Problems associated with past research on the fertility-development issue are identified in this article, and a model of the macro-level determinants of fertility -- a model informed by the revised theory of fertility transition -- is specified. This model is estimated both cross-sectionally and longitudinally. In addition, a model is specified that assesses the importance of a family planning program effort on fertility change in less developed countries. The effects on fertility change of other variables is controlled. Using crude birth rates for 117 countries, the model is operationalized and the revised model is applied to 1974 rates. A longitudinal model of 1955-1959 to 1974 change in rates is then tested. The final model for 81 less developed countries from the original set of 117 countries includes a measure of family planning effort. Results support the view that high levels of modernization increase motivation to control fertility, but they also show that excessive reliance on developmental change in less developed countries to bring about fetility declines would prolong unnecessarily the current period of rapid population growth. The dominant role of modernization in the models that lack data on family planning programs only facilitates understanding of the past. Modernization is not the only road to future lower fertility. Modernization, abortion, and family planning programs are explicit policy relevant variables. It was found that legalized abortion has a large and independent impact on lowering birth rates and that family planning programs also reduce unwanted births in less developed countries. These programs were the most important factor related to change in 1955-1959 to 1974 crude birth rate.