Factors affecting the sex ratio in humans: multivariate analysis of the Italian population.

Several studies have shown that the human secondary sex ratio is affected by a wide range of biological and environmental factors. Here, we describe a partitioning of the sex ratio variability as observed in the Italian population over the last two generations. This period has seen drastic changes in the environmental conditions of Italy. As a consequence, demographic and biological variables that can affect the sex ratio have also changed dramatically. In an attempt to isolate any specific effect, we used a stepwise multiple regression to analyze the covariation over time of the sex ratio and of relevant parameters, such as stillbirth rate, maternal age, firstborn proportion, and birth order. The results show that a quadratic function of the firstborn proportion and mother's age is a fairly good predictor of sex ratio values.