Lost Daughters and Fragile Sons: Patterns of Differential Parental Investment Across Thirty-five Countries

Survivorship of children is unsurprisingly dependent upon numerous variables, not least of which is the role that preferential treatment plays in biasing the birth and survival of sons and daughters across cultures. This study draws upon an evolutionary approach by examining the "Trivers-Willard hypothesis" concerning condition-dependent sex allocation and differential parental investment. The central idea is that within a polygynous social mating structure - where reproductive variance is higher for males than for females as an intrinsic function of polygyny - mothers in optimal condition (defined by high status, good health, and abundant resources) are more likely to produce and invest in male offspring whereas mothers in poor condition (defined by low status, poor health, and resource deprivation) are more likely to produce and invest in female offspring so as to maximize potential lifetime reproductive success. Previous research on humans concerning this hypothesis tends to be restricted to one cultural group and thereby limited in sample size. For this study, nationally representative household survey data collected by the Demographic and Health Surveys (DHS+) program across 35 countries was used to test biological, resource-oriented, and behavioral aspects affecting maternal condition, sex allocation, and parental investment in humans. Country samples ranged from 732 to 21,839 women interviewed within South Asia, Sub-Saharan Africa, the Caribbean/Latin America, and the Near East/North Africa. The units of analysis were the mothers and their lastborn child (N = 128,039 woman-child pairs). A sequence of hierarchical regressions theoretically pre-specified a causal model concerning four constructed scales measuring maternal socioeconomic resources, maternal biological condition, prenatal care for the lastborn child, and health-seeking for the lastborn child. In startling contrast to the predictions of the original hypothesis, analysis of the overall model revealed small, yet stable, cross-regional main effects suggesting that - for all four regions – maternal biology predicts lastborn daughters while maternal resources predict lastborn daughters for each region, with the exception of Sub-Saharan Africa. Moreover, protective/preventative health-related behaviors predict lastborn sons within South Asia and the Near East/North Africa, while prenatal care and health-seeking are differentially attributed to the prediction of sons and daughters within Sub-Saharan Africa and the Caribbean/Latin America