automatically through wage-price flexibility. The Great Depression of the 1930s — with 25% US unemployment lasting a decade — showed that economies could get stuck below full employment. Keynes's General Theory of Employment, Interest and Money (1936) explained why, and how government action could break the deadlock.

The Keynesian framework — short-run assumptions

To focus on demand-side determination of output, the model makes several simplifying assumptions:

• Short run — capital stock, technology and labour force are fixed.
• Fixed price level — firms have idle capacity, so they meet extra demand by raising output, not prices.
• Two-sector economy initially — households and firms only (no government, no foreign trade).
• Demand-determined output — whatever buyers want at the prevailing price, firms will produce.
• Saving and investment are functions of income and interest rate respectively (but treated as autonomous here).

Aggregate Demand (AD)

Aggregate Demand is the total ex-ante (planned) expenditure on final goods and services in the economy during a year. In the two-sector model:

In the four-sector model used in later chapters, AD = C + I + G + (X − M), where G is government expenditure and (X − M) is net exports.

Aggregate Supply (AS) is the total value of final goods and services that firms plan to produce — which equals total planned income (Y). In equilibrium AD = AS = Y.

The consumption function

Keynes's "fundamental psychological law": as income rises, consumption rises but by less than the increase in income. Mathematically:

Marginal Propensity to Consume (MPC) = ΔC/ΔY — the fraction of an additional rupee of income that is consumed.

Average Propensity to Consume (APC) = C/Y — the fraction of total income consumed at a given level of income. APC falls as income rises (Engel's curve intuition).

The saving function

Since Y = C + S (income is either consumed or saved), the saving function is the mirror image of the consumption function.

Marginal Propensity to Save (MPS) = ΔS/ΔY = (1 − MPC). Therefore MPC + MPS = 1.

Average Propensity to Save (APS) = S/Y = 1 − APC.

Investment

For this chapter investment is treated as autonomous (Ī) — independent of income. In reality investment depends on the rate of interest and expected returns, but holding these constant gives a clean equilibrium model.

Combined, aggregate demand becomes:

This is a straight line on the AD-Y diagram with intercept (C̄ + Ī) and slope c (the MPC).

Equilibrium output — the income-expenditure approach

Equilibrium occurs where AD = Y, i.e. where planned spending equals planned output. On the diagram this is the intersection of the AD line and the 45° line (along which AD = Y).

If AD > AS: firms find inventories falling unexpectedly → they raise output → Y rises until equilibrium.
If AD < AS: inventories pile up → firms cut output → Y falls until equilibrium.

Equivalent approach — Saving = Investment

The same equilibrium can be derived using saving and investment. In a two-sector economy, the leakage (saving) must equal the injection (investment) for equilibrium:

This is identical to the income-expenditure result — a useful check, and the basis for the S-I diagram (saving rises with Y; investment is horizontal; intersection gives equilibrium Y).

The investment multiplier

The most powerful idea in Chapter 4. A small increase in autonomous investment (or autonomous consumption) raises equilibrium income by a multiple of the initial change.

Intuition. Suppose investment rises by ₹100. That ₹100 becomes someone's income. With MPC = 0.8, they spend ₹80, which becomes someone else's income. That person spends ₹64; the next ₹51.2; and so on. The sum of the infinite series is:

Inflationary & deflationary gaps

The equilibrium output Y* found above need not equal the full-employment level of output (YF). Two situations arise:

Deflationary gap (recessionary gap)

AD falls short of what is needed to achieve full employment. Y* < YF. There is unemployment of labour and idle capacity. Government should stimulate AD — through higher G, lower T, or expansionary monetary policy.

Inflationary gap

AD exceeds full-employment output. Y* would lie above YF, but since output cannot rise beyond YF, the excess demand pulls up prices. Government should contract AD — through lower G, higher T, or tight money.

Policy implications

Chapter 4's model underpins modern fiscal policy:

• Counter-cyclical spending. In a downturn, government borrows and spends; in a boom, it consolidates. India's MGNREGA, capital expenditure boost in Union Budgets, and the Atmanirbhar Bharat packages of 2020-21 are real-world applications.
• Targeted transfers. Direct Benefit Transfers (DBT), PM-KISAN, free foodgrains under PMGKAY all leverage high MPC at the bottom of the income distribution.
• Public investment crowd-in. Infrastructure (highways, railways, power) creates demand today and supply tomorrow — the Keynesian multiplier is highest for capex, not revex.
• Limits of the model. Open economy leakages (imports), price flexibility, supply constraints and Ricardian equivalence dampen the multiplier. RBI estimates India's fiscal multiplier on capex at ~2.45 vs only ~0.45 for revex.