The substitution effect follows relative price

It always moves consumption toward the relatively cheaper good.

If the price of bus travel falls while other prices stay fixed, bus travel becomes cheaper relative to driving or ride-share. Holding the consumer at the original utility level, the substitution effect increases bus trips. For an own-price increase, the substitution effect reduces the good’s quantity demanded.

The substitution effect isolates the change in relative price. When bus fare falls, one bus trip sacrifices fewer units of other goods. Even if the consumer were compensated or charged just enough to remain at the original satisfaction level, the new relative price would encourage a bundle with more bus travel. For the good whose own price changes, substitution always moves opposite the price change.

The effect also changes consumption of alternatives. If bus travel and ride-share are substitutes, cheaper bus travel leads the consumer away from ride-share through substitution. The exact size depends on preferences and available alternatives, but the relative-price direction is determinate.

Graphically, a price change rotates the budget line. To isolate substitution, economists imagine a compensated budget line with the new slope that just reaches the old indifference curve. The movement along that old utility level is the substitution effect. An introductory question usually tests direction rather than construction.

Do not describe substitution as “the consumer has more money left.” That phrase concerns purchasing power and belongs to the income effect. Substitution can be identified even when a hypothetical compensation removes the purchasing-power change. Its defining cause is the altered opportunity cost between goods.

Hold satisfaction conceptually fixed

Coffee price falls while tea price stays fixed. Imagine adjusting the consumer’s income so the old satisfaction level is just attainable at the new relative prices. The consumer chooses more coffee and less tea along that old utility level. That movement is substitution. Any additional change caused by greater real purchasing power is counted separately.

The substitution effect does not depend on whether the good is normal or inferior. Those labels determine the income effect. This is why a price decrease always creates a substitution push toward the focal good even when the income effect later pushes in the opposite direction.

For labor supply, leisure has an opportunity price equal to the wage. A higher wage makes an hour of leisure more expensive in forgone earnings, so substitution favors more work and less leisure. The same relative-price reasoning applies even though one “good” is time.

Substitution effects also appear when the price of a related good changes, but the formal own-price decomposition follows the focal good’s price. If tea price falls while coffee price stays fixed, substitution can reduce coffee consumption because tea became relatively cheaper. Keep the focal quantity and changed price explicit in the final sentence.

The direction follows relative price even when the consumer’s nominal income never changes.

That sentence is the fastest check against an option that mistakenly attributes substitution to purchasing power.