Which of the following factors affects individual’s demand for a commodity ? 1. Price of the commodity 2. Income of the consumer 3. Prices of related goods Select the correct answer using the code given below :

1. Introduction to Consumer Choice and Utility (basic)

Welcome to your first step in mastering Demand Theory! To understand how markets work, we must first look at the individual. At its heart, the Theory of Consumer Behaviour is about the problem of choice. Every day, you have a specific amount of money (income) and a variety of goods you want to buy. The challenge is deciding how to spend that income to achieve maximum satisfaction. This choice is guided by two main pillars: what you like (your preferences) and what you can afford (determined by your income and the prices of goods) Microeconomics (NCERT class XII 2025 ed.), Chapter 2, p.8.

Economists measure this satisfaction using a concept called Utility. However, our desire for a specific good isn't constant. According to the Law of Diminishing Marginal Utility (LDMU), as you consume more and more units of a commodity, the extra satisfaction (marginal utility) you get from each additional unit starts to decline Microeconomics (NCERT class XII 2025 ed.), Chapter 2, p.10. For example, the first glass of water on a hot day provides immense satisfaction, but the fourth or fifth glass provides much less.

This psychological reality is exactly why the demand curve usually slopes downward. Because the "worth" or marginal utility of each extra unit falls, a consumer is only willing to buy more if the price drops. If a shop charges ₹40 for a juice box, you might buy one. But because the second box gives you less extra utility than the first, you’ll likely only buy that second box if the price is lower than ₹40 Microeconomics (NCERT class XII 2025 ed.), Chapter 2, p.11. Thus, the relationship between price and the quantity you demand is typically negative Microeconomics (NCERT class XII 2025 ed.), Chapter 2, p.23.

Sources: Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p.8; Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p.10; Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p.11; Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p.23

2. The Law of Demand and Demand Curves (basic)

At the heart of consumer behavior lies the Law of Demand. It states a simple, intuitive truth: other things being equal (ceteris paribus), there is a negative or inverse relationship between the price of a commodity and the quantity demanded. In simpler terms, when the price of a good rises, people generally buy less of it; when the price falls, they buy more Microeconomics (NCERT class XII 2025 ed.), Chapter 2, p.24.

This relationship is visually captured by the Demand Curve. When we plot price on the vertical (Y) axis and quantity on the horizontal (X) axis, the curve slopes downward from left to right. Why does this happen? Economists point to two main psychological and economic drivers:

• Diminishing Marginal Utility: As you consume more of a good, the satisfaction (utility) you derive from each additional unit decreases. Therefore, you are only willing to buy more if the price drops Microeconomics (NCERT class XII 2025 ed.), Chapter 2, p.10.
• Income Effect: When the price of a good (like bananas) drops, your "real income" or purchasing power effectively increases, allowing you to buy more with the same amount of money Microeconomics (NCERT class XII 2025 ed.), Chapter 2, p.24.

It is crucial to distinguish between a movement along the curve and a shift of the curve. A change in the price of the good itself causes a movement along the existing demand curve (change in quantity demanded). However, other factors like a change in the consumer's income or the prices of related goods (substitutes and complements) will cause the entire demand curve to shift Microeconomics (NCERT class XII 2025 ed.), Chapter 2, p.25. For instance, if your income rises, your demand for "normal goods" increases at every price point, shifting the curve to the right.

Sources: Microeconomics (NCERT class XII 2025 ed.), Theory of Consumer Behaviour, p.10; Microeconomics (NCERT class XII 2025 ed.), Theory of Consumer Behaviour, p.24; Microeconomics (NCERT class XII 2025 ed.), Theory of Consumer Behaviour, p.25

3. Market Demand and its Derivation (intermediate)

In our previous steps, we focused on how a single consumer makes choices. However, in a real economy, we look at the Market Demand, which represents the total quantity of a good that all consumers in the market are willing to buy at a specific price. To understand this, we must first recognize that an individual's demand is shaped by three primary factors: the price of the commodity, the consumer's income (which determines if a good is 'normal' or 'inferior'), and the prices of related goods like substitutes and complements Microeconomics (NCERT class XII 2025 ed.), Chapter 2, p.24, 28. When these individual preferences are combined, they form the collective pulse of the market.

