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BACKGROUND: Natural and man-made disasters as well as declining economies appear to coincide with reduced odds of male live births among humans (i.e. lower secondary sex ratio). This association has been attributed to excess death of males in gestation and to reduced conception of males. We attempt to empirically discriminate between these two attributions by testing the hypotheses that the attacks of September 11, 2001 were followed in California first by higher fetal death sex ratios and later by lower sex ratios among very low weight births and total live births. METHODS: We apply interrupted time-series methods to the fetal death, very low birth weight, and secondary sex ratios. The methods control for trends, seasonal cycles, and other forms of autocorrelation that could induce spurious associations. RESULTS: Findings support the excess death explanation in that the fetal death sex ratio reached …