·Economics & Markets
Section 1
The Core Idea
When a price changes, demand doesn't just move — it splits. The substitution effect is the shift between goods caused by changed relative prices: as one good becomes cheaper relative to others, you substitute toward it. The income effect is the change in quantity demanded because your real purchasing power changed: a price cut makes you effectively richer, so you may buy more of that good or reallocate across your basket. The total observed change in quantity is the sum of both. For normal goods the two reinforce each other: a price fall raises quantity via substitution (switch into this good) and via income (you have more real income). For inferior goods the income effect opposes the substitution effect: a price fall still tempts substitution toward the good, but your higher real income may push you toward superior alternatives. When the income effect dominates, you get the backward-bending labour-supply curve or a Giffen good — quantity demanded falls when price falls.
John Hicks and Roy Allen formalised the decomposition in the 1930s using indifference curves and compensated demand. The practical payoff: you can't interpret a demand response without asking how much is "switch because relative prices changed" versus "spend differently because I feel richer or poorer." Tax policy, minimum wages, and pricing strategy all depend on which effect dominates for which segment. A wage increase can reduce hours worked if the income effect is large — workers hit their target income and take more leisure. A subsidy on a staple food can reduce consumption of that staple among the poorest if the income effect pushes them toward slightly better substitutes. The same price move can have opposite effects in different populations.
Labour supply is the classic application. A higher wage makes work more attractive relative to leisure (substitution effect: work more) but also raises real income (income effect: consume more leisure if leisure is normal). For low wages the substitution effect usually dominates — people work more when the wage rises. For high wages the income effect can dominate — the backward-bending supply curve. Tax policy has the same structure: cutting marginal tax rates has a substitution effect (work more; keep more of the next dollar) and an income effect (you're richer; work less). The Laffer curve and the debate over revenue impact turn on the relative size of these effects. In consumer demand, Giffen goods are the edge case: a price rise for an inferior staple (e.g. rice for very poor households) can increase quantity demanded because the income effect — "I'm poorer, I substitute toward the cheapest staple" — outweighs the substitution effect away from the now-costlier good.
