Pockets of high fertility persisted in some areas of the American South through the Great Depression. Most other areas of the country adopted modem fertility patterns considerably earlier in the century; these “laggard” areas are clear exceptions to the national demographic revolution in family building. In this paper we attempt to identify the factors that account for the persistently high fertility in some southern regions. We use county-level data for 1940 to assess the utility of three theoretical models of fertility: structural, diffusion-innovation, and health. Differences by race are also considered, in view of the distinctly different histories of whites and African-Americans in the south. Our findings suggest that unicausal explanations for the persistence of high fertility are too simplistic; all three theoretical perspectives receive empirical support. Considerable similarity is observed in the findings for blacks and for whites. Yet important differences also emerge, especially the more powerful effects of structural variables on white fertility. We conclude that the evidence indicates the need for “diversity” in the study of demographic behavior. Not only should we examine a variety of causal mechanisms for demographic phenomena; we also should consider the varying utility of those mechanisms across different social groups.