Reduced form explanations of population growth are decomposed into fertility and mortality. These reduced form equations are estimated first from three cross sections of 68 low income countries from 1972 to 1989, and then reestimated from changes occurring within these countries over time, by fixed effect methods. Adult female schooling is the most important factor related to lower fertility, mortality, and population growth, as is the availability of calories per capita. Reducing the share of the labor force in agriculture is also linked to lower fertility and mortality, and somewhat slower population growth. Nonhuman capital wealth, measured here by net fuel exports as a share of GDP, is associated with higher fertility and population growth. All of these patterns observed in both the cross section and the fixed effect estimates are anticipated from human capital models of lifetime fertility, and replicated in • microeconomic empirical studies. Family planning programs are associated with slower population growth across countries, but contrary to expectation, family planning is unrelated to demographic changes occurring over the last two decades in the less developed countries.