The law of demand states that quantity demanded increases when price decreases, but why? Two reasons why the demand curve slopes downward are the substitution effect and the income effect. The income effect states that when the price of a good decreases, it is as if the buyer of the good's income went up. The substitution effect states that when the price of a good decreases, consumers will substitute away from goods that are relatively more expensive to the cheaper good. Learn about the role of the income effect and the substitution effect on the shape of the demand curve in this video.