What Do We Learn from Unit Roots in Macroeconomic Time Series?

It is often argued that the presence of a unit root in aggregate output implies that there is no "business cycle": the economy does not return to trend following a disturbance. This paper makes this notion precise, but then develops a simple aggregative model where this relation is contradicted. In the model output both has a unit root, and displays repeated short-run fluctuations around a deterministic trend. Some summary statistical evidence is presented that suggests the phenomena described in the paper is not without empirical basis.