Indifference curves and budget lines

When the price of a good falls, the total change in quantity demanded (the price effect) can be decomposed into two parts: the substitution effect and the income effect.

The substitution effect is the change in quantity demanded due to the good becoming relatively cheaper than other goods, holding real income constant. It is always positive for a price fall: the consumer switches toward the now-cheaper good. The income effect is the change in quantity demanded due to the rise in real income (purchasing power) that a price fall brings. Its direction depends on the type of good.

For a normal good, the income effect is positive — higher real income leads the consumer to buy more. So the substitution and income effects reinforce each other, and quantity demanded rises strongly when price falls. This gives a clearly downward-sloping demand curve.

For an inferior good, the income effect is negative — as real income rises, the consumer buys less of the good. This works against the substitution effect. However, for most inferior goods the substitution effect outweighs the negative income effect, so quantity demanded still rises when price falls (just by less than for a normal good).

For a Giffen good — a strongly inferior staple that takes a large share of a poor consumer's income — the negative income effect is so large it outweighs the substitution effect. So a price fall leads to a fall in quantity demanded, giving an upward-sloping demand curve. Giffen goods are very rare because these conditions are seldom met.

Thus the income/substitution decomposition explains not only the normal law of demand but also the unusual cases of inferior and Giffen goods, showing why the strength (and even the direction) of the demand response depends on the type of good.

The indifference curve model analyses consumer choice using indifference curves (combinations of two goods giving equal utility) and a budget line (affordable combinations), with the consumer in equilibrium where the budget line is tangent to the highest attainable indifference curve. Whether it is the best way to understand consumer choice depends on its strengths against rival approaches and its realism.

Strengths of the model. A major advantage over marginal utility theory is that it does not require utility to be measured cardinally — it relies only on consumers being able to rank combinations (ordinal utility), which is more realistic. It elegantly explains consumer equilibrium and, crucially, allows the price effect to be decomposed into income and substitution effects. This decomposition explains the demand for normal, inferior and Giffen goods in a way marginal utility theory handles less neatly, and it shows clearly how income and price changes affect choice. As a logical framework, it is powerful and internally consistent.

Limitations. The model assumes consumers can consistently and rationally rank all combinations (complete, transitive preferences) and maximise utility with full information — assumptions challenged by behavioural economics, which shows that real consumers use heuristics, are influenced by framing, habit, emotion and advertising, and make systematic errors. The model is also static (ignoring changing preferences), usually handles only two goods, ignores social influences (bandwagon/snob effects), and uses indifference curves that cannot be observed or measured empirically, limiting its testability.

Comparison with alternatives. Compared with marginal utility theory, the indifference curve model is generally superior because it needs only ordinal preferences and gives the income/substitution decomposition. Compared with behavioural economics, however, it is less realistic: behavioural models capture actual decision-making (bounded rationality, biases) far better, which is increasingly important for policy (nudges, merit/demerit goods).

Judgement. The indifference curve model is the best of the traditional, rational-choice models of consumer behaviour — more general and less restrictive than marginal utility theory, and uniquely good at decomposing price effects. But it is not the single best way to understand consumer choice overall, because it shares the rationality assumptions that behavioural economics has shown to be systematically violated. The most defensible conclusion is that the indifference curve model is the best theoretical framework for the logic of rational choice, but a full understanding of real consumer behaviour requires combining it with behavioural economics — so 'the best way' is true only within the rational-choice tradition, not absolutely.

This pattern — quantity demanded falling when price falls — describes a Giffen good, and can be explained using the income and substitution effects.

Rice is a staple food that takes up a large share of the poorest households' income, and it is a strongly inferior good (when they can afford better, they buy less rice and more of other foods).

When the price of rice falls, two effects occur. The substitution effect makes rice relatively cheaper, encouraging households to buy more rice (as usual). But the price fall also raises their real income — because rice is such a large part of their spending, the increase in purchasing power is significant. Since rice is strongly inferior, this higher real income leads them to buy less rice (and instead buy more nutritious, preferred foods such as meat or vegetables). This is the negative income effect.

For these households, because rice is strongly inferior and dominates their budget, the negative income effect outweighs the positive substitution effect. The net result is that a fall in the price of rice leads them to buy less rice — the defining behaviour of a Giffen good. This is why, very unusually, the demand curve for rice among the poorest can slope upward.

In the indifference curve model, a consumer is in equilibrium where their budget line is tangent to the highest attainable indifference curve — the affordable combination of two goods that gives the most utility. A change in income or in a price moves the budget line and so changes equilibrium.

A change in income (with prices constant) causes a parallel shift of the budget line. A rise in income shifts it outward, making more of both goods affordable; the consumer reaches a new tangency on a higher indifference curve (more utility). How consumption of each good changes depends on whether the goods are normal (consumption rises) or inferior (consumption falls as income rises). Tracing equilibria as income changes gives the income–consumption curve, from which the effect of income on demand (and hence whether a good is normal or inferior) can be read.

A change in the price of one good (with income and the other price constant) pivots the budget line on the axis of the good whose price changed. If good X becomes cheaper, the line pivots outward along the X-axis, and the consumer reaches a new tangency, generally buying more X. This price effect can be decomposed into the substitution effect (X is relatively cheaper, so buy more) and the income effect (real income has risen). Tracing equilibria as the price changes derives the consumer's demand curve for that good.

Comparison and judgement. Both changes move the consumer to a new equilibrium, but they do so differently: an income change shifts the line parallel (changing affordability of both goods equally), while a price change pivots it (changing relative prices as well as real income). The price change is richer because it combines a substitution and an income effect, allowing the model to explain normal, inferior and Giffen goods. Overall, the model shows clearly how both income and prices determine consumer choice — its key strength — though it relies on the assumption that consumers rank preferences consistently and rationally, which limits how literally its predictions should be taken.