A Diagrammatic Exposition of Weak Complementarity and the Willig Condition

This article provides a graphical explanation for one of the most important restrictions to utility functions used in revealed preference approaches for measuring the demand for public goods and product quality—weak complementarity. It also describes how the Willig condition is an important element along with weak complementarity in measuring Hicksian consumer surplus for changes in public goods or quality using Marshallian demand curves.