| Abstract: |
Argue that the age-at-death in a prehistoric cemetery was more likely to reflect changes in fertility and population growth than in mortality so that the "life expectancies" derived from counting the dead in cemetery populations told us little about mortality or the real life expectancy of individuals in the population. Shows that mean skeletal age is approximately equivalent to the reciprocal of the birth rate and is not correlated with the death rate. Thus when, in a temporal series, mean skeletal age increases it means that the birth rate decreased, not that the epxpectation of life increased. In a stationary population the mean age at death is equal to the inverse of the birth rate and to the inverse of the death rate as well as to the expectatio of life at birth. Skeletal age reflects expectation of life only if the death rate happens to be precisely equal to the birth rate. This not likely. pg 491 Mean skeletal age depends only on the proportion of individuals of the entire sample (which may span several generations) who die at a given age, not on the proportion of individuals of a given age (the birth cohort) who die at a particular time. The resulting error from using mean age at death to estimate expectation of life depends primarily on the age structure of the population and hence the birth rate. pg 492 In order to calculate the expectation of life at birth in a nonstationary population it is necessary to have some estimate of the total size of each living cohort. When dealing with skeletal samples that span several generations, the assumption of the stationary population more justified. As Weiss (1973) and Weiss and Simouse (1976) have shown, many skeletal populations are sampled from very long time periods, thus smoothing out stochastic variations in the age distribution of the population. pg 493 Since skeletal age is not related to mortality, evidence for decreased mortality rates must come from sources other than mean age at death. Given the nature of archaeological record, the only sources will probably be studies on the quality of the environment and the availability and quantity of food. Studies on the narue of infectious disease in human populations have shown that with the advent of agriculture and sedentism the incidence of infectious diseases increases that with the advent of agriculture and sedentism the incidence of infectious diseases increases. When a population becomes sedentary, mortality does in fact decrease despite increased morbidity due to disease. Presumably this is due to the availability of more reliable and better food among the sedentary Bushmen and to additional demands imposed upon the immune system of mobile Bushmen by constant exposure to new disease strains. pg 493-494 There are two schools of thought on the reasons for higher growth rates with the shift to an agricultural subsistence. The first is that the increased growth rates were primarily the result of an increase in the fertility levels of the populations. This article indicates that birth rates agter the transition were lower than those of the Upper Paleolithic and do not support this view. Sample error may be factor. The second view is that the increased growth rates were primarily a result of a decrease in the mortality rates These arguments are based on assumptions about the effect of sedentism, such as a decrease in the level of infant mortality, not on actual esitmates from data from the archaeological record. This does not imply that growth rates increased
the increase in density may be due simply to the maintenance of a constant growth rate. pg 495 The increase in mean age at death provides no information about expectation of life at birth or death rates. Evidence of a decline in health are seen in th epopulations studied by Angel (1971:111), even though the mean age at death has increased. The observation is consistent with the veiw that mean age at death is not correlated with the death rate. This model shows that "expectation of life at birth" is actually the mean age at death and is nearly independent of the death rate and the real expectation of the life at birth. Consequently, conclusion about the general level of mortality of a skeletal population derived from mean age at death are unreliable and meaningless. On the other hand. the interpretation of the mean age at death as the reciprocal of the birth rate provides insights about fertility or populations, and in some cases reconciles previous inconsistencies between inferences based upon skeletal samples and inferences based on other kinds of archaeological data.
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