In 1929, Warren S. Thompson published a three-part classification of world populations according to their fertility levels and growth rates that explained the progressive passage from one group to another in terms of economic and social factors. American demographers, preoccupied by the Great Depression, paid insufficient attention to this early formulation of demographic transition theory. During 1928-31, Robert Kuckzinsky systematically analyzed the historical evolution of mortality and fertility in Europe and introduced the term "transition" in reference to eastern Europe. In 1944-45, Frank Notestein and Kingsley Davis presented the theory of demographic transition in the form that came to be nearly universally accepted. All societies, it was believed, would pass through the three stages, from a preindustrial to a postindustrial demographic equilibrium. Mortality was presented as a dependent variable under economic control, while fertility was a dependent variable under social control. Demographic transition theory would provide the conceptual framework for UN demographic projections and the justification for family planning programs for the massive agricultural populations of Asia. As the theory developed, the relationship between development and demographic transition was inverted; it was argued that rapid growth constituted an insurmountable obstacle to industrialization or any kind of modernization. Fertility had to be reduced in poor countries by any means possible to permit their economic advancement. Family planning programs in developing countries were supported, and major resources were devoted to KAP studies and the World Fertility Survey. The struggle to control fertility became the most urgent objective. It was not until the 1974 UN World Population Conference in Bucharest that the American delegation abandoned the extremist position of the preceding decade and acknowledged that population policies are not substitutes for development policies.