The Law of Demand Versus Diminishing Marginal Utility

Diminishing marginal utility is neither necessary nor sufficient for downward sloping demand. Yet upper-division undergraduate and beginning graduate students often presume otherwise. This paper provides two simple counter examples that can be used to help students understand that the Law of Demand does not depend on diminishing marginal utility. The examples are accompanied with the geometry and basic mathematics of the utility functions and the implied ordinary/Marshallian demands