Date of Award


Degree Type


Degree Name

Master of Science



Major Professor

Ken Orvis

Committee Members

Shih-Lung Shaw, Anita Drever


It has been hypothesized that humans may exert facultative, adaptive control over their sex of their offspring through the action of the endocrine system. No conclusive evidence of this has been found, although varying hormonal levels in parents at the time of conception may partly influence the sex of the child (James 1986, 1987b, 1999). A decline in the human sex ratio at birth (SRB) observed in the U.S. and some other countries has been attributed by some investigators to widespread environmental exposure to endocrine disrupting chemicals.

The many factors hypothesized to influence the SRB make testing this attribution difficult, but one suggestion has been to explore the geographic and temporal pattern of SRB to determine if a sentinel health event signaled by abnormal SRB is present (Davis et al. 1998). This thesis explores the possibilities of geographic analysis of SRB at various scales, focusing on the local geographic scale of the U.S. county to determine whether patterns of explainable variation exist. It tests the basic geographic-patterning assumption, the hypothesis that hormonal mediated influences such as local socioeconomic conditions, adult reproductive sex ratio, urban versus farm environment, and racial composition may influence the SRB, and looks for posited geographic patterning that might be indicative of hormonally active agents working in the human environment.

This set of hypotheses is tested in univariate and multivariate logistic regression models combining complete U.S. individual birth record datasets for 1970, 1980 and 1990 with selected U.S. Census county-level statistics that were chosen to represent hypothesized socio-environmental, hormonally mediated influences. Separate models were constructed for white and black births, and variables of birth order, plurality, and seasons of birth were included in multivariate models to control for these confounding individual influences on the SRB.

Results show that geographic patterning is strongly evident at the county level and this approach in general works well to elucidate the influence on SRB of these external hormonally medicated factors. SRB in white populations significantly decreased with increases in county urban population proportion in 1980 and 1990 and with increases of the percentage of families living below the poverty line in 1970 and 1990. The change in odds ratio for white male births was barely detectable, however, and was less than that found for individual characteristics such as birth order and plurality. Black population SRB was not as influenced by externally hormonally mediated factors as white SRB, except in 1980. Little clear evidence for the presumed effects of endocrine disruptors was found.

The results support further study of externally hormonal mediated influences on the SRB at local geographic scales. In particular, geographic patterning is strongly evident but varies locally in magnitude and sign, and spatially in pattern, between sampling dates. This suggests that not all significant factors are accounted for in this analysis, and that more work needs to be done to weigh the independent influences of individual biological factors and those external factors that might vary with changes in social, economic, and age-distribution conditions. A significant influence of SRB seasonality in the 1970 sample year also suggests that changes in temperature, light, rainfall patterns and other environmental signals that might stimulate hormonal influence of the SRB should be explored.

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