Demand curves usually slope downward because of the income and substitution effects of a price change of a good and the law of diminishing marginal utility.

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The income effect occurs when a decrease in price increases the purchaser's real income, enabling them to buy more of a good. The substitution effect occurs when a decrease in price makes a good more attractive relative to other goods. The law of diminishing marginal utility states that as a person consumes more of a good, the marginal utility they derive from each additional unit consumed decreases. These three factors combine to create a demand curve that is downward sloping.

Demand Curves Usually Slope Downward Because of the Income and Substitution