Population homeostasis in sub-Saharan Africa.

Global population growth remains one of the major challenges of the twenty-first century. This is particularly true for African countries which have been undergoing their demographic transitions. To investigate whether predicted increasing population density and urbanization can help to stabilize African population, we construct a database for 84 georeferenced Demographic and Health Survey (DHS) samples including 947,191 individuals in sub-Saharan Africa and match each location with gridded population density from NASA. We apply a proportional hazard model to evaluate the quantitative impact of local population density on the transitions from childlessness to motherhood, and from celibacy to marriage. Moving from the 5th to the 95th percentile of population density increases the median age at first birth by 2.2 years. This roughly decreases completed fertility by half a child. The same increase in population density increases the median age at first marriage by 3.3 years. These findings contribute to the understanding of why fertility has not dropped in Africa as fast as expected. One part of the answer is that population density remains low. Yet the total effect of increased density on fertility remains limited and counting on it to stabilize the population would be unrealistic.

Leaders and Laggards in Life Expectancy Among European Scholars From the Sixteenth to the Early Twentieth Century.

When did mortality first start to decline, and among whom? We build a large, new data set with more than 30,000 scholars covering the sixteenth to the early twentieth century to analyze the timing of the mortality decline and the heterogeneity in life expectancy gains among scholars in the Holy Roman Empire. The large sample size, well-defined entry into the risk group, and heterogeneity in social status are among the key advantages of the new database. After recovering from a severe mortality crisis in the seventeenth century, life expectancy among scholars started to increase as early as in the eighteenth century, well before the Industrial Revolution. Our finding that members of scientific academies-an elite group among scholars-were the first to experience mortality improvements suggests that 300 years ago, individuals with higher social status already enjoyed lower mortality. We also show, however, that the onset of mortality improvements among scholars in medicine was delayed, possibly because these scholars were exposed to pathogens and did not have germ theory knowledge that might have protected them. The disadvantage among medical professionals decreased toward the end of the nineteenth century. Our results provide a new perspective on the historical timing of mortality improvements, and the database accompanying our study facilitates replication and extensions.

Fertility and Childlessness in the United States.

We develop a theory of fertility, distinguishing its intensive margin from its extensive margin. The deep parameters are identified using facts from the 1990 US Census: (i) fertility of mothers decreases with education; (ii) childlessness exhibits a U-shaped relationship with education; (iii) the relationship between marriage rates and education is hump-shaped for women and increasing for men. We estimate that 2.5 percent of women were childless because of poverty and 8.1 percent because of high opportunity cost of childrearing. Over time, historical trends in total factor productivity and in education led to a U-shaped response in childlessness rates while fertility of mothers decreased.