Two things happen the moment a price moves
By now you can find a consumer's best basket: where the budget constraint just kisses the highest reachable indifference curve, the point we called consumer equilibrium. But a question still hangs in the air from the very first rung of this domain: when the price of one good falls, exactly WHY do people usually buy more of it? "They just do" is not an answer a careful economist will accept.
The honest answer is that a single price change is secretly two changes bundled together. Drop the price of coffee, and first, coffee becomes cheaper relative to tea — even if you were no richer, you'd lean toward the one that now buys more satisfaction per dollar. Second, your same wallet now stretches further; you are, in real terms, a little richer than you were a minute ago. Untangling these is the [[income-and-substitution-effects|income and substitution effects]], and it is the keystone that holds up the whole demand curve.
The substitution effect: lean toward whatever got cheaper
Picture a thought experiment economists love. Coffee's price falls, but at the same instant an imaginary accountant claws back just enough money so you are no better off than before — same satisfaction, same indifference curve, only the relative prices have changed. What do you do? You still shuffle toward coffee, because cup-for-cup it now delivers more bang per dollar than tea. That pure tilt toward the relatively cheaper good, with real income stripped out, is the substitution effect.
Here is the beautiful part: the substitution effect ALWAYS points the same way. When a good gets relatively cheaper, this effect always nudges you to buy more of it; when it gets relatively dearer, always less. No exceptions, ever. It is simply you re-balancing toward the better deal — the same logic you met as the substitutes story, now seen from the inside of a single choice.
The income effect: the stealth raise that can go either way
Now give the imaginary accountant the day off and hand back that clawed-back money. The price cut leaves you genuinely better off — your real income rose. The income effect is how you respond to that extra buying power alone, with relative prices already accounted for. And unlike its tidy twin, this effect's direction depends entirely on what KIND of good we are talking about.
For a normal good, feeling richer makes you want more of it — the income effect pushes the same way as the substitution effect, and the two march together. For an inferior good — think instant noodles, the bus instead of a taxi — feeling richer makes you want LESS; you trade up to something nicer. Here the income effect quietly pulls against the substitution effect. The labels normal, inferior, and the rare Giffen good are exactly the good types sorted by which way their income effect leans, measured by income elasticity of demand.
Notice we are finally paying off a promise from the demand rung. Back there we mentioned, in passing, that income usually shifts the whole demand curve rightward — "but some goods, like instant noodles, go the other way." THIS is that other way, dissected. The income effect is the gear that decides the direction.
Adding the two up: where the demand curve is born
The total response to a price change — what you actually see in the shop — is just the substitution effect plus the income effect, stacked. Walk it through for a price cut and you can read off every case in the table.
PRICE FALLS -> substitution income total demand curve? -------------------------------------------------------------------------- Normal good buy MORE buy MORE buy MORE slopes down (normal) Inferior good buy MORE buy less buy MORE slopes down (sub. wins) Giffen good (rare) buy MORE buy LESS buy LESS slopes UP (income wins)
Read the bottom row slowly, because it is the famous twist. A Giffen good is an inferior good so dominant in a poor household's budget that the income effect doesn't just oppose the substitution effect — it overwhelms it. When the price of the staple rises, the family is now so much poorer in real terms that they can no longer afford the occasional treat that broke up the diet, and they fall back on EVEN MORE of the staple. Price up, quantity up: a demand curve that slopes the wrong way. The most credible real evidence comes from a study of rice for very poor households in Hunan, China — and even that finding is debated.
Why this is the keystone, and how big the effects really are
Step back and see what you've built. The demand curve slopes downward not by decree but for a reason you can now spell out: the substitution effect always pulls toward the cheaper good, and for the overwhelming majority of goods the income effect either agrees or is too weak to overrule it. The downward slope is the visible sum of two invisible forces — and the only way to flip it is the freakish Giffen case where a starving budget makes income win.
How big is each piece in practice? It depends on how much of your budget the good eats. Spend a tiny share — salt, paper clips, a pack of gum — and a price change barely touches your real income, so the income effect is almost nothing; nearly all the response is substitution. Spend a huge share — rent, the family's staple grain — and the income effect can roar. That is precisely why Giffen behaviour, if it appears at all, hides among staples that devour a poor household's spending, never among trinkets.
Stay honest about what this is and isn't. The clean split into two effects is a model, not a measurement you can watch happen — the imaginary accountant who freezes real income is a thought experiment, and economists even argue about the exact right way to do the freezing. It also leans, as ever, on ceteris paribus: hold tastes and other prices still. What the decomposition buys you is not false precision but genuine understanding — a reason, not just a rule, behind the most-drawn line in economics. And with that, the consumer's story is complete: from satisfaction and the magic of one more, through budgets and the best basket, to the two hidden effects behind every price tag. Next the lens turns around to face the seller.