Secular movements in sex ratios of adults and of births in populations during the past half-century.

There is some evidence for a small overall negative correlation across populations between sex ratio (proportion male) at birth and adult sex ratio. There seems to be no systematic correlation within populations across time of sex ratio at birth with adult sex ratio during the past 50 years. So even if adult sex ratios play some part in determining the overall level of the sex ratio at birth, they apparently have played little role in the recent widespread secular changes in sex ratio at birth. It is shown here that there is a strong cohort effect in adult sex ratios: if a woman is in a marriage squeeze (i.e. in a cohort with a relative abundance of women) at age 15 years, she will remain in such a squeeze for the rest of her reproductive life. In England and Wales, the maternal age-specific sex ratios at birth moved roughly in parallel across time during the years 1950-1995. This suggests that sex ratio at birth is not a cohort phenomenon (as it would be if it were affected by adult sex ratio) but is subject to some agents which change with time and affect women (parents) of all ages roughly equally.