Trends and implications of variability in demographic rates
Keywords: Humanities and Sciences.
Biology.

This thesis analyzes the impacts and trends of demographic transitions, with an emphasis on the role of variability in demographic rates. The thesis investigates two types of variation: the variation across age for mortality and fertility, and the variation across quantity for fertility. The research employs diverse demographic and statistical methods to examine not only the human population but also other animal and plant species. The thesis covers a wide range of topics, ranging from population-level to individual-level analyses, including population resilience, kinship networks, and individual financial decision-making. Chapter 2 focuses on the variation in age at death for the human population and its impact on the decision to claim a pension. This analysis is conducted for both individuals and couples based on modern portfolio theory, using the U.S. Social Security system as an example. Chapter 3 moves to the variation in age at reproduction and expands the scope by including a wide range of species and multiple life history traits. Specifically, this chapter investigates the covariation of the mean and variation in the age of reproduction (measured by generation time and reproductive dispersion), and population resilience (measured by damping time) based on phylogenetic generalized least squares regressions and phylogenetic principal component analyses. Chapter 4 contributes to fertility research by examining the distribution of lifetime reproductive success (LRS) to gain insights into the individual variability of reproductive outcomes. The findings provide novel perspectives on fertility trends, emphasizing the influence of fertility propensity and female survival on reproductive outcomes. This analysis also sheds light on factors contributing to childlessness and the changing patterns of fertility in U.S. cohorts. Chapters 5 and 6 delve into an extensive analysis of kinship networks, investigating the dynamics of kinship networks in response to changes in fertility and mortality. In a time-invariant setting, Chapter 5 finds a quadratic relationship between the net reproductive rate and the number of living kin when the specified individual Focal is at demographically dense ages. This relationship offers a reasonable approximation of kinship size under demographic transitions. Chapter 6 extends the analysis to a time-varying framework and explores the effects of the speed of demographic transitions on both the number and age composition of kin. Both these chapters show a significant effect of the change in the age pattern of fertility and mortality rates (or the variation across age) on kinship networks. Overall, by developing a deeper understanding of the complex mechanisms underlying fertility and mortality change, this thesis aims to understand the dynamic of kinship networks under the demographic transition and identify strategies for improving individual and population outcomes in an uncertain world. For humans, this involves examining outcomes such as the age at which individuals claim a pension and the fiscal stability of pension systems. For other species, the focus is on understanding population responses to changing environments as well as different life history strategies