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Aggregate Demand, Aggregate Supply & Propensity to Consume and Save

Master Aggregate Demand Supply and Propensity for CBSE Class 12 Economics. Covers AD=C+I autonomous vs induced expenditure, AS=Y=C+S, consumption function C=a+bY, APC and MPC definitions and properties, APS and MPS, all four key identities MPC+MPS=1 APC+APS=1, deriving saving function from consumption function, breakeven income, factors affecting propensity to consume, verified numerical schedule with board-level questions.

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What determines how much a nation produces in a year? Why does output sometimes fall below potential even when resources are available? The answers lie in Aggregate Demand — the total spending in the economy — and its relationship with Aggregate Supply — the total income generated. When these two are in balance, the economy is in equilibrium. But before we can find that equilibrium, we must understand exactly how households decide how much to consume and how much to save out of their income — the subject of this note. The concepts of APC, APS, MPC, and MPS are the building blocks of all income-employment theory and the most numerically tested ideas in the entire macroeconomics syllabus. For CBSE Class 12, this sub-unit carries a significant share of the 12 marks allocated to Determination of Income and Employment — expect 4–6 marks of numericals and short answers in every board paper.

1. Aggregate Demand (AD)

Aggregate Demand (AD) is the total expenditure that all sectors of the economy — households, firms, government and the foreign sector — are willing and able to make on final goods and services at a given level of income during a given period of time.

In a two-sector economy (households + firms, no government, no foreign trade):

$AD = C + I$

Where C = Private consumption expenditure and I = Investment expenditure (assumed autonomous — independent of income).

Components of AD

Component	What it is	Depends on?
Consumption (C)	Household spending on goods and services	Income (Y) — rises as income rises; C = a + bY
Investment (I)	Firms' spending on capital goods	Autonomous — fixed by interest rates, business expectations, NOT by current income

Autonomous vs Induced Expenditure

Type	Definition	Example
Autonomous Expenditure	Expenditure that does NOT depend on current level of income — remains constant when income changes	Investment (I); Autonomous consumption (a) — even at zero income, households spend on necessities
Induced Expenditure	Expenditure that CHANGES with income — rises when income rises, falls when income falls	Induced consumption (bY) — as income rises, households spend more on consumption

2. Aggregate Supply (AS)

Aggregate Supply (AS) is the total value of goods and services produced — equivalently, the total income generated in the economy at a given level of employment.

In the Keynesian framework for a closed two-sector economy:

$AS = Y = C + S$

Where Y = National Income, C = Consumption, S = Saving.

AS is a 45° line through the origin — every unit of income produced (output/AS) equals a unit of income received. AS always equals Y by definition.

3. Consumption Function

The Consumption Function (introduced by J.M. Keynes) describes the relationship between income and consumption:

$C = a + b Y$

Where:

a = Autonomous consumption — consumption at zero income (positive: households borrow or dissave to survive)
b = MPC (Marginal Propensity to Consume) — the slope of the consumption function
Y = National Income
0 < b < 1 (each additional rupee of income is partly consumed, partly saved)

Numerical Schedule: C = 50 + 0.8Y

Y (₹)	C = 50+0.8Y	S = Y−C	APC = C/Y	APS = S/Y	MPC	MPS
0	50	−50	—	—	—	—
100	130	−30	1.30	−0.30	0.8	0.2
200	210	−10	1.05	−0.05	0.8	0.2
250	250	0	1.00	0.00	0.8	0.2
300	290	10	0.97	0.03	0.8	0.2
400	370	30	0.93	0.07	0.8	0.2

The highlighted row (Y=250) is the breakeven point — where C = Y, S = 0, APC = 1.

4. Propensity to Consume — APC and MPC

Average Propensity to Consume (APC)

$APC = \frac{C}{Y}$

APC is the fraction of total income spent on consumption. Key properties:

APC falls as income rises — as income grows, households save a larger proportion.
APC > 1 at low income levels — when income is very low, households spend more than their income (dissaving — borrow or draw down savings).
APC = 1 at breakeven — where C = Y (entire income consumed, saving = 0).
APC < 1 at higher incomes — households save a positive amount.
APC is never zero (consumption never falls to zero).

Marginal Propensity to Consume (MPC)

$MPC = \frac{Δ C}{Δ Y}$

MPC is the change in consumption per unit change in income — the slope of the consumption function.

0 < MPC < 1 always — each additional rupee of income leads to less than one rupee of additional consumption.
MPC is constant in a linear consumption function (same slope everywhere).
MPC is the slope of the consumption curve.
For rich countries: MPC tends to be lower. For poor countries: MPC tends to be higher (necessities take priority).

