In 2002, more American counties (985) experienced natural decrease than at any time in the nation's history. The incidence of natural decrease has diminished since then, but remains near record levels. It is most common in rural areas remote from metropolitan centers. Spatial concentrations exist in the Great Plains, Corn Belt, and East Texas, with scattered pockets in the Ozark-Ouachita Uplands, Upper Great Lakes, and Florida. A multivariate spatial-error regression model demonstrates that natural decrease is a consequence of the complex interaction between fertility, mortality, and migration over a protracted period and is symptomatic of fundamental changes in the demographic structure of an area. Age-structure changes resulting from protracted, age-specific migration are a primary cause of natural decrease. Temporal variations in fertility also have a significant impact, but counties experiencing natural decrease do not have fertility levels below the national average.