At the macroeconomic level the principal aim is to develop an interactive economic-demographic model of U.S. experience over the past five decades that centers on the causes and effects of the long swing in fertility and its relation to "relative income." We plan too to develop a more comprehensive picture of the effects of the swing by analyzing several social as well as economic variables, and to do at least some preliminary work on some of these questions for a few other developed countries with similar experience. At the microeconomic level the proposed research includes the nature and trend during economic development of the prevalence of deliberate fertility control, the relation of this to trends in fertility differentials by socioeconomic status, and the factors lying behind the SES-fertility differentials both for those who do and those who do not regulate their fertility. Among other things this involves theoretical and empirical work on natural fertility, the relevance of the "bequest motive" to agricultural populations, and the mechanisms of interdependent preferences underlying the relative income variable. The principal data base for empirical research in this segment of the work will be household survey data from less developed countries, with special attention to rural areas. These various topics should help to evaluate and extend the microeconomic theory of fertility and to develop further our analysis of the evolution of a society from a natural fertility regime to conditions approximating a "perfect contraceptive society."