On generating birth rates from skeletal populations.

Sattenspiel and Harpending (1983, American Antiquity 48(3): 489-498) have stated that the life expectancy at birth (e0(0] which paleodemographers calculate from skeletal population data is actually the mean age at death (ad) of the population. Yet, only when a population is neither growing or declining (i.e., is stationary) are these two statistics equivalent. They further assert, that the mean age at the death (ad) is more accurately interpreted as a measure of the fertility of the population. While we support their statement that since paleodemographic calculations use skeletal evidence of death, these do not a priori produce life expectancy values, we disagree that the inverse of the birth rate is a substitute for the average age at death (ad). The following pages demonstrate that: 1) An exact expression for the relationship between ad and 1/b can be derived using standard stable population theory, wherein ad = 1/b is shown to be a special case. 2) There are only two cases when ad = 1/b is an identity. 3) Whereas empirically ad and 1/b appear to correspond closely, this is an artifact of heavy mortality at early ages, which is a characteristic of the populations being considered. 4) Without insights into the behavioral dynamics of the situation any assessment of the demographics of the population is questionable.