5. Saving Function and Propensity to Save

Since S = Y − C and C = a + bY:

$S = Y - C = Y - (a + b Y) = - a + (1 - b) Y$

The Saving Function is: $S = - a + MPS \cdot Y$

Where autonomous saving = −a (negative — at zero income, household dissaves by amount a) and MPS = 1 − MPC = slope of the saving function.

Average Propensity to Save (APS)

$APS = \frac{S}{Y}$

APS rises as income rises — higher income → larger fraction saved.
APS is negative at low incomes (dissaving — households spend more than income).
APS = 0 at breakeven income (S = 0).

Marginal Propensity to Save (MPS)

$MPS = \frac{Δ S}{Δ Y} = 1 - MPC$

MPS = the slope of the saving function = the fraction of each additional rupee of income that is saved.

The Four Key Relationships — Always True

1. $MPC + MPS = 1$
2. $APC + APS = 1$
3. $MPC = 1 - MPS$ and $MPS = 1 - MPC$
4. $APC = 1 - APS$ and $APS = 1 - APC$

Deriving C from S and S from C

Given	Derive	Method
C = a + bY	S = −a + (1−b)Y	S = Y − C: intercept changes sign; MPS = 1−MPC
S = −a + mY	C = a + (1−m)Y	C = Y − S: intercept changes sign; MPC = 1−MPS
S = −80 + 0.3Y	C = 80 + 0.7Y	Intercept: −(−80) = 80; MPC = 1−0.3 = 0.7

6. Factors Affecting Propensity to Consume

Keynes identified several factors that determine the psychological propensity to consume:

Factor	Effect on MPC
Income level	Lower income households have higher MPC (spend more of each rupee on necessities)
Distribution of income	More equal distribution → higher aggregate MPC (poor spend more than rich per rupee)
Fiscal policy	Tax cuts increase disposable income → consumption rises
Rate of interest	Higher interest rate encourages saving → lower MPC
Future expectations	Optimistic expectations → higher consumption; pessimistic → lower

APC, APS, MPC, MPS — Quick Calculation Reference

Given information	Find	Formula / Step
C and Y values	APC	APC = C ÷ Y
Two Y, C pairs	MPC	MPC = ΔC ÷ ΔY
MPC known	MPS	MPS = 1 − MPC
APC known	APS	APS = 1 − APC
Saving function S = −a + mY	Consumption function	C = a + (1−m)Y; change sign of intercept, MPC = 1−m
Consumption function C = a + bY	Breakeven income	Set C = Y: a + bY = Y → Y* = a ÷ (1−b)

Board Exam Power Tips — Avoid These Common Mistakes

MPC + MPS = 1 always; APC + APS = 1 always. These are the two most-tested identities. If MPC = 0.75, then MPS = 0.25. If APC = 1.3, then APS = −0.3. Getting the complement wrong loses easy marks.
APC can be greater than 1, equal to 1, or less than 1 — but MPC is always between 0 and 1. APC > 1 means dissaving (C > Y). MPC > 1 is impossible (you cannot consume more than an additional rupee of income). Students sometimes confuse these.
MPC is constant in a linear consumption function; APC changes with income. In C = a + bY, b (= MPC) is the constant slope. APC = C/Y = a/Y + b — it changes as Y changes. This is why MPC ≠ APC (except at the breakeven point where S = 0).
Autonomous consumption (a) is always positive. Even at zero income, people need food, clothing, shelter — they borrow or dissave. 'a' is always positive in the consumption function.
To derive saving function from consumption function: change sign of intercept; MPS = 1 − MPC. C = 80 + 0.7Y → S = −80 + 0.3Y. Students often forget to change the sign of the intercept or use the wrong MPS.
Breakeven income: set C = Y, solve for Y. C = a + bY = Y → a = (1−b)Y → Y* = a/(1−b). At this income, S = 0 and APC = 1. Below this income, dissaving; above, positive saving.

Practice Questions (CBSE Board Level)

Q1 (1 mark): If MPC = 3/4, find MPS. Also, state the relationship between MPC and MPS.

Explanation:

MPS = 1 − MPC = 1 − 3/4 = 1/4 = 0.25

Relationship: $MPC + MPS = 1$ (always). The sum of Marginal Propensity to Consume and Marginal Propensity to Save is always equal to exactly 1. This is because any additional unit of income ( $Δ Y$ ) can only be either consumed ( $Δ C$ ) or saved ( $Δ S$ )—there is no other economic use for it. Therefore, $Δ C + Δ S = Δ Y$ , which divided by $Δ Y$ yields $MPC + MPS = 1$ .

Q2 (4 marks): The consumption function is $C = 40 + 0.75 Y$ . When income ( $Y$ ) is ₹400, calculate: (i) Consumption, (ii) Saving, (iii) APC, (iv) APS. Verify that APC + APS = 1.

