Sex ratio at birth was studied in relation to grandparental age, to enquire into the effect of the accumulation of X-linked recessive lethal mutations. From the data of the tenth census of the Italian population two samples of women having a live-born only child were examined: the first from some provinces of northern and central Italy, the second from the island of Sardinia.
Besides maternal grandfather's and grandmother's ages, the mother's age and some socioeconomic factors such as the woman's educational level, her urbanization index and time were included in a multiple regression analysis model. In both areas the results show a significant decrease of the sex ratio as grandpaternal age increases, a weaker effect of grandmaternal age, and a strong decrease when maternal age increases; this last effect is probably due to physiological factors, in addition to the genetic ones.
Differences between the partial regression coefficients of the two areas, though not significant, reflect the different socioeconomic and cultural conditions, which affect the sex ratio. From the partial regression coefficients on the grandpaternal age an estimate of the mutation rate was obtained.