Effective demand principle

Topic: Aggregate Demand, Income Determination and the Multiplier · NCERT: Class 12, Ch 4 "Determination of Income and Employment"

Meaning

The effective demand principle says that when the price level is fixed and supply is perfectly elastic, total output (national income) is set only by aggregate demand, meaning total planned spending. Output goes to the level where planned spending equals planned output: Y* = (C̄ + Ī)/(1 − c), or Y* = Ā/(1 − c).

It matters because it overturned the classical view. Output is limited by demand, not supply. So an economy can get stuck below full employment, and the government may need to raise demand [4].

Explanation

How it works: demand leads, output follows

• Keynes's starting idea: national income (Y), output (O) and expenditure or effective demand (D) are equal, so Y = O = D [2].
• Aggregate demand (AD): total planned spending. In a two-sector model (households and firms only), AD = C + I.
• Consumption function: C = C̄ + cY.
  • C̄ is autonomous consumption. People spend this amount even when their income is zero.
  • c is the marginal propensity to consume (MPC). It is the share of each extra rupee of income that people spend.
• Investment: I = Ī. This is autonomous investment. It is fixed and does not depend on income.

• So AD = C̄ + Ī + cY. This is a line that starts at (C̄ + Ī) on the vertical axis. Its slope is c, which is less than 1.

• Aggregate supply (AS): the total output firms plan to produce. On the graph it is the 45° line. Every point on this line has equal values on both axes, so output supplied = income.

• Equilibrium: output settles where ex ante (planned) AD = ex ante AS. On the graph, this is where the AD line cuts the 45° line.
• The AD line is flatter than the 45° line, so the two lines cross only once.
• At that point, planned spending buys exactly what firms planned to produce.

Why supply simply follows demand: the fixed-price condition

• Chain of reasoning:
• The economy has unused resources, such as idle machines, empty buildings and workers without jobs.
• So the law of diminishing returns does not apply. (This law says that each extra worker added to a fixed factory adds less output than the one before.)
• So firms can produce more without any rise in marginal cost (the cost of making one more unit).

• Costs do not rise, so prices do not rise. Supply is perfectly elastic: output can go up or down without any change in price.

• Sticky prices: the IMF lists sticky prices as a core Keynesian idea. Prices, and wages even more, change only slowly [4].

• So in the short run, a change in AD mainly changes real output and employment, not prices [4].

Worked example

• Take C = 40 + 0.8Y and I = 10.
• AD = 40 + 0.8Y + 10 = 50 + 0.8Y.
• Set AD = Y: Y = 50 + 0.8Y, so 0.2Y = 50.

• Y* = 50/0.2 = 250.

• Check with saving:

• C = 40 + 0.8 × 250 = 240.
• Saving S = Y − C = 250 − 240 = 10, which equals I = 10.

• At equilibrium, planned saving = planned investment.

• The same result comes from the saving function S = −C̄ + (1 − c)Y = −40 + 0.2Y. At Y = 250, S = 10.

• 1/(1 − c) is the investment multiplier. In this example it is 1/0.2 = 5.

How output moves to equilibrium: unplanned inventory

• Inventory: stocks of unsold and unfinished goods that firms hold.
• If AD < Y (excess supply):
• Goods stay unsold, so unplanned inventory piles up.
• Firms cut output.

• Y falls towards Y*.

• If AD > Y (excess demand):

• Firms sell from their stocks, so inventories fall below the planned level.
• Firms raise output.

• Y rises towards Y*.

• NCERT exercise: Ā = ₹50 crore, MPS = 0.2 (so c = 0.8), Y = ₹4,000 crore.

• MPS (marginal propensity to save) is the share of each extra rupee that is saved. MPS = 1 − MPC.
• AD = 50 + 0.8 × 4,000 = ₹3,250 crore. This is less than Y.
• So there is excess supply of ₹750 crore. Stocks pile up and output falls.

• Equilibrium income = 50/0.2 = ₹250 crore.

• Key point: if AD is below the economy's productive capacity (what it could produce), the result is unemployment and depression [2]. The economy can rest at an underemployment equilibrium: a stable income level at which some workers still have no jobs.

In India

• Theory concept with an Indian example: India has no law or official target linked to this principle. But the RBI tracks the signals the principle predicts.
• RBI's OBICUS survey: the Order Books, Inventories and Capacity Utilisation Survey covers the manufacturing sector. It gives a quick picture of demand conditions [5].
• Q1:2020-21 (April–June 2020, COVID-19 lockdown), 50th round, 462 companies [5]:
• Capacity utilisation (CU), the share of factory capacity actually used, fell to 47.3%, down from 69.9% in Q4:2019-20 [5].
• Seasonally adjusted CU fell to 48.2%, down from 68.2% [5].

• The finished-goods inventory to sales ratio rose to 23.6%, up from 15.1% [5].

• What this shows:

• Sales fell faster than stocks. This is the textbook AD < Y → unplanned inventory piles up case.
• Low CU means idle resources. This is the same condition behind the fixed-price, perfectly elastic supply assumption.
• So output was limited by demand, not by capacity. That is effective demand at work.

Don't confuse with

• Say's law: "supply creates its own demand" [3]. Supply leads and the economy settles at full employment. The effective demand principle reverses this: demand leads and output follows, and the economy may settle below full employment.
• Ex post equality of S and I: actual saving always equals actual investment, because unplanned inventory change counts as investment. Equilibrium under effective demand needs planned (ex ante) AD = planned AS, and this happens only at Y*.
• Full employment equilibrium: in the classical view, the economy reaches full employment by itself if wages are flexible [4]. Effective demand equilibrium can be an underemployment equilibrium.
• Price adjustment: in this model, excess supply is cleared by cutting output, not by lowering prices, because prices are fixed.

Prelims Hooks

• Keynes set out the effective demand principle in The General Theory of Employment, Interest and Money (1936), written in response to the Great Depression [4].
• Y = O = D: Keynes treated national income, output and effective demand as equal [2].
• Y* = (C̄ + Ī)/(1 − c). With C = 40 + 0.8Y and I = 10, Y* = 250, and planned S = I = 10.
• Trap: the principle holds only under fixed prices and perfectly elastic AS, which needs unused resources. The 45° line represents aggregate supply.
• Trap: when AD < Y, the result is unplanned inventory accumulation → output falls. Prices do not fall in this model.
• Say's law = "supply creates its own demand" [3]. The effective demand principle = output is determined by AD alone.

Mains Points

• Demand-led slowdowns and fiscal policy: in Q1:2020-21, CU was 47.3% and the finished-goods inventory to sales ratio was 23.6% [5]. Low capacity use plus piled-up stocks means output was limited by demand. This supports government spending to lift demand, as the effective demand principle suggests [4]. It also frames the debate between counter-cyclical fiscal policy (spending more in a slowdown) and strict fiscal consolidation (cutting the deficit) during a downturn.
• Limits of the principle: it holds only when there is slack (idle capacity and idle workers).
• Near full capacity, extra demand raises prices, not output.
• This is why the RBI watches capacity utilisation as an early sign of inflation pressure.

• Policy should fit the stage of the business cycle.

• Keynes vs classical economics: Say's law says markets correct themselves. Keynes said that low demand can cause long-lasting unemployment [2][4]. Demand comes from both private and public decisions, so the state has a role when private demand falls short [4]. Inventory surveys like OBICUS help spot demand shortfalls early, before GDP data comes out [5].

Related concepts

• Fixed price assumption
• Aggregate supply
• Equilibrium income

Read more

• Equilibrium income: the 45° line and the effective demand principle