Survivorship of children is dependent upon numerous variables, including the role that preferential treatment may play in biasing the birth and survival of sons and daughters across cultures. This study draws upon an evolutionary approach by examining a theory referred to as the "Trivers-Willard hypothesis" concerning condition-dependent sex allocation and differential parental investment. 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 behavioural aspects affecting maternal condition, sex allocation, and parental investment in humans. The units of analysis for this study were the mothers and their lastborn child (N=128,039 woman-child pairs). A series of hierarchical regressions were executed to empirically investigate the TW hypothesis in humans. Scales were developed for maternal socioeconomic resources (MSR), maternal biological condition (MBC), prenatal care for the lastborn (PCL), and health-seeking for the lastborn child (HSL). MSR was measured by relative household economic status, woman's and partner's education, and residence in an urban/rural setting. MBC was defined by body mass index, pregnancy status and duration, and breast-feeding status. PCL was an index for type of prenatal care received, number of prenatal visits, and assistance during delivery of the lastborn child. Lastly, HSL measured indicators of treatment for diarrhea and immunizations received by the lastborn child. Across the 35 countries, the analyses did not support the Trivers-Willard hypothesis. However, there is evidence of regional and country level differences.