On the Implications of Age-Specific Fertility for Sibships and Birth Spacing

Abstract

Demographic analyses of fertility customarily focus on age-specific birth rates, implicitly assuming that, at every age, fertility is the same for women of every parity. This chapter looks at the implications of that common assumption, and finds that they are far-reaching. From a set of age-specific birth rates, one can determine the likelihood that sibships of any size and gender composition arise in a cohort. Calculations for several fertility levels and a variety of sibship constellations are given. Further, overall cohort experience can be divided into subcohorts based on their ultimate number of children, and those subcohorts can be further broken down into the pathways by which that ultimate parity is reached. Such an analysis allows a detailed examination of fertility timing and birth spacing. Illustrative calculations indicate that higher parity subcohorts have children of every birth order earlier and move to higher parities faster than lower parity subcohorts. Because of the sequential nature of fertility, parity homogeneity casts a long shadow.