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Demography is the study of human population and its changes due to deaths, births, marriages and divorces, and migration. The term demos denotes people in Greek—the term demography literally means the systematic study of people. In the early twenty-first century the discipline encompasses a broad array of subject matters, covering, among others, economic, social, public health, and political issues. This entry will provide a brief overview of the uses of demographic data and key trends.

Contemporary demographers focus on two broad areas. The first is the size, composition, and characteristics of populations. The second focus is on processes that influence population change. Demographers need data to conduct analyses: censuses, surveys, and civil and vital registrations are sources of population-related information. Censuses collect data or information at the household level and are conducted at regular intervals. Surveys based on scientific sampling principles are conducted as needed for a variety of purposes, but participation by individuals is voluntary. The registration of births, deaths, marriages, and divorces, along with mandatory reporting requirements for communicable diseases or for residence changes, serve as excellent sources of data on fertility, mortality, nuptiality, and migration. An additional source of population data includes estimations, which are increasingly sophisticated calculations based on information not directly related to the sources listed here.

In addition to the size and spatial aspects of settlement, demographers examine and analyze factors related to the composition of populations, namely age groupings and race/ethnic and gender distributions. Characteristics of populations, such as education and economic status, are also studied. Generally these factors are examined from a single point in time or cross-sectional perspective as well as in reference to dynamic changing processes (longitudinal).

Data on the size, composition, and characteristics of populations are used for a variety of public and private purposes. Drawing political boundaries for elections, determining commercial investment decisions, and assessing the prevalence of health problems are three examples of uses of demographic data. For example, the number of congressional districts in the United States has remained constant at 435 for nearly a century, despite a tripling of the nation's population. The allocation of congressional districts is based on a state's share of the national population.

Another example of the use of demographic analyses occurs when states, counties, and cities make decisions about infrastructure investments, such as the location of new fire stations, schools, and public libraries. Furthermore, private firms use population information such as education level and incomes to target households for marketing purposes or to estimate the need for commercial services. Demographic data are also used to calculate rates of prevalence of disease in order to assess the magnitude and the need for intervention. The incidence of communicable disease—for example, tuberculosis—is standardized (e.g., per 10,000 population), facilitating comparisons across administrative boundaries and helping to focus attention on health disparities between areas and population subgroups.

Demography addresses the processes that change populations. Three related factors affect population change:

  • Fertility, which measures the average number of children born to a woman (or populations) during child-bearing ages;
  • Mortality, which is the process by which deaths occur in populations; and
  • Migration, which is the movement of individuals or groups that involves a permanent or semipermanent change of residence across administrative and political boundaries, that is, across county, state, and national boundaries.

The twentieth century was characterized by what has been termed a demographic transition, in which mortality rates dropped dramatically due to advances in sanitation, access to health care, rapid socioeconomic changes, including a change in the status of women and altered attitudes toward contraception. The reduction in mortality was followed by declines in fertility rates. The fertility decline was evident in Europe, North America, and Oceania throughout the latter part of the twentieth century, with fertility rates at or below the replacement level of 2.1 children per women. By the year 2000, most Asian and Latin American countries had entered a transition toward lower fertility, and by the start of the twenty-first century, some low-fertility countries had adopted policies to reverse the declines. Many of the countries that had not shown fertility declines were located in sub-Saharan Africa. In addition to high infant mortality rates, mortality among the working-age population in Africa was also high as a result of the Acquired Immune Deficiency Syndrome (AIDS) epidemic, which was having a profound and detrimental impact.

By the early twenty-first century, the population aged sixty-five and above in low-fertility countries was increasing at a much faster rate than those of lower age ranges because of increased life expectancy combined with low birth rates. Since older populations typically need more health care along with support during retirement, increases in elderly populations put a strain on health care, social security, and pension systems. The aging population had a significant impact on health care systems and workforce recruitment and training and initiated a debate on how to support an aging population through publicly funded social security schemes. These debates occurred at a time when the size of the younger working population was shrinking.

In low-fertility countries, immigration mediated the impact of fertility declines. In North America and Oceania, large-scale immigration began in the nineteenth century, and net migration became a significant and increasing component of the population increase by the early twenty-first century. Other Western countries have had a history of emigration, that is, an outflow of people rather than immigration. But from the 1960s onward, an influx of African and Asian immigrants to Europe, both legal and illegal, gave rise to social and political pressures.

While fertility levels are declining in Asia, Latin America, and the Middle East, population size will continue to grow for the foreseeable future because large proportions of the population are in the younger reproductive ages. Emigration plays only a small role in reducing population growth in these regions. The challenge for many of these countries is to provide education, workforce training, and employment to their predominantly young populations, and failure to meet these needs may lead to political instability and sometimes acts of violence. Most spheres of human activity—political, social, and economic—now involve consideration of the size, composition, and characteristics of the population. This is unlikely to change as nations confront the issues discussed here.

Ramdas Menon

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