Demography

Demography is the study of how populations are structured and change due to the interplay of births, deaths, and migration. In narrow terms, ‘formal demography’ refers to the scientific study of human populations with a focus on their size, structure, distribution, and development. Defined more broadly as ‘population studies,’ demography also studies the causes and consequences of population compositions and changes, and draws on neighboring disciplines, such as sociology, economics, anthropology, and epidemiology. Scholars working in this tradition can be designated as social, economic, cultural, or health [Page 259]demographers, underscoring the field's multidisciplinary nature. There is increasing overlap between the concepts and methodologies of demography and epidemiology. However, separate histories, missions, professional discourses, and cultures have hindered dialogue between the two fields.

Just as epidemiology can be traced to John Snow's account of cholera in 19th-century London, so the origin of formal demography lies in John Graunt's pioneering 1662 analysis of London's ‘Bills of Mortality.’ This yielded a crude life table, revealed much about demographic changes occurring within British society, and drew attention to the need for population statistics in public administration. The origins of population studies can be traced to Thomas Malthus, who published his first essay on the ‘principle of population’ in 1798. His thesis, that population growth threatened prosperity because it ultimately outran increases in food supplies, stimulated interest in the relationships between population and resources.

Demography has since built a rich empirical research tradition and a substantial body of conceptual knowledge. In the 20th century, its dominant theoretical preoccupation has been with demographic transition theory. This refers to the movement of death and birth rates in a society from a pretransitional stage, where both are high, to a posttransitional stage, where both are low. In between is the demographic transition itself, a period of rapid and substantial population growth due to births exceeding deaths, as characterized by most developing countries in the latter half of the 20th century. Birth and death rates eventually converge as the transition nears an end, resulting in little growth through natural increase (the excess of births over deaths). Although largely a set of generalizations from observed trends with limited explanatory and predictive power, demographic transition theory has proved a useful framework for describing and comparing population change over time and space. It also gave rise to both epidemiologic and health transition theory.

In the late 1960s, the problem of world population growth dominated demographic thought. The world's population growth rate peaked at 2% inthelate1960s, equivalent to a doubling time of 34 years (i.e., at a 2% rate of growth, the population would double in 34 years). Birth rates have since fallen nearly everywhere, but there remains a clear distinction in both population growth rate and age structure between the industrialized and affluent countries of Europe, North America, Australia, New Zealand, and Japan, and many nonindustrialized countries in Central and South America, Africa, and Asia. The latter continue to exhibit relatively high population growth rates and youthful age structures, meaning a high proportion of their population is less than 18 years of age. In contrast, many industrialized countries have a stable or negative growth rate and aging populations. The United States has a positive growth rate due mostly to its sustained high rates of immigration, with immigrants primarily younger than the native born and more likely to have children. Successive cohorts of small family size eventually lead to population decline because to maintain a stable population size, an appreciable proportion of couples must have families larger than two children to counterbalance those unmarried, married but sterile, voluntarily childless, and one-child families. This is the situation facing much of Europe and East Asia today. In 2005, fertility had reached below the replacement level of 2.1 children per womanin44developedcountries. In 15 countries fertility had fallen to below 1.3, levels unprecedented in human history.

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