Theory of Consumer Behaviour Class 12 NCERT Guide

What Consumer Behaviour Studies

Consumer behaviour studies how a consumer allocates limited income among different goods to maximise satisfaction. The chapter begins with the idea of rational choice: a consumer compares benefits and costs before buying. If utility is measured in numerical units, we use the cardinal utility approach; if satisfaction is ranked instead of measured, we use the ordinal or indifference curve approach. In CBSE answers, the key is not to memorise curves blindly. A demand decision is built from three facts: wants are unlimited, resources are limited, and additional units usually give less extra satisfaction. This is why Marginal Utility falls, why an indifference curve is normally convex, and why the budget line represents all affordable combinations.

Key Terms You Must Know

Utility

The want-satisfying power of a commodity. It is subjective, so the same good may give different satisfaction to different consumers.

Total Utility

The total satisfaction obtained from consuming a given number of units of a commodity. It generally rises at a diminishing rate at first.

Marginal Utility

The additional utility obtained by consuming one more unit. Formula: MU = change in TU divided by change in quantity.

Law of Diminishing Marginal Utility

As a consumer consumes more units of a commodity, keeping other things constant, the extra satisfaction from each additional unit tends to decline.

Indifference Curve

A curve showing different combinations of two goods that give the consumer the same level of satisfaction.

Budget Line

A line showing all combinations of two goods that a consumer can buy when income and prices of the two goods are given.

Consumer Equilibrium

A situation where the consumer gets maximum satisfaction from given income and prices, and has no tendency to change the chosen combination.

How Consumer Equilibrium Is Reached

1. Start with preferences — The consumer ranks combinations of two goods. Combinations on a higher indifference curve are preferred because they give more satisfaction, while combinations on the same curve are equally satisfying.
2. Add the budget restriction — The budget line shows what is affordable. Its equation is PxX + PyY = M, where M is income, Px is price of good X, and Py is price of good Y. The slope is Px/Py in absolute terms.
3. Find the best affordable point — Equilibrium occurs where the highest possible indifference curve touches the budget line. At this tangency point, MRSxy = Px/Py, meaning the consumer’s willingness to substitute X for Y equals the market exchange ratio.
4. Check the second condition — At equilibrium, MRS must be diminishing. This is why the indifference curve should be convex to the origin; otherwise tangency may not represent a stable maximum satisfaction point.

Worked Examples with Steps

• Example 1: From a Total Utility schedule, TU for 1, 2, 3, 4 units is 20, 36, 48, 56 utils. Find MU and comment on the law.
  Step 1: MU of first unit = TU of 1st unit = 20.
  Step 2: MU of second unit = 36 - 20 = 16.
  Step 3: MU of third unit = 48 - 36 = 12.
  Step 4: MU of fourth unit = 56 - 48 = 8.
  Final answer: MU values are 20, 16, 12, 8. Since MU falls as consumption rises, the schedule illustrates the Law of Diminishing Marginal Utility.

• Example 2: A consumer has income ₹100. Price of X is ₹10 and price of Y is ₹20. Write the budget equation and find intercepts.
  Step 1: Use PxX + PyY = M.
  Step 2: Substitute values: 10X + 20Y = 100.
  Step 3: If Y = 0, then 10X = 100, so X = 10. This is the X-axis intercept.
  Step 4: If X = 0, then 20Y = 100, so Y = 5. This is the Y-axis intercept.
  Final answer: Budget equation is 10X + 20Y = 100; intercepts are X = 10 and Y = 5.

• Example 3: At a point on an indifference curve, MRSxy = 4. Price of X is ₹8 and price of Y is ₹4. Is the consumer in equilibrium?
  Step 1: Equilibrium requires MRSxy = Px/Py.
  Step 2: Calculate price ratio: Px/Py = 8/4 = 2.
  Step 3: Compare: MRSxy = 4 but Px/Py = 2.
  Step 4: Since the consumer is willing to sacrifice 4 units of Y for one more X, but market requires only 2 units of Y, X gives relatively more satisfaction.
  Final answer: The consumer is not in equilibrium and should consume more X and less Y until MRS falls to 2.

Common Mistakes and Board Exam Tips

• Do not write that utility is always measurable in reality. The cardinal approach assumes measurability, while the ordinal approach only ranks preferences.
• In budget line questions, use the correct formula PxX + PyY = M. Many students wrongly add quantities without multiplying by prices.
• Remember that the slope of the budget line depends on relative prices, not income alone. A change in income shifts the line; a change in price rotates it.
• For consumer equilibrium under the indifference curve approach, write both conditions: MRSxy = Px/Py and MRS is diminishing.
• Do not confuse Total Utility and Marginal Utility. TU may rise even when MU is falling, as long as MU remains positive.

How to Write Strong CBSE Answers

For 3-mark answers, define the concept, state the formula or condition, and explain one implication. For 4- or 6-mark answers, draw a labelled diagram whenever asked about budget line, indifference curve, or equilibrium. Label axes as Good X and Good Y, show the budget line, and mark the equilibrium point clearly. In numerical answers, write each substitution step because CBSE marking often gives marks for method even if the final arithmetic has a small error.

Practice Questions with Solutions

• Q: A consumer gets Total Utility of 10, 18, 24, 28 and 30 utils from 1 to 5 units of a good. Calculate Marginal Utility.
  A: Step 1: MU of first unit = 10.
  Step 2: MU of second unit = 18 - 10 = 8.
  Step 3: MU of third unit = 24 - 18 = 6.
  Step 4: MU of fourth unit = 28 - 24 = 4, and MU of fifth unit = 30 - 28 = 2.
  Final answer: MU values are 10, 8, 6, 4 and 2 utils.

• Q: Explain why an indifference curve slopes downward.
  A: Step 1: An indifference curve shows combinations of two goods giving the same satisfaction.
  Step 2: If the consumer gets more of Good X and satisfaction must remain the same, the consumer must give up some units of Good Y.
  Step 3: Therefore, X and Y move in opposite directions along the same curve.
  Final answer: An indifference curve slopes downward because more of one good must be balanced by less of the other good to keep satisfaction constant.

• Q: A consumer has income ₹120. Price of Good X is ₹12 and price of Good Y is ₹6. Find the budget line equation and maximum units of each good.
  A: Step 1: Use the equation PxX + PyY = M.
  Step 2: Substitute values: 12X + 6Y = 120.
  Step 3: If all income is spent on X, Y = 0, so X = 120/12 = 10.
  Step 4: If all income is spent on Y, X = 0, so Y = 120/6 = 20.
  Final answer: Budget line is 12X + 6Y = 120; maximum X = 10 units and maximum Y = 20 units.

• Q: State whether the consumer is in equilibrium when MRSxy = 3, price of X = ₹15 and price of Y = ₹5.
  A: Step 1: Equilibrium condition is MRSxy = Px/Py.
  Step 2: Calculate Px/Py = 15/5 = 3.
  Step 3: Compare MRSxy = 3 with Px/Py = 3.
  Final answer: The consumer is in equilibrium because MRSxy equals the price ratio.

• Q: Distinguish between movement along a budget line and shift of a budget line.
  A: Step 1: Movement along the same budget line means the consumer chooses a different affordable combination while income and prices remain unchanged.
  Step 2: A shift of the budget line occurs when income changes while prices remain the same.
  Step 3: If income increases, the line shifts outward parallel; if income decreases, it shifts inward parallel.
  Final answer: Movement is a change in chosen combination on the same line, while shift is a change in purchasing capacity due to income change.

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