Analysis of the human sex ratio by using overdispersion models.

Data from nineteenth-century Germany are reanalyzed to determine how the probability for having a child of each sex changes with family size. The data, collected by A. Geissler, concern some 1 million birth registrations and 3.7 million births occurring in Saxony between 1876 and 1885. Three models are fitted to the data. "The multiplicative and beta-binomial models provide similar fits, substantially better than that of the double-binomial model. All models show that both the probability that the child is a boy and the dispersion are greater in larger families. There is also some indication that a point probability mass is needed for families containing children uniquely of one sex." excerpt