Identification with External Instruments in Structural Vars

IV methods have become the leading approach to identify the effects of macroeconomic shocks. Conditions for identification generally involve all the shocks in the VAR even when only a subset of them is of interest. This paper provides more general conditions that only involve the shocks of interest and the properties of the instrument of choice. We introduce a heuristic and a formal test to guide the specification of the empirical models, and provide formulas for the bias when the conditions are violated. We apply our results to the study of the transmission of conventional and unconventional monetary policy shocks