Changes in the human sex ratio at birth—defined as the percentage of newborns that are boys—are associated with the presence of air and water pollutants, but are not predictably associated with seasonality or weather, according to a new study of more than 6 million births in the US and Sweden. The study, led by Andrey Rzhetsky of the University of Chicago, is publishing December 2nd in the journal PLOS Computational Biology.