Paleodemography, like demography, is concerned with mortality, fertility, population structure and life expectancy (among other things). Unlike demography proper, paleodemography rarely has written records such as birth and death certificates to draw on and has to rely on skeletal indicators of age, gender, etc. This has lead to some heavy criticisms of the field (“A Farewell to Paleodemography”, “Paleodemography: Not Quite Dead Yet”, etc. and yes the latter tittle is a Monty Python reference) which I don’t intend to discuss here. Suffice to say, the field has had to develop some interesting methodologies to overcome some of the limitations in the data of interest.
A case in point is the recent article in PNAS examining the affect of the plague on mortality in England (see below for full citation). The paper is both a theoretical examination of the possible bias in catastrophic die offs and a specific case study. The theoretical issue is actually an interesting question. I’ll let DeWitte and Wood explain the basic problem:
But in fact people die for a reason, and those dying at any given age are demonstrably not a random sample of the living population at risk of death at that age… Although all people die eventually, their health characteristics just before the time of death are unlikely to be representative even of their own physical condition over most of their lifetimes. The dying, on average, are less healthy than the rest of the living.
Think about that for a minute. If, say, the skeletal sample recovered from Dickson Mounds is not a random sample of the living population at risk of death at the time, how can we say anything about that population (in terms of structure, etc.)? One way is to look at catastrophic die offs such as those caused by disease, war, or climatic factors such as tsunamis. Presumably such events capture a unbiased snapshot of the demographic and health characteristics at the time of the event. In order to examine the question, DeWitte and Wood examine mortality in an English and two Danish cemeteries that date to 1347-1351. Theoretically, one would expect that sick or malnourished people would have higher mortality than healthy people when infected by the plague. In both the English and the Danish samples sickness (or frailty to be technically accurate) was measured based on pathological lesions on the skeleton. According to DeWitte and Wood:
The purpose here was to test whether the Black Death killed people indiscriminately–i.e., regardless of frailty as indicated by the presence of skeletal lesions–or whether Black Death mortality behaved like normal, nonepidemic mortality in which individuals with the highest frailty were at the highest risk of death.
This is where the Danish sample (from Viborg and Odense) comes in. In order to answer the question a comparison sample from the same time period, but one unaffected by the plague, was needed. Although the two Denmark samples were not a perfect match to the English sample, they were a relatively good match. The results were interesting and somewhat counterintuitive. In the Danish sample frailty was strongly associated with excess mortality, as one would expect. In the English sample, although frailty was associated with excess mortality, it was less than in the Danish sample. Think about that for a minute. Here is how Dewitte and Wood explain the results:
This situation would produce a reduction in the excess mortality of individuals with lesions in East Smithfield (compared with that in Denmark), not because individuals with lesions were less likely to die during the Black Death than under normal conditions, but because the risk of death for otherwise healthy people increased during the epidemic. In other words, the results suggest that the Black Death did not kill indiscriminately, but it did discriminate less sharply than death normally does.
What this means is that catastrophic die offs are not unbiased. Studies of the 1995 earthquake in Japan, Native American massacres, and mass famine also bear this out. Over and above the implications for bioarchaeology and paleodemography, this also has some implications for paleoanthropology – the First Family was a catastrophic die off…
DeWitte, S.N., Wood, J.W. (2008). Selectivity of Black Death mortality with respect to preexisting health. Proceedings of the National Academy of Sciences, 105(5), 1436-1441. DOI: 10.1073/pnas.0705460105
Filed under: Paleodemography