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A significant decline in the proportion of male births has been observed in recent decades in several western countries, and several researchers have raised the question of the possible role of environmental pollution in this trend. In the present analysis, the authors examined the effect of environmental factors (pollution from pesticides, urban pollution, and industrial pollution) on the sex ratio (proportion of males) of Italian singleton 1st births for the years 1989-1993, controlling for the age of the mother. The data show a slight increase in the sex ratio from north to south; however, no reduction in the proportion of males was seen in areas with higher pesticide consumption or intense industrial pollution. A small reduction in sex ratio was found in the major urban centers. The analysis was based on individual information on birth order and maternal age, as well as on ecological data on environmental pollution. No significant association was found between environmental pollution and the proportion of male births in Italy during the study period., IN HUMANS, THE SEX RATIO at birth is subject to secular variations for reasons not currently understood. (1) The question has recently stimulated considerable scientific interest, because a significant decline [...]