Evolutionary Demography

Evolutionary demography has grown rapidly in recent years, as the biological topics of life history evolution and evolution in population with complex life cycles have benefitted from and contributed to a broader focus on evolutionary biodemography. This chapter provides a critical summary of the central ideas and methods. The authors emphasise theoretical methods, starting with the main ideas that have attracted attention in the field, the assumptions behind these, and efforts to relax those assumptions, and provide a short account of some new directions. The chapter begins with the classic work of Peter Medawar and William Hamilton and discusses the connections, applications, assumptions, and limitations related to their ideas and results, e.g. sensitivity and corresponding elasticity of growth rate on fertility and survival. It highlights extensions to variable environments and the large body of theory around that topic. Next the chapter discusses how these theoretical methods are related to analyses and theories of post-reproductive life, via the general concept of ‘borrowing fitness’. Finally, the chapter discusses nonlinear models of mutation and selection and density-dependent models.

Birth order and gender : differential parental investment in Medieval England and France in the eleventh, twelfth, and thirteenth centuries

This dissertation addresses the question: will parents invest differently in their children based on gender and birth order? Using parental investment theory and four major sets of outcome variables--child survival, parental investment (through wealth, land, and titles), marriage, and reproductive success--this question will be examined in an historic population, medieval England and France in the eleventh, twelfth, and thirteenth centuries. The results presented in this dissertation shows that parents were willing to invest in their offspring differentially with a preference for sons over daughters and older children over younger children. Historic populations provide a microcosm in which to study human behavior. The findings in this dissertation have implications for both evolutionary ecology and evolutionary demography.

Financial opportunity costs and deaths among close kin are independently associated with reproductive timing in a contemporary high-income society

Evolutionary demography predicts that variation in reproductive timing stems from socio-ecologically contingent trade-offs between current and future reproduction. In contemporary high-income societies, the costs and benefits of current reproduction are likely to vary by socioeconomic status (SES). Two influential hypotheses, focusing on the parenthood ‘wage penalty’, and responses to local mortality have separately been proposed to influence the timing of parenthood. Economic costs of reproduction (i.e. income loss) are hypothesized to delay fertility, especially among high childhood SES individuals who experience greater opportunities to build capital through advantageous education and career opportunities. On the other hand, relatively low childhood SES individuals experience higher mortality risk, which may favour earlier reproduction. Here, we examine both hypotheses with a representative register-based, multigenerational dataset from contemporary Finland ( N = 47 678). Consistent with each hypothesis, the predicted financial cost of early parenthood was smaller, and mortality among close kin was higher for individuals with lower childhood SES. Within the same dataset, lower predicted adulthood income and more kin deaths were also independently associated with earlier parenthood. Our results provide a robust demonstration of how economic costs and mortality relate to reproductive timing. We discuss the implications of our findings for demographic theory and public policy.