Demand curves are made up of points at which the quantity consumed of a good or service by a consumer is balanced against its price. This usually reflects a situation in which consumers will pay less given a greater quantity of a good on sale, and more when there is a lower quantity around. Explaining how these points arise is therefore different from explaining the causes of demand—which are, broadly, income, fashion taste and preference, and the price and availability of substitute and complementary products. It is also different from explaining the slope of the demand curve, which is a matter of elasticity (and which can be affected by non-price hedonic factors such as effective advertising, or addiction.) There are two convincing theories of why the points which make up a demand curve arise. One is based on the theory of marginal utility. The second is based on the idea of consumer indifference. Marginal utility theory holds that a consumer will look to the usefulness der...