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Natural decrease is no longer rare in the United States. By 1989, 34 percent of all U.S. counties had experienced at least one year of it. Natural decrease is most common in rural areas remote from metropolitan centers. Regional concentrations of natural decrease exist in the Great Plains, the Corn Belt, and East Texas with scattered pockets in the Ozark-Ouachita Uplands, Upper Great Lakes, and Florida. Natural decrease is caused by age structure distortions stimulated by protracted, age-specific migration. Although temporal variations in fertility also contribute to natural decrease, these variations are not due to below average fertility. Natural decrease is symptomatic of fundamental changes in the demographic structure of an area.
Kenneth M. Johnson (Sun,) studied this question.