Introduction: evolutionary theory and the search for a unified theory of fertility.

Demography and evolutionary biology share common origins but have divergent emphasis on the role of theory in understanding population phenomena. A unified theory of fertility would be beneficial in explaining variation in demographic characteristics across geographic and temporal gulfs and in integrating disparate perspectives. The six papers in this thematic collection represent a nascent but vital field: human evolutionary demography. These papers examine the ways in which evolutionary theory can inform, strengthen, and focus research on topics of long-standing interest to demographers by explicitly modeling the relationship of socioecological variables to life history traits. The papers demonstrate that an understanding of human life history evolution and the use of evolutionary theory as an organizing framework can lead to a productive reassessment of five areas, which are of long-standing concern to demographers, and which conventional demographic approaches have had limited success in understanding. These are conflicts of interest between parents and children and between men and women, the allocation of resources to competing and/or alternative forms of investment in reproduction and parenting, resource flow within the household, demographic transitions and particularly the fertility transition associated with economic development, and variation in life history characteristics such as fertility and mortality across populations. Future research integrating models of trait-environment correlation with models of individual information processing and decision-making will help identify areas of focus, revitalize current models, and play a leading role in the development of a unified theory of fertility applicable across societies and times.