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.

The Trivers-Willard hypothesis predicts a sex ratio bias contingent on maternal condition in species characterized by variation in male reproductive success. A male-biased sex ratio among mothers in good condition, and a female-biased sex ratio among mothers in poor condition is expected. Studies in humans have thus far provided mixed answers to the question of whether or not sex ratio is affected by maternal condition. The present study assessed whether or not the introduction of a western cash economy influenced the observed secondary sex ratio in Nepal's Khumbu region. Because acculturated villages provided better access to the cash economy and to health facilities, residence in an acculturated village was used as a proxy for "good" maternal condition. I analyzed demographic data gathered by survey in 1971 and 1982. The sample included 734 children from the 1971 survey and 1598 children from the 1982 survey. Using Poisson regression I analyzed the extent to which the sex ratios in age-stratified groups differed between the acculturated and unacculturated villages. In the 1971 dataset, the younger women in the acculturated villages displayed a significantly higher (p=.014) proportion of male offspring. It is likely that older women were subjected to minimal acculturation effects during their child-bearing years and among these data there was a lack of significant deviation between acculturated and unacculturated post-menopausal women. The rapid overall increase in acculturation between 1971 and 1982 likely made conditions in the two sets of villages much more similar by 1982. The results of this study underscore the impact that the transition to a market economy had on women in Nepal's Khumbu region. / Anthropology

The Trivers-Willard hypothesis (TWH) states that parents in good conditions will bias the sex ratio toward sons and parents in poor conditions will bias the sex ratio toward daughters. The present study contributes to literature in several ways: a large, general, country population data set (N= 1 401 851) from modern contemporary society; first study in the Czech Republic; an inclusion of air pollution into the TWH estimation; and a more detailed focus on stillbirths. With the natality microdata from the Czech Statistical Office and data concerning the level of air pollution in the Czech Republic from the Czech Hydrometeorological Institute, I analyze if the biological and socio-economics status of mothers and the characteristics of our surroundings (air pollution) affect the sex of children. The results are insignificant or not robust across specifications. I identified three hypotheses which are most likely the reason for the insignificant results: a non-inclusion of the biological and socio-economical status of a father, insufficient diversity or evolutionarily novel environment in the Czech Republic. As a conclusion, the presented evidence suggests that stillbirths are random in the Czech Republic and that the sex ratio is not affected by the socio-economics status of mothers or the characteristics of our surroundings (pollution).