The proposed research will investigate correlates and determinants of fertility differentials in 1900 among the U.S. population. At the time, the American population was in a transition from high to low fertility and was characterized by substantial internal variation in reproductive patterns. The basic data source will be a sample of 100,924 individuals which were sampled from the U.S. census manuscripts. The research will determine the major social and demographic predictors of fertility, test multi-variate causal models of fertility variation, and study differentials in fertility among nationality groups as an aspect of immigrant assimilation.