Class 12 Economics Revision Notes: Theory of Consumer Behaviour

Key Terms & Definitions

Utility

The want-satisfying power of a commodity. It is subjective and varies from person to person.

Total Utility (TU)

The sum total of utility derived from the consumption of all units of a commodity.

Marginal Utility (MU)

The additional utility derived from the consumption of one more unit of a commodity. MU = ΔTU/ΔQ or TU(n) - TU(n-1).

Law of Diminishing Marginal Utility (LDMU)

States that as more and more units of a commodity are consumed, the marginal utility derived from each successive unit tends to decrease.

Indifference Curve (IC)

A curve showing different combinations of two goods that yield the same level of satisfaction (utility) to the consumer.

Marginal Rate of Substitution (MRS)

The rate at which a consumer is willing to give up units of one good (Y) to obtain one more unit of another good (X), while keeping the total utility constant. MRSxy = ΔY/ΔX.

Budget Line (Price Line)

A graphical representation showing all possible combinations of two goods that a consumer can buy with a given income and given prices of the goods. Equation: Px.X + Py.Y = M.

Understanding Consumer Equilibrium

Consumer equilibrium refers to a situation where a consumer spends their given income on the purchase of one or more commodities in such a way that they maximise their total satisfaction. This concept is central to the theory of consumer behaviour and can be explained using two main approaches: the Cardinal Utility Approach (based on numerical measurement of utility) and the Ordinal Utility Approach (based on ranking preferences).

In the Cardinal Approach, satisfaction is measured in 'utils'. A consumer is in equilibrium when the marginal utility per rupee spent is equal across all goods, i.e., MUx/Px = MUy/Py. If MUx/Px > MUy/Py, the consumer will shift consumption towards good X until the equality is restored. This approach also incorporates the Law of Diminishing Marginal Utility which states that as consumption of a good increases, the additional satisfaction from each extra unit decreases.

The Ordinal Approach, also known as Indifference Curve Analysis, is considered more realistic. It assumes that utility cannot be measured numerically but can be ranked. Here, consumer equilibrium occurs at the point where the budget line is tangent to the highest possible indifference curve. At this point, the slope of the indifference curve (MRSxy) is equal to the slope of the budget line (Px/Py). So, the equilibrium condition is MRSxy = Px/Py. This tangency point represents the most preferred combination of goods that the consumer can afford. The second-order condition for equilibrium requires that the indifference curve must be convex to the origin at the point of tangency, which implies a diminishing MRS. Any other point on the budget line would be on a lower indifference curve, meaning less satisfaction, or would be unattainable.

Cardinal vs. Ordinal Utility Approach

Worked Examples

• {"title":"1. Budget Line Equation","bodyMarkdown":"Suppose a consumer has an income (M) of ₹100. Price of Good X (Px) is ₹10 per unit, and Price of Good Y (Py) is ₹5 per unit.\n\nQuestion: Write the budget line equation and find the maximum units of X and Y that can be purchased.\n\nSolution:\nBudget Line Equation: Px.X + Py.Y = M\n10X + 5Y = 100\n\nIf the consumer spends all income on X: 10X = 100 => X = 10 units.\nIf the consumer spends all income on Y: 5Y = 100 => Y = 20 units."}
• {"title":"2. Marginal Rate of Substitution (MRS)","bodyMarkdown":"A consumer is indifferent between (1X, 10Y) and (2X, 7Y).\n\nQuestion: Calculate the MRSxy.\n\nSolution:\nMSSxy = (Change in Y) / (Change in X)\nChange in Y = 10 - 7 = 3 units\nChange in X = 2 - 1 = 1 unit\nMRSxy = 3/1 = 3.\nThis means the consumer is willing to give up 3 units of Y to get 1 additional unit of X."}

Key Points to Remember

• Utility is a subjective concept, not objectively measurable.
• Total Utility increases as long as Marginal Utility is positive.
• Total Utility is maximum when Marginal Utility is zero.
• Total Utility decreases when Marginal Utility is negative.
• Indifference Curves are always downward sloping, convex to the origin, and never intersect.
• Higher Indifference Curve represents a higher level of satisfaction.
• MRS diminishes as we move down an Indifference Curve due to the Law of Diminishing Marginal Utility.
• The slope of the Budget Line is Px/Py, representing the market rate of exchange.
• Consumer Equilibrium (Ordinal Approach) occurs where MRSxy = Px/Py and the IC is tangent to the Budget Line.
• Changes in income shift the Budget Line parallel; changes in price rotate the Budget Line.

Exam Tip for Consumer Behaviour

Pay close attention to the diagrams for Indifference Curves and Budget Lines, especially for consumer equilibrium. Practice drawing them accurately and labeling all axes and curves. Clearly state the conditions for consumer equilibrium for both single commodity (MUx = Px) and two commodities (MUx/Px = MUy/Py or MRSxy = Px/Py) along with their respective assumptions. A common mistake is forgetting the second-order condition (IC must be convex) for the ordinal approach. Clearly explain why tangency is the equilibrium point (i.e., at any other point, MRS ≠ Px/Py leading to disequilibrium).

Practice Questions with Solutions

• Q1: What happens to Total Utility when Marginal Utility is zero? A1: Total Utility reaches its maximum point when Marginal Utility is zero.
• Q2: State two properties of an Indifference Curve. A2: Indifference Curves are downward sloping, convex to the origin, and never intersect each other (any two are sufficient).
• Q3: What is the slope of the Budget Line? A3: The slope of the Budget Line is the price ratio of the two goods, i.e., Px/Py.
• Q4: How does consumer equilibrium differ in the Cardinal and Ordinal Utility approaches? A4: Cardinal: MUx/Px = MUy/Py. Ordinal: MRSxy = Px/Py (and IC convex at tangency).

Frequently Asked Questions