Mary K. Shenk: University of Missouri Department of Anthropology

Evolutionary anthropologists have given significant attention to the global phenomenon of the demographic transition, especially the remarkable decreases in fertility that characterize it. The literature is crowded with competing theories and sub-theories, and scholars often call for more comprehensive, better-controlled studies that would allow us to distinguish between competing causal models—yet only limited comparative work has previously been done. This paper compares evolutionary models emphasizing decreasing risk, changing motivations for parental investment, and changing forms of cultural transmission as motivations for fertility decline. The goal is to determine which model, or combination of models, produces the most robust explanation of a rapid, recent demographic transition in rural Bangladesh.

Models are compared using an evidence-based statistical approach employing model selection techniques derived from likelihood theory. This approach allows quantification of the relative degree of support the data give to alternative models, even when model predictions are not mutually exclusive. Data come from a new survey conducted in rural Bangladesh in 2010 designed specifically for comparative testing. Results suggest that models emphasizing changing motivations for parental investment, including increasing payoffs to investment as well as increasing costs of and competition between children, are the best predictors of fertility decline. Yet findings also indicate that a full explanation involves variables from multiple models, and that important causal synergies underlie key predictors such as education, suggesting that multiple causal pathways are likely to be implicated in the rapidity and degree of recent demographic transitions.