Demography is the scientific study of the size, composition, and distribution of human populations, and their changes resulting from fertility, mortality, and migration. Demography is concerned with how large (or small) populations are, that is, their size; how the populations are composed according to age, sex, race, marital status, and other characteristics, that is, their composition; and how populations are distributed in physical space (e.g., how urban and rural they are), that is, their spatial distribution (Bogue 1969). Demography is also interested in the changes over time in the size, composition, and distribution of human populations, and how these result from the processes of fertility, mortality, and migration.
The term demography is from the Greek demos (population) and graphia (writing). It is believed to have first appeared in print in 1855 in the book Elements ofHuman Statistics or Comparative Demography by the Belgian statistician Achille Guillard (1799-1876) (Borrie 1973, p. 75; Rowland 2003, p. 16).
Some demographers argue that demography is best treated as a subdiscipline or specialization of sociology owing to its organizational relationship with sociology (Moore 1959, p. 833). However, the organizational affinity in universities between demography and sociology is not universal. In some Eastern European universities, demography is organizationally linked with economics, and in some Western European universities, with geography. In many countries (e.g., China), demography is taught in a separate university department.
The American sociologist Kingsley Davis (19081997), who served at different times as president of both the Population Association of America and the American Sociological Association, wrote in 1948 in his classic sociology textbook, Human Society, that "the science of population, sometimes called demography, represents a fundamental approach to the understanding of human society" (1948, p. 551). The relationship between sociology and demography is hence a fundamental one: "Society is both a necessary and sufficient cause of population trends" (1948, pp. 553-554).
There are only two ways to enter a population, by birth and by in-migration. There are two ways to leave a population, by death and by out-migration. Thus, a population is often defined by demographers according to the specific needs of the research and researcher. Samuel Preston and his colleagues have written that the "term ‘population’ refers to a collection of items, for example, balls in an urn. Demographers use the term in a similar way to denote a collection of persons alive at a specified point in time who meet certain criteria" (2001, p 1). For example, the population of interest may be that of students attending a specific university during a specific year. In this situation, the students are born (i.e., enter) into the population when they enroll, and they die (i.e., leave) when they graduate.
Generally, demographers use vital registration (birth and death) records to count births and deaths in a population to determine fertility and mortality rates. The more difficult demographic process to measure is migration because in most countries registration records are not maintained when persons migrate into or out of the population. Data gathered around the world from decennial census and sample surveys are also used by demographers to examine demographic and sociodemographic issues.
Demographic techniques allow for the calculation of population projections, which specify the future size of the population by utilizing specific assumptions about the parameters driving the future fertility, mortality, and migration of the population. Population projections for all the countries around the world are periodically calculated by demographers at the United Nations and other international organizations and are made publicly available. Such projections are often used by government agencies and private firms to plan the infrastructure of cities, such as the number of schools, hospitals, airports, and parks that would be needed in the future in order for the cities to be able to function properly.
Demography is concerned not only with the observation and description of the size, composition, and spatial distribution of human populations and the changes resulting from fertility, mortality, and migration. Demography is also concerned with developing explanations for why the demographic variables operate and change in the ways they do: That is, why do some populations increase in size and others decrease? Why do some become older and others become younger? Why are some more urban and others more rural?
One paradigm in demography, known as formal demography, uses only demographic variables, such as age and sex, as independent variables to answer the above questions. Another paradigm, known as social demography, uses such nondemographic variables as marital status, race, education, socioeconomic status, occupation, household size, and type of place of residence—variables drawn mainly from sociology, economics, psychology, geography, anthropology, biology, and other disciplines—to answer the questions.
To illustrate, formal demographers might address differences in populations in their birth rates and death rates by considering their differences in age composition or in sex composition. Younger populations typically have higher birth rates than older populations; and populations with more females than males will usually have lower death rates than populations with more males than females (Poston 2005). Social demographers might address the above differences in populations in their birth rates and death rates by examining differences among them in, say, their socioeconomic status. Usually, populations with high socioeconomic status will have lower birth rates and death rates than populations with low socioeconomic status.
Demographic data may be introduced to provide some perspective for distinguishing between these two approaches. Human populations have different levels of fertility. Countries thus differ with respect to their total fertility rates (roughly defined as the average number of children born to a woman during her childbearing years). In 2004 Poland and Romania had very low fertility rates of 1.2, among the lowest in the world. Conversely, Niger,Guinea-Bissau, and Yemen had very high fertility rates of 8.0, 7.1, and 7.0, respectively—the highest in the world (Population Reference Bureau, 2004). Why do these fertility differences exist? Why do Niger, Guinea-Bissau, and Yemen have fertility rates that are so much higher than those of Poland and Romania? To answer this question, the social demographer would go beyond purely demographic issues of age and sex composition and would focus on the processes of industrialization and modernization.
