Before the stable population can be fully characterized, the age-specific fertility rates needed to satisfy equation (1-4a) must be determined. For most anthropological data (and certainly for archaeological data) this information is not available. A method must be devised to estimate the rates of B(X), based on the assumption of a growth rate r and the mortality information.

The relative amount of fertility in females at each age during the childbearing period follows a generally similar pattern for most human populations: fertility begins at a relatively low rate at the age of menarche—in fact, there is generally a period of adolescent sterility (Montagu 1957), then it rises rapidly to a maximum from age 20 to age 30, after which it falls gradually to a minimum just before menopause. This general pattern is virtually universal among human females. We can use this degree of similarity to model our anthropological fertility rates.