The process of finding the market demand from individual demand curves is known as Horizontal Summation. Imagine two consumers, A and B. At a price of ₹10, Consumer A wants 2 units and Consumer B wants 3 units. To find the market demand at that price, we simply add their quantities together (2 + 3 = 5 units). We call this 'horizontal' summation because on a standard graph, quantity is measured along the horizontal axis. By repeating this for every possible price point, we derive the Market Demand Curve Microeconomics (NCERT class XII 2025 ed.), Chapter 2, p.27.

Generally, the market demand curve also slopes downward. This is because as the price falls, not only do existing consumers typically buy more, but new consumers who previously found the good too expensive may now enter the market. As long as individual demand curves are downward-sloping, their horizontal sum—the market demand curve—will also follow this trend, reflecting a negative relationship between price and total quantity demanded Microeconomics (NCERT class XII 2025 ed.), Chapter 6, p.75.

Factor	Individual Demand	Market Demand
Basis	Single consumer's behavior.	Aggregate of all consumers in the market.
Derivation	Based on utility and budget.	Derived via Horizontal Summation of individual curves.
Slope	Usually downward (Law of Demand).	Downward-sloping as more consumers participate at lower prices.

Sources: Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p.24, 27, 28; Microeconomics (NCERT class XII 2025 ed.), Chapter 6: Market Equilibrium, p.75

4. The Supply Side: Law of Supply and Market Equilibrium (intermediate)

To understand how markets actually function, we must look at the mirror image of demand: The Law of Supply. While consumers seek to maximize utility by buying more at lower prices, producers (firms) aim to maximize profit by selling more when prices are high. This results in a positive relationship between the price of a commodity and the quantity supplied. Unlike the downward-sloping demand curve, the supply curve typically slopes upward, indicating that as the market price rises, firms are willing to offer more of the good for sale Microeconomics (NCERT class XII 2025 ed.), Market Equilibrium, p.79.

The Market Equilibrium is the "sweet spot" where these two opposing forces meet. It occurs at the price where the quantity that firms want to sell is exactly equal to the quantity that consumers want to buy Microeconomics (NCERT class XII 2025 ed.), Market Equilibrium, p.73. At this point, the market is said to be "cleared," and there is no inherent pressure for the price to change. Graphically, this is the point (E) where the supply and demand curves intersect Microeconomics (NCERT class XII 2025 ed.), Market Equilibrium, p.81.

However, markets are dynamic and often face disequilibrium. When the prevailing market price is not at the equilibrium level, two scenarios can occur:

Scenario	Market Condition	Outcome
Price > Equilibrium	Excess Supply (Surplus)	Firms have unsold stock and will lower prices to attract buyers Microeconomics (NCERT class XII 2025 ed.), Market Equilibrium, p.86.
Price < Equilibrium	Excess Demand (Shortage)	Consumers cannot find enough goods, leading them to bid prices up until equilibrium is restored.

It is also important to note that the equilibrium itself can shift. For instance, if the cost of inputs (like labor or raw materials) increases, the supply curve will shift, leading to a new equilibrium with a higher price and lower quantity Microeconomics (NCERT class XII 2025 ed.), Market Equilibrium, p.79.

Sources: Microeconomics (NCERT class XII 2025 ed.), Market Equilibrium, p.73; Microeconomics (NCERT class XII 2025 ed.), Market Equilibrium, p.79; Microeconomics (NCERT class XII 2025 ed.), Market Equilibrium, p.81; Microeconomics (NCERT class XII 2025 ed.), Market Equilibrium, p.86

5. Price Elasticity of Demand (exam-level)

While the Law of Demand tells us the direction in which quantity demanded moves when price changes (usually inversely), it doesn't tell us the magnitude. Price Elasticity of Demand (PED) bridges this gap. It is a quantitative measure of how responsive the demand for a good is to a change in its price. Formally, it is defined as the percentage change in demand for the good divided by the percentage change in its price Microeconomics (NCERT class XII 2025 ed.), Chapter 2, p.28.

Elasticity isn't just one fixed number for every product; it varies based on how "necessary" or "substitutable" a good is. We generally categorize elasticity into five types based on the numerical value (absolute value) of the coefficient:

Numerical Value	Terminology	Description
eD = 0	Perfectly Inelastic	Demand does not change at all with price (e.g., life-saving drugs).
0 < eD < 1	Inelastic	% change in demand is less than % change in price (e.g., salt, fuel).
eD = 1	Unitary Elastic	% change in demand equals % change in price.
1 < eD < ∞	Elastic	% change in demand is greater than % change in price (e.g., luxury cars).
eD = ∞	Perfectly Elastic	Infinitesimal price change causes infinite demand change (theoretical).

