The author poses the question as to what causes the fluctuations in the birthrate and in population increments in the socialist countries. In order to arrive at an answer, he first analyzes demographic development in Czechoslovakia during the period 1869-1980. By comparing the differences between demographic development in Slovakia and the Czech lands, he shows the changes in demographic processes are not essentially determined by national, historical, or other factors, but are above all a complex reflection of changes in the economic mechanism. In the 2nd part of the article, the author examines in detail economic processes influencing natality and the development of population, as well as the mechanism of mutual influence which exists between demographic and economic processes. He points out that the different causes of demographic development cannot be derived from the level of the living standard, but from its changes (and/or changes in economic development). By analyzing data for the period 1948-80, the author arrives at a more precise definition of these changes in the economy. They include, for instance, changes in the rate of growth of the national economy, changes in the ratio of accumulation and consumption, changes in the ratio of national income formation and use, changes in the structure of the living standard, changes in the development of the economy and consumption in the direction of equilibrium or disequilibrium. At the same time, the author shows how these changes act toward their mutual synthesis. In each stage of development, the decisive factor for the size of population increments and natality is whether the ratio between resources and consumption is adequate, i.e., whether the relevant structure of the living standard is in accord with it. The author also examines the time lag between influences caused by changes in the economy and natality; he demonstrates that changes in the economy are effective only in this context if they overcome a threshold of sensitivity or if they accumulate in a specific manner. (author's modified)