Diminishing Marginal Utility We buy things because we feel the product is useful to us, but as we use more and more of a product we experience The principle of diminishing marginal utility states that the satisfaction we gain from buying a product lessens as we buy more of the same product. As we use more of a product, we are not willing to pay as much for it. Therefore, the demand curve is downward sloping. Review What You Learned Explain the relationship between the demand schedule and demand curve. Explain how the slope of the demand curve can be explained by the principle of diminishing marginal utility. II. Demand II: "Factors Affecting Demand" Introduction A number of factors will cause demand to increase or decrease. We learned before that the demand curve is just a representation of the quantities that people an willing to buy at all possible market prices. However, something might happen to change people's willingness and/or ability to buy. These changes are usually of two types