Consumption & Investment: Functions, Trends & Determinants

What is Consumption?

Consumption refers to the spending by households on goods and services to satisfy their wants and needs. It is the largest component of Aggregate Demand (AD) in an economy.

Key Features of Consumption:

• Necessity: People consume to survive and improve their living standards.
• Dependence on Income: Higher income → Higher consumption (but not always in the same proportion).
• Non-Production Use: Unlike investment, consumption does not add to future production capacity.

Types of Consumption:

1. Durable Goods (e.g., cars, refrigerators) – Last long, bought occasionally.
2. Non-Durable Goods (e.g., food, clothes) – Consumed quickly.
3. Services (e.g., education, healthcare) – Intangible but essential.

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Consumption Function

The consumption function shows the relationship between consumption (C) and disposable income (Yd).

Mathematical Representation:

$$C=C_0+b{Y}_d$$

• $C$ = Total consumption
• $C_0$ = Autonomous consumption (spending even if income is zero, e.g., basic needs)
• $b$ = Marginal Propensity to Consume (MPC) – The fraction of additional income spent
• $Y_d$ = Disposable income (income after taxes)

Example:

If $C_0=100$ and $b=0.8$, then:

• If $Y_d=500$, $C=100+0.8(500)=500$
• If $Y_d=1000$, $C=100+0.8(1000)=900$

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Marginal Propensity to Consume (MPC)

MPC measures how much consumption changes when income changes.

$$MPC=\frac{\Delta C}{\Delta Y_d}$$

Example: If income rises from 500 to 700 and consumption rises from 400 to 500: $$MPC=\frac{500-400}{700-500}=\frac{100}{200}=0.5$$

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Average Propensity to Consume (APC)

APC shows the proportion of income spent on consumption.

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

Example: If $C=600$ and $Y_d=1000$: $$APC=\frac{600}{1000}=0.6$$

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Factors Affecting Consumption

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Consumption vs. Saving

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Investment Function

Investment refers to spending by businesses on capital goods (machines, buildings, infrastructure) to increase future production.

Types of Investment:

1. Gross Investment – Total spending on new capital (including replacement).
2. Net Investment – Gross investment minus depreciation (actual addition to capital stock).
3. Autonomous Investment – Independent of income (e.g., new technology adoption).
4. Induced Investment – Depends on income (e.g., more factories if demand rises).

Investment Function:

$$I=I_0+iY$$

• $I$ = Total investment
• $I_0$ = Autonomous investment (does not depend on income)
• $i$ = Marginal Propensity to Invest (MPInvest) – How much investment changes with income
• $Y$ = National income

Example: If $I_0=200$ and $i=0.1$, then:

• If $Y=1000$, $I=200+0.1(1000)=300$

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Factors Affecting Investment

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Accelerator Principle

The accelerator principle explains how changes in income affect investment.

• If income rises, firms need more capital → Investment increases sharply.
• If income falls, firms reduce production → Investment drops quickly.

Example: If income grows by 10%, but firms need 20% more machines to meet demand, investment accelerates.

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NEB Board-Style Questions (Practice)

Short Answer Questions (5 marks each)

1. Define consumption function. Explain the relationship between consumption and income with the help of a numerical example.
2. What is MPC? How is it different from APC? Calculate MPC if consumption rises from 400 to 500 when income rises from 600 to 800.
3. Differentiate between autonomous and induced investment. Give two examples of each.
4. Explain the accelerator principle with an example.
5. How do interest rates and business expectations affect investment decisions?

Long Answer Questions (10 marks each)

6. Explain the consumption function with a diagram. What are the main determinants of consumption? How does government policy influence consumption?
7. Discuss the factors affecting investment in an economy. How does the accelerator principle explain the relationship between income and investment?
8. Compare and contrast consumption and investment. Why is investment more volatile than consumption?
9. A household earns Rs. 50,000 per month. If its consumption is Rs. 40,000, calculate:
   • APC
   • APS (Average Propensity to Save)
   • If income rises to Rs. 60,000 and consumption rises to Rs. 45,000, calculate MPC and MPS (Marginal Propensity to Save).
10. How does a rise in interest rates affect consumption and investment? Use diagrams to support your answer.

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Exam Tip

✅ Diagrams are crucial! Always draw:

• Consumption function (45° line diagram)
• Investment function (horizontal line for autonomous investment)
• Accelerator principle (steep slope for investment vs. income)

✅ Memorize formulas:

• $C=C_0+b{Y}_d$
• $MPC=\frac{\Delta C}{\Delta Y_d}$
• $I=I_0+iY$

✅ Understand real-world applications:

• Government policies (tax cuts, subsidies) affect consumption.
• Business cycles (boom vs. recession) explain investment volatility.

✅ Practice numerical problems – NEB often tests calculations of MPC, APC, and investment changes.

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Visual Summaries

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This note covers all key concepts in Unit 6 with examples, diagrams, and exam-style questions to help you score full marks in NEB! 🚀