The spatial units of analysis employed in urban and regional research display considerable internal heterogeneity in terms of the social, demographic, and economic variables used to describe them. One implication of this has been overlooked in the literature, namely, that aggregate rates may have an implicit dynamic of change. Differential internal rates modify the composition of the aggregate, changing the relative importance of subareas in determining the aggregate rate. To demonstrate this, methods for decomposing the change in growth rates due to heterogeneity are developed and applied to nonmetropolitan growth rates between 1950 and 1980. Internal heterogeneity was found to be an important, and sometimes even a dominant, component of change. Furthermore, the analysis sheds considerable light on the reasons for the changes in aggregate rates that marked the nonmetropolitan turnaround.