In economics and particularly in consumer choice theory, the substitution effect is one component of the effect of a change in the price of a good upon the amount of that good demanded by a consumer, the other being the income effect.
When a good's price decreases, if hypothetically the same consumption bundle were to be retained, income would be freed up which could be spent on a combination of more of each of the goods. Thus the new total consumption bundle chosen, compared to the old one, reflects both the effect of the changed relative prices of the two goods (one unit of one good can now be traded for a different quantity of the other good than before as the ratio of their prices has changed) and the effect of the freed-up income. The effect of the relative price change is called the substitution effect, while the effect due to income having been freed up is called the income effect.
The same concepts also apply if the price of one good goes up instead of down, with the substitution effect reflecting the change in relative prices and the income effect reflecting the fact the income has been soaked up into additional spending on the retained units of the now-pricier good.
Suppose the initial situation is given by the graph with the indicated (and never-changing) indifference curves shown and with budget constraint BC1 and with the consumer choosing point A because it puts him on the highest possible indifference curve consistent with BC1. The position and slope of the budget constraint are based on the consumer's income and on the prices of the two goods X and Y. If the price of Y falls, the budget constraint pivots to BC2, with a greater Y-intercept because if all income were spent on Y more of it could be purchased at the now-lower price. The overall effect of the price change is that the consumer now chooses the consumption bundle at point C.
But the move from A to C can be decomposed into two parts. The substitution effect is the change that would occur if the consumer were required to remain on the original indifference curve; this is the move from A to B. The income effect is the simultaneous move from B to C that occurs because the lower price of one good in fact allows movement to a higher indifference curve.
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