Consumption and Saving Functions Explained: MPC, MPS, APC, APS

The consumption function is the heart of Keynesian economics. It describes the single most predictable relationship in macroeconomics: as income rises, consumption rises too — but not by as much. Understanding this relationship, and its mirror image in saving, unlocks the entire Keynesian framework.

Marginal Propensity to Consume (MPC)

MPC = ΔC ÷ ΔY

MPC measures how much of each additional rupee of income is spent on consumption.

Example: Income rises by ₹100. Consumption rises by ₹80.
MPC = 80 ÷ 100 = 0.8

Properties of MPC

• Always between 0 and 1 → 0 < MPC < 1
• A person with MPC = 1 spends every extra rupee; MPC = 0 saves every extra rupee
• Higher-income groups tend to have lower MPC — the rich save a larger fraction of additional income than lower-income households

Average Propensity to Consume (APC)

APC = C ÷ Y

APC measures total consumption as a proportion of total income — not just what happens at the margin, but the overall picture.

Properties of APC

• Can exceed 1 at very low incomes — if someone earns ₹1,000 but spends ₹1,200 (by borrowing), APC = 1.2
• Falls as income rises — higher earners spend a smaller proportion of their income
• APC > MPC at all positive income levels — because autonomous consumption (a) always pulls the average up above the marginal rate

APC vs MPC: Quick Comparison

Deriving the Saving Function

Since all income is either consumed or saved:

Y = C + S
Therefore: S = Y − C

Substituting C = a + bY:

S = Y − (a + bY)
S = −a + (1 − b)Y

This gives us the saving function, where:

• −a is autonomous dissaving (at zero income, saving is negative because people are borrowing to consume)
• (1 − b) is the Marginal Propensity to Save (MPS)

Marginal Propensity to Save (MPS)

MPS = ΔS ÷ ΔY = 1 − MPC

MPS measures how much of each additional rupee of income is saved.

Example: MPC = 0.75 → MPS = 1 − 0.75 = 0.25
For every ₹100 of extra income, ₹75 is consumed and ₹25 is saved.

The Key Identity

MPC + MPS = 1

This must always hold — additional income can only be consumed or saved. There is no third option.

Average Propensity to Save (APS)

APS = S ÷ Y = 1 − APC

APS measures total saving as a proportion of total income.

The Key Identity

APC + APS = 1

Again, total income is entirely accounted for by consumption and saving.

All Four Measures: Summary Table

Always true:
MPC + MPS = 1
APC + APS = 1

Worked Numerical Example

Given:
Autonomous consumption = ₹200
MPC = 0.8
Income level = ₹1,000

Step 1: Calculate consumption
C = 200 + 0.8 × 1,000 = 200 + 800 = ₹1,000

Step 2: Calculate saving
S = Y − C = 1,000 − 1,000 = ₹0

Step 3: Calculate APC
APC = C ÷ Y = 1,000 ÷ 1,000 = 1.0

Step 4: Calculate APS
APS = 1 − APC = 1 − 1.0 = 0

Now let income rise to ₹1,500:

C = 200 + 0.8 × 1,500 = ₹1,400
S = 1,500 − 1,400 = ₹100
APC = 1,400 ÷ 1,500 = 0.93 (fallen from 1.0)
APS = 100 ÷ 1,500 = 0.067

The Paradox of Thrift: A Warning About Saving

Saving is virtuous for an individual — but if everyone tries to save more simultaneously, a paradox emerges:

1. Households increase saving → Consumption falls
2. Falling consumption → Lower aggregate demand
3. Lower AD → Lower national income
4. Lower income → Less total saving overall

The individual's rational behavior produces a collective outcome no one wanted. This is the Paradox of Thrift — one of the most counterintuitive and important insights in Keynesian economics.

Policy implication: During a recession, urging everyone to save is the wrong advice. It deepens the downturn. Government spending (dissaving by the public sector) can counteract the contractionary effect.

Exam Tips

For short-answer questions:

• Always state the formula before solving
• Verify your answer by checking MPC + MPS = 1

For derivation questions:

• Show the full algebraic steps when deriving S = −a + (1−b)Y from C = a + bY

Common MCQ traps:

• APC can exceed 1 — MPC cannot
• MPS = 1 − MPC (not MPC = 1 − MPS... same thing, but write it right)
• At break-even income, APC = 1 and APS = 0

Continue reading: The Investment Multiplier: How ₹200 Crore of Investment Creates ₹800 Crore of Income

Topics covered: Consumption function, Saving function, MPC, MPS, APC, APS, Autonomous consumption, Paradox of thrift, Keynesian identities | CBSE Class 12 Economics, CUET Preparation