The income effect follows real purchasing power

Its direction depends on whether the good is normal or inferior.

A price decline allows the same money income to purchase more, raising real purchasing power. For a normal good, this income effect raises consumption and reinforces substitution. For an inferior good, higher real purchasing power reduces consumption, so the income effect opposes substitution.

Real income refers to the bundle of goods the consumer can afford, not merely the number printed on a paycheck. When one price falls and nominal income is unchanged, the old bundle costs less, so purchasing power rises. When a price rises, purchasing power falls because the old bundle may no longer be affordable.

For a normal good, consumption moves with real income. A price decrease raises real income and therefore raises consumption through the income effect. For an inferior good, consumption moves oppositely: the same price decrease raises real income but lowers demand through this channel. The substitution effect still increases consumption of the cheaper good.

Most inferior goods still obey the law of demand because the substitution effect is stronger. A Giffen good is a rare, strongly inferior good for which the opposing income effect exceeds substitution, causing quantity demanded to fall when price falls. A Giffen good is not simply any inferior good and is not the same as a prestige good purchased because a high price signals status.

One price decline, two effects

The price of instant noodles falls. Substitution makes noodles cheaper relative to other foods, raising noodle purchases. The price decline also raises real income. If noodles are inferior, that income effect lowers noodle purchases. If substitution is larger, the net quantity still rises.

Reverse the price change to check understanding. If noodle price rises, substitution lowers noodle consumption. Real purchasing power falls. For an inferior good, lower real income increases noodle consumption, so income again opposes substitution. Ordinary inferior demand still falls because substitution dominates.

A Veblen or prestige effect is conceptually different. Some buyers may value a good partly because its high price signals status, meaning a preference determinant changes with perceived price. Giffen behavior arises from a large negative income effect under a tight budget, not from prestige. Answer choices sometimes use the labels interchangeably. Keep the mechanisms separate.

The decomposition explains the law of demand rather than replacing it. For normal goods, both effects support downward-sloping demand. For most inferior goods, substitution remains stronger. Only the demanding Giffen conditions reverse the ordinary result, so do not label any low-quality or inexpensive product Giffen without evidence of a dominant income effect.

When a stem asks only for the substitution effect, ignore the good’s normal or inferior status. When it asks for the income effect, identify both the direction of real purchasing power and the good’s classification. When it asks for the total effect, combine the two and note any ambiguity if their relative sizes are not given.