An interesting nuance is that elasticity can change along a single demand curve. On a linear (straight-line) demand curve, the elasticity is not constant. It is infinite (∞) where the curve touches the vertical (price) axis, zero (0) where it touches the horizontal (quantity) axis, and exactly one (1) at the midpoint Microeconomics (NCERT class XII 2025 ed.), Chapter 2, p.30. However, certain special curves have constant elasticity, such as a rectangular hyperbola, where the elasticity is 1 at every single point Microeconomics (NCERT class XII 2025 ed.), Chapter 2, p.31.

Finally, elasticity has a direct relationship with Total Expenditure (Price × Quantity). If a good has inelastic demand, a price increase actually leads to an increase in total expenditure because the drop in quantity is too small to offset the higher price. Conversely, if demand is elastic, a price increase leads to a decrease in total expenditure Microeconomics (NCERT class XII 2025 ed.), Chapter 2, p.35.

Sources: Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p.28; Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p.30; Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p.31; Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p.35

6. Impact of Income and Related Goods on Demand (exam-level)

While the price of a commodity is the most visible driver of demand, two other powerful forces act behind the scenes: consumer income and the prices of related goods. Understanding these allows us to move beyond simple price-quantity relationships to a more realistic demand function. In economics, we categorize goods based on how demand reacts when a consumer's purchasing power increases. For normal goods, demand moves in the same direction as income—as you earn more, you buy more. Conversely, inferior goods (like coarse cereals) see a drop in demand as income rises, because consumers switch to higher-quality alternatives Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p. 24.

Interestingly, the classification of a good isn't permanent. A specific item might be a normal good for a consumer at low income levels (as they can finally afford it), but becomes an inferior good as their income grows further and they look for even better substitutes Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p. 25. Beyond income, demand is also dictated by the price of other goods in the market, categorized as either substitutes or complements.

Type of Related Good	Definition	Impact on Demand	Example
Substitutes	Goods used in place of one another.	Price of Good A ↑, Demand for Good B ↑	Tea and Coffee
Complements	Goods consumed together.	Price of Good A ↑, Demand for Good B ↓	Cars and Petrol

Finally, we must distinguish between necessities and luxuries. Necessities, like basic food items, are usually price inelastic—their demand doesn't fluctuate much even if prices climb. Luxuries, however, are highly sensitive to price changes Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p. 31. These factors—own price, income, and related prices—collectively define the Demand Function, which is the ultimate tool for predicting consumer behavior.

Sources: Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p.24; Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p.25; Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p.31

7. Solving the Original PYQ (exam-level)

Now that you have mastered the building blocks of consumer behavior, this question brings them all together through the concept of the Demand Function. Think of demand not as a single number, but as a dynamic relationship between a consumer's desire for a good and the economic environment surrounding it. The factors you have studied—price sensitivity, purchasing power, and market inter-dependencies—are the exact variables UPSC is testing here to see if you can look beyond just the sticker price to the broader theory of Microeconomics (NCERT class XII 2025 ed.) > Chapter 2: Theory of Consumer Behaviour.

To arrive at the correct answer (C), let’s walk through the logic: First, the Price of the commodity is the most immediate factor; according to the Law of Demand, price changes cause a movement along the demand curve. Second, Income determines the consumer's budget constraint; as you learned, demand for normal goods rises with income, while inferior goods see a decrease. Third, Prices of related goods shift the demand curve entirely. Whether it is substitutes (where a price rise in one increases demand for the other) or complements (where they are consumed together), these external prices are vital. Since all three factors are primary determinants in the functional relationship of demand, 1, 2 and 3 must all be included.

UPSC often uses distractor traps like options (A), (B), and (D) to catch students who focus too narrowly. A common mistake is to associate "demand" only with the Law of Demand (Price), which would lead a student to incorrectly choose (D). However, in economic theory, "demand" refers to the entire schedule of quantities a consumer is willing to buy at various prices, incomes, and market conditions. Options that exclude income or related goods are incomplete and fail to capture the multi-dimensional nature of consumer choice as explained in Microeconomics (NCERT class XII 2025 ed.) > 2.6.2 Factors Determining Price Elasticity of Demand for a Good.