Explanation:

(i) Consumption: $C = 40 + 0.75 (400) = 40 + 300 = ₹ 340$

(ii) Saving: $S = Y - C = 400 - 340 = ₹ 60$

(iii) APC (Average Propensity to Consume): $A P C = \frac{C}{Y} = \frac{340}{400} = 0.85$

(iv) APS (Average Propensity to Save): $A P S = \frac{S}{Y} = \frac{60}{400} = 0.15$

Verification: $APC + APS = 0.85 + 0.15 = 1.00$ ✓

Q3 (2 marks): The saving function is $S = - 80 + 0.3 Y$ . Derive the consumption function. Find the breakeven level of income.

Explanation:

Deriving the Consumption Function:
Since $Y = C + S$ , we know $C = Y - S$ .
$C = Y - (- 80 + 0.3 Y)$
$C = Y + 80 - 0.3 Y$
$C = 80 + 0.7 Y$
(Alternatively: Autonomous consumption $a$ is the negative of autonomous savings $- (- 80) = 80$ , and $MPC = 1 - MPS = 1 - 0.3 = 0.7$ ).

Breakeven income:
The breakeven point occurs when entire income is consumed ( $C = Y$ ), meaning Savings ( $S$ ) = 0.
Setting $S = 0$ :
$- 80 + 0.3 Y = 0$
$0.3 Y = 80$
$Y = \frac{80}{0.3} = ₹ 266.67$

Q4 (MCQ — 1 mark): At the breakeven level of income:

A) APC = 0
B) APC > 1
C) APC = 1
D) MPC = 1

Answer: C) APC = 1

Explanation: At the breakeven point, total consumption equals total income ( $C = Y$ ), meaning saving is exactly zero ( $S = 0$ ). Therefore, the Average Propensity to Consume ( $APC = C / Y$ ) evaluates to $Y / Y = 1$ . The APS is 0 at this point. Note that MPC (the marginal slope) remains constant at its fractional value (e.g., 0.75) and does not change to 1.

Q5 (3 marks): Explain mathematically why APC falls as income increases, while MPC remains constant (assuming a linear consumption function).

Explanation:

For a standard linear consumption function: $C = a + b Y$ (where ' $a$ ' is autonomous consumption and ' $b$ ' is MPC).

Why APC falls:
By definition, $A P C = \frac{C}{Y}$ . Substituting the function:
$A P C = \frac{a + b Y}{Y} = \frac{a}{Y} + b$
As income ( $Y$ ) continuously rises, the fraction $\frac{a}{Y}$ becomes progressively smaller (since a constant is being divided by a growing denominator). Therefore, the overall value of $(\frac{a}{Y} + b)$ decreases. Intuitively, as individuals get richer, the fixed survival consumption ( $a$ ) accounts for a smaller and smaller percentage of their total income.

Why MPC is constant:
By definition, $M P C = \frac{Δ C}{Δ Y}$ . In the linear equation $C = a + b Y$ , ' $b$ ' is the slope of the line. The slope of a straight line is constant everywhere. Therefore, the proportion of additional income spent on consumption remains exactly the same at all income levels.

Q6 (MCQ — 1 mark): APS can be negative. This means:

A) Saving is less than income
B) Consumption exceeds income (dissaving)
C) MPS is negative
D) MPC exceeds 1

Answer: B) Consumption exceeds income (dissaving)

Explanation: APS is calculated as $S / Y$ . Since income ( $Y$ ) is always positive, APS can only be negative if Savings ( $S$ ) is a negative number. Negative savings ( $S < 0$ ) occur mathematically when $C > Y$ . This means households are spending more than they are earning, surviving by borrowing money or drawing down past accumulated savings. In economics, this phenomenon is called dissaving, and it typically occurs at very low income levels (below the breakeven point).

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Aggregate Demand, Aggregate Supply & Propensity to Consume and Save

1. Aggregate Demand (AD)

Components of AD

Autonomous vs Induced Expenditure

2. Aggregate Supply (AS)

3. Consumption Function

Numerical Schedule: C = 50 + 0.8Y

4. Propensity to Consume — APC and MPC

Average Propensity to Consume (APC)

Marginal Propensity to Consume (MPC)

5. Saving Function and Propensity to Save

Average Propensity to Save (APS)

Marginal Propensity to Save (MPS)

The Four Key Relationships — Always True

Deriving C from S and S from C

6. Factors Affecting Propensity to Consume

APC, APS, MPC, MPS — Quick Calculation Reference

Practice Questions (CBSE Board Level)

Related Study Material

Practice Questions

Topic Information

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