On the Fundamentalness of Nonfundamentalness in DSGE Models

This note warns against the use of noncausal VARs as a reliable test for indeterminacy. By means of a simple example, we show that determinate models may well entail nonfundamental ARMA equilibrium reduced forms - which only (and uniquely) depend on the fundamental structural shocks -, whereas indeterminate ones may actually be sunspot-free and possess fundamental (i.e. invertible) equilibrium representations. Hence, detecting a causal representation of the data cannot be interpreted as evidence of determinacy.