Sex ratio and parental age gap.

We tested the hypothesis that a large age difference between parents can shift the sex ratio at birth in favor of males, as Manning et al. (1997) suggested in their analysis of English and Welsh data. Among children born in Lombardy (northern Italy) in 1990 and 1991, we observed an anomalous excess of males born to a particular subsample of parents with a wide age gap (> 15 yr) between them; in the overall sample the father-mother age gap does not significantly contribute to the determination of the child's sex.