The evolutionary demography of sex ratios in rural Bangladesh

In this chapter, we use evolutionary models of sex ratio variation to examine offspring sex ratios in Matlab, Bangladesh, from the 1960s to 2010, during which time sex ratios have shown a decreasing male bias. Evolutionary models lead us to examine particular aspects of family ecology, yielding predictions both unique from and similar to those proposed in the demographic literature. We examine three evolutionary models-the costs of reproduction model, the Trivers-Willard hypothesis, and the local resource competition and enhancement models. Our results support both the Trivers-Willard and local resource competition/enhancement model, but results for the costs of reproduction model are weak. In general, we find that variables associated with higher wealth, status, and engagement in agriculture are linked to more male-biased offspring sex ratios, while higher fertility, older age at marriage, and higher women's education are associated with less male-biased sex ratios. We also examine how Bangladesh fits into the larger cultural area of South Asia known in the press and policy circles for its high sex ratios. We compare the correlates of sex ratio in Matlab to those in other parts of the region and discuss why sex ratios in Bangladesh are less male biased, despite shared cultural characteristics, such as son preference and dowry, which some argue "cause" high sex ratios in parts of India and Pakistan. We conclude with a discussion of the utility of evolutionary models and offer policy recommendations for the region.