Why are Beveridge-Nelson and Unobserved-component decompositions of GDP so Different?

This paper reconciles two widely-used decompositions of GDP into trend and cycle that yield starkly different results. Beveridge-Nelson (BN) implies that a stochastic trend accounts for most of the variation in output, while Unobserved-Components (UC) implies cyclical variation is dominant. Which is correct has broad implications for the relative importance of real versus nominal shocks. We show the difference arises from the restriction imposed in UC that trend and cycle innovations are uncorrelated. When this restriction is relaxed, the UC decomposition is identical to the BN decompositions. Furthermore, the zero correlation restriction can be rejected for U.S. quarterly GDP, with the estimated correlation being -0.9.