Which Structural Parameters Are "Structural"? Identifying the Sources of Instabilities in Economic Models

The objective of this paper is to identify which parameters of a model are stable over time. Existing procedures can only be used to test whether a given subset of parameters is stable, and cannot be used to find which subset of parameters is stable. We propose a new procedure that is informative on the nature of instabilities affecting economic models, and sheds light on the economic interpretation and causes of such instabilities. Furthermore, our procedure provides clear guidelines on which parts of the model are reliable for policy analysis and which are possibly mis-specified. Our empirical findings suggest that instabilities during the Great Moderation were mainly concentrated in Euler and IS equations as well as in monetary policy. Such results offer important insights to guide the future theoretical development of macroeconomic models.