The Evolutionary Demography of Human Fertility

Demography is the study of population processes including births (fertility), deaths (mortality), and migration. These processes are critical to an understanding of evolution since natural selection is a product of differential fertility and mortality.  Yet evolution has also shaped the physiological, psychological, behavioral, and social processes by which all organisms—including humans—reproduce, age, die, and seek new territories.  Our work focuses on the demography of modern humans using the lens of evolutionary theory, seeking to understand how contemporary patterns of fertility (when people have children and how many children they have), and related decisions about how to invest in each child, compare to the past, as well as why—and when—such patterns have changed.

Previous work, including an NSF-funded project in rural Bangladesh, has focused on the question of the demographic transition—why human fertility rates have declined so rapidly over the last two centuries. It also yielded analyses on the effects of father absence on age at marriage and first birth, the causes of variation in family-level sex ratios, the patterns of and motivations for consanguineous marriage, and the policy implications of our results. A subsequent working group funded by SAR and NESCent led to a special issue of Philosophical Transactions of the Royal Society B including new theoretical work on the demographic transition and other topics related to human fertility.

Current research in this area, pursuing the themes of the original grant and working group, includes articles on the demographic transition, inheritance, and perceptions of mortality. A second project focuses on the psychological mechanisms underlying fertility decision-making using experimental methods and data from both university students and smaller cross-cultural populations around the world. This is a very ambitious endeavor, including a large number of collaborators and a strong focus on methodological rigor, and is likely last for several years.