Why did demographers begin studying fertility more closely in industrialized countries during the late 1800s?

For most of the 19th century, demographic studies primarily focused on mortality. Demographers began to study fertility and reproduction with equal interest only after observing a significant decline in birth rates across industrialized nations during the second half of the 1800s.1

This shift in focus was driven by the emergence of differential fertility, which raised significant questions regarding social selection and the evolution of intelligence—a topic that garnered widespread interest through the works of figures such as Charles Darwin and Francis Galton.1

While early demography was rooted in the analysis of vital statistics and mortality tables, the industrial era necessitated a deeper understanding of why families in urbanized, industrial societies were voluntarily choosing to have fewer children. This inquiry eventually contributed to the development of demographic transition theory, which sought to explain how socioeconomic factors like urbanization, increased literacy, and the changing economic value of children influenced these fertility trends.213