The Economics of Exclusion Restrictions in IV Models

We explore a key underlying assumption, the exclusion restriction, commonly used in interpreting IV estimates in the presence of heterogenous treatment effects as a local average treatment effect (LATE). We show through a series of simple examples that in some commonly featured cases that this assumption is likely to be violated among inframarginal agents, i.e. the always- and never-takers. This violation of the exclusion restriction will generally confound the LATE interpretation of the associated IV results. We discuss potential adjustments to IV estimates in the presence of this bias.