The causal machinery underlying sex determination is directly relevant to many questions relating gender and family composition to social and economic outcomes. In recent work, Oster (2005) highlighted a correlation between parental hepatitis B carrier status and child sex. One of the results in her original paper was especially important, because it spoke directly to causality, providing evidence that hepatitis B causes male-skewed sex ratios at birth. Oster's original paper generated several reevaluations that have done away with one aspect of her findings, but the specific result speaking to causality has never been reevaluated and was never retracted. This note finally settles the one remaining discrepancy between the original paper and the reevaluations it inspired, by presenting evidence that the causal analysis in the original paper was erroneous. Understanding sex determination is critical for social science research, as well as an end in itself in biology and medicine. However, my results suggest that it is extremely unlikely that chronic Hepatitis B infection plays any causal role; social scientists would be well advised to direct their attention to more plausible, more relevant, and better established hypotheses.