Dr. Mary Shenk (University of Missouri) and Dr. Mary Towner (Oklahoma State University) will use nested datasets from Matlab, Bangladesh, to examine evolutionary and comparative models of the demographic transition, a global phenomenon in which high fertility and mortality are replaced by low fertility and mortality. Most research links the demographic transition to economic development, but the underlying mechanisms still inspire much interdisciplinary debate; as a unifying framework, evolutionary theory shows great potential for helping to clarify both the motivations and mechanisms involved. The goals of the project are to advance theoretical and empirical understanding of the demographic transition using evolutionary theory, and to provide the first comprehensive comparative analyses of key evolutionary and non-evolutionary models. These goals will be met through three specific aims: (1) use large-scale, longitudinal data from Bangladesh to evaluate three influential evolutionary models of the demographic transition, focusing on risk, parental investment, and cultural evolution; (2) use an evidence-based statistical approach to compare evolutionary models with their non-evolutionary counterparts; and (3) collect detailed new data on a subsample of the Matlab population to provide in-depth information to facilitate the comparison of multiple models.
The demographic transition will have significant social impacts in the 21st century. High birthrates hinder economic development in poor nations, while population aging and low fertility undermine safety nets and contribute to social tensions in developed nations. Evolutionary approaches have the potential to revolutionize the study of demographic behavior, yet work in evolutionary demography has been limited in scope and is only familiar to a small community of scholars. This project aims to alter this pattern by testing evolutionary models using population-level data, comparing them to models from other fields, and publishing the results across disciplines. The project will address policy implications by disseminating findings to the study population, policy journals, and the development work community. It will also provide research training to students and forge durable ties between collaborating institutions.
What are the causes of the demographic transition, an ongoing global phenomenon wherein high fertility and mortality rates are gradually replaced by low fertility and mortality rates? This question has inspired decades of interdisciplinary debate. The demographic literature usually links the transition, which became widespread in the 19th and 20th centuries, to the financial, social, and technological changes associated with development. The causal mechanisms underlying it, however, are still largely unresolved. As a unifying framework, evolutionary theory shows great potential for helping to untangle such mechanisms. The primary purpose of our project was to advance theoretical and empirical understanding of the demographic transition using the perspective of evolutionary theory and to determine what utility we gain from this perspective in comparison to other approaches. Our project has also supported numerous graduate and undergraduate researchers investigating related research questions about the relationships between fertility and father absence, sex ratios, childhood mortality, birth spacing, and breastfeeding. Moreover, our field research is ongoing and will be further detailed in a companion project summary (Shenk, 2015). Evolutionary theory offers an important addition to economic and demographic studies of human behavior. It provides a framework for understanding why humans behave in certain ways rather than others, with the resulting ability to unify studies of many different types of behaviors in many different societies under one theoretical rubric. The relevance and potential importance of evolutionary approaches are especially apparent in areas of study that relate to reproduction. How many children a person should have, how much care each child should receive, and how social and ecological factors affect these decisions are questions about which evolutionary theory provides clear predictions and which evolutionary anthropologists have been exploring for years. Our project included three specific aims: 1) rigorously evaluate three influential evolutionary models of the demographic transition, 2) compare evolutionary and non-evolutionary models of the demographic transition, and 3) collect detailed new data to augment an existing large, longitudinal, mid-transition dataset To address these aims, we combined a large, existing dataset with a smaller, more detailed new dataset collected specifically for the proposed research. Population-level demographic data came from an existing longitudinal dataset collected and held by the International Center for Diarrheal Disease Research, Bangladesh (ICDDR,B). Drawn from a complete sample of the population of rural Matlab, Bangladesh, the data cover a period of forty years during which total fertility rates fell by more than 3.5 children per woman and mortality rates decreased by 50-70% (ICDDR,B 2003, 2008). We augmented this dataset with detailed new records on a subsample of the Matlab study population, including (a) open-ended interviews used for theme analysis as well as contextual and interpretive information, and (b) a survey of 944 women collecting additional family and individual level information. Each survey and interview was linked to the demographic data in the full sample. The combination of longitudinal, large-sample demographic data with detailed, individual-level data is unique in allowing simultaneous, comparative tests of the primary evolutionary and non-evolutionary models of the demographic transition. This comparison of models of the demographic transition has already been a key result of our project. 1 The literature is crowded with competing theories, including causal models that emphasize (a) mortality and extrinsic risk, (b) the economic costs and benefits of investing in self and children, and (c) the cultural transmission of low-fertility social norms. We use detailed demographic data from recent fieldwork to determine which models produce the most robust explanation of the rapid, recent demographic transition in rural Bangladesh. To rigorously compare models, we use an evidence-based statistical approach employing model selection techniques derived from likelihood theory. This approach allows us to quantify the relative evidence the data give to alternative models, even when model predictions are not mutually exclusive. Results indicate that fertility, measured as either total fertility or surviving children, is best explained by models emphasizing economic factors and related motivations for parental investment. Our results also suggest important synergies between models, implicating multiple causal pathways in the rapidity and degree of recent demographic transitions. The demographic transition is an important global phenomenon likely to have significant long-term impacts on the world in the 21st century. High birthrates are a key deterrent to economic development in less developed nations, while population aging combined with below-replacement fertility is beginning to undermine social safety nets and contribute to social tensions in more developed nations. An improved comparative understanding of the causes of the demographic transition is necessary if we are to effectively shape population policy for both the developing and the developed world. 1 Shenk, M. K., Towner, M. C., Kress, H. C., & Alam, N. (2013). A model comparison approach shows stronger support for economic models of fertility decline. Proceedings of the National Academy of Sciences, 110(20), 8045-8050.
