Paleodemography is the calculation of prehistoric human population structure, including life expectancy, mortality rates at different ages, and general health and well-being indicators. Such data are gathered from measured skeletons excavated from cemeteries. Historic paleodemography can use information from tombstones, cemetery records, hospital records, census data and other historic records as well: but in the absence of such written documentation, only physical investigation of human remains is suitable.
Population pyramids such as that used in traditional demographic studies are also used in paleodemographic studies, to illustrate the percentage of males and females in different categories. The image illustrated here is a population pyramid for Spain in 1900, based on census data.
Age Estimation
Scholars estimate the ages of individual skeletons based on the most reliable markers of age, such as dental development, annual increments in dental cementum (which accrue every year rather like growth rings in trees), closure of joint connections such as long-bone epiphyses and cranial sutures and changes in the surfaces where the pelvis meets other bones. These measures are collected on skeletons and compared to published averages based on large human skeletal reference collections. The variation of a particular measurement for any one individual can be quite substantial; diet or health or climate can affect the time when teeth erupt or when bones fuse together.
Determining the age of a person who died after reaching full maturity is notoriously difficult to pin down, and often such persons are characterized as "adult", rather than falling into a particular age range.
Disease and Cause of Death Identifications
Some diseases within a prehistoric population can be identified by dental or bony lesions which indicate slowness of growth relating from biological stress, such as malnutrition. Pre-mortem (before death) or peri-mortem (around the time of death) damage to the skeleton could represent accidents or warfare, of course; arthritis and other chronic diseases are often identified in cemetery populations. However, many diseases do not leave markers on bone, and of those that do leave markers, the markers only appear after the disease has reached an extremely advanced stage.
What can be detected in a skeleton is not disease so much as bone structure affected by working conditions. Teeth are often damaged when they are used as tools over many decades; bones respond to muscle changes that may have resulted after an individual spent their life performing repetitive physical tasks.
The study of diseases in past populations is called paleopathology.
Ethnicity
Determining the ethnicity of a dead person is not as easy as you might think. For one thing, despite the political weight of ethnic groups in the modern world, the truth is, there's not a lot of genetic evidence behind them. In spite of CSI methods illustrated in TV shows, facial reconstructions are not easy to get right. It's rarely easy to tell what ethnic group a person has once the soft tissues are gone; not to mention that we have no real way of knowing what people considered an "ethnic group" in prehistoric communities.
In addition, even today ethnic affiliations are often partly or even primarily a question of choice: how you call yourself is more important, culturally speaking, then what your genetic or bone structure says, which is why censuses ask people what ethnicity they are. So how do you determine that archaeologically? You look for strangers, outsiders in the communities: that is, burials that don't follow the usual pattern. Variant burial methods (some ethnic groups might have had particular rituals they followed) or grave goods (materials placed within a grave by relatives) are used as clues to an individual's outsider status, but may also be indications of social ranking, age, gender and role in the community and have little to do with outsider status.
Systemic Problems with Paleodemography
Paleodemography needs to be understood as an estimate of population structure, rather than as an exact science. Apart from the standard archaeological problems associated with preservation and context, the single most compelling problem with paleodemographic measures is---the people being measured are dead. By definition many or most of the people being studied were probably ill or under some sort of stress immediately prior to their death and that is what is recorded in their bones. Thus, a demographic study on a living population with regard to population structure, disease rate and mortality rate is likely to be far more reliable than that performed on a cemetery population from the same community.
Secondly, it is highly unlikely that a single cemetery represents all the members of a particular community. For example, in many prehistoric societies, neonates and infants were not buried with the juveniles and adults of a community. Sometimes cemeteries are used for a period, abandoned and reused later; sometimes they are used for only certain classes of individuals or family groups.
Finally, cemeteries are the endpoint of the life of a community, and as such they don't clearly illustrate important issues such as population size changes, inmigration or outmigration, or changes in mortality rates over time. Those characteristics must be identified very carefully indeed.
Methodological Advances
The growing field of genetics may have some assistance in supporting many of the paleodemographic efforts. Genetic structures of viruses are being detailed, and identified microbial DNA in human bone has been used to recognize the presence of pathogens long before the resulting diseases would have affected osteological tissue.
Analysis of stable isotopes in human bone have also been brought to bear on paleodemographic studies. These studies can allow the researcher to identify the locations where specific individuals were born, spent their childhood, and lived in the years prior to their deaths. That might make it easier to identify outsiders.
Sources
Appleby JEP. 2011. Bodies, burials and ageing: accessing the temporality of old age in prehistoric societies. Oxford Journal Of Archaeology 30(3):231-246.
Ubelaker DH. 2008. Paleodemography. In: Pearsall DM, editor-in-chief. Encyclopedia of Archaeology. New York: Academic Press. p 1767-1771.
Wood JW. 2005. Paleodemography. In: Kempf-Leonard, K, editor-in-chief. Encyclopedia of Social Measurement. New York: Elsevier. p 1-5.
