Comparative HistoryDemography is the study of population statistics, their relationship to social systems and human ecology. Demography is concerned with three main variables: fertility (births), mortality (deaths), and migration (movement of people). Age‐sex pyramids are used to visualize the effect of interaction between these dynamic variables and to make comparisons between populations and their subsets. The age‐sex pyramid displays the proportion of individuals by sex within each age category. To compare populations, demographers assess relative mortality hazards and fertility rates per age category. It was once assumed that the age‐sex pyramid primarily reflected relative mortality hazards. Survivorship curves, life tables, and statistics such as life expectancy at birth or mean age at death were commonly calculated from the age distribution and compared between populations. Recently it has been recognized that mortality is a diffuse process that occurs across the entire spectrum of ages and it appears to affect the shape of the pyramid secondarily. Fertility has a greater impact on the shape of the age pyramid because it occurs at a single moment in the lifespan for everyone in the population and fertility rates determine the size of each cohort (a group of individuals in a population who are born and enter the population at the same time). Essentially, this fertility‐centered approach to demography indicates that statistics such as mean age at death actually reflect fertility more than mortality, although at first this is counterintuitive.... ...
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