Another example focuses on what demographers refer to as the percentage rate of natural increase/decrease, that is, the difference between the birth rate and the death rate. In 2004 both Russia and Bulgaria had a rate of -0.6 percent: that is, the difference between their crude birth and death rates was about -6/1000 or -0.6/100. In contrast, the rate in both Madagascar and Saudi Arabia was 3.0 percent. In these countries, the difference between their birth and death rates was 30/1000 or 3/100.
Why are these four countries growing at such drastically different rates? Why do Russia and Bulgaria have negative growth rates, and why do Madagascar and Saudi Arabia have positive rates? The formal demographer might develop an answer by considering the birth rates of these countries. The numbers of babies born per 1,000 population in 2004 in Russia, Bulgaria, Madagascar, and Saudi Arabia were 10, 10, 43, and 32, respectively. The latter two countries have higher rates of growth than the former two countries because their birth rates are so much higher. The social demographer would first consider the birth rate differentials, but would then go beyond this demographic consideration to an answer involving nonde-mographic factors that may be influencing the birth rates. Perhaps the economy has something to do with it (poorer countries have higher birth rates). Perhaps the level of industrialization of the country has an impact (the more industrialized countries generally have lower birth rates). Perhaps the role of women compared to men is having an effect (countries with more gender equity tend to have lower birth rates).
Whatever the reasons, the social demographer extends the answer beyond demographic reasons. Social demography is broader in scope and orientation than formal demography. Preston has noted, for example, that demography includes "research of any disciplinary stripe on the causes and consequences of population change".
Given the impact of industrialization in the reduction of fertility and mortality and the international migration flows from less developed to more developed countries around the world, it is a common practice among demographers to observe separately the demographic processes in less developed countries from those in more developed countries. The issues that concern demographers often vary depending on the level of industrialization of each country. In less developed countries, high levels of fertility, high levels of infant mortality, a high prevalence of HIV/AIDS, and high levels of out-migration to more developed countries tend to be some of the main demographic concerns. In more developed countries, low fertility patterns, women having babies at later ages, populations with below replacement levels of fertility, and large numbers of migrants from less developed countries are some of the main issues being examined by demographers.
A frequent concern in demography is the extent to which changes in individual-level behavior have an effect on aggregate processes (Preston et al. 2001). For example, if it suddenly became normative for individuals in a population to become smokers once they reach a certain age, then the demographer would want to find out to what extent the life expectancy at age x would be affected, as well as the death rate for that population. Similarly, regarding fertility, if women in a certain country decided to have children at older ages, then the concern becomes to what extent such behavior can have an effect on the total fertility rate, on the growth rate, and on whether the population will be maintained at a replacement level of fertility (which in populations with low levels of mortality is around 2.1 children per woman).
Demographers also are often concerned with how social policy could impact the aggregate population processes. In China, for example, demographers have identified a relationship between the enforcement of fertility policies and increasing levels of social and economic development and the sex ratio at birth (Poston et al. 1997; Poston and Glover 2005). The sex ratio at birth is the number of males born per 100 females born and is around 105 in most societies. Since the 1980s in China it has been significantly above 105. In 2000 China’s sex ratio at birth was near 120. The rapid reduction of fertility in China, along with the long-standing preference for sons, has led to the selective abortion of female fetuses, and a sex ratio at birth above normal levels. As a consequence, in China there will not be enough women in the population for the next few decades for Chinese men to marry. This is a major effect of societal modernization and fertility-control policies (Poston and Morrison 2005).
Demographers do not always agree about the boundaries and restrictions of their field. John Caldwell stated the problem succinctly: "What demography is and what demographers should be confined to doing remains a difficult area in terms not only of the scope of professional interests, but also of the coverage aimed at in the syllabuses for students and in what is acceptable for journals in the field" (1996, p. 305).
Other demographers argue for a broader approach, noting that demography is not a specialization of sociology, or of any discipline, but a discipline in its own right. Consider the definition of demography in the popular demography textbook Population: An Introduction to Concepts and Issues by John Weeks: Demography is "concerned with virtually everything that influences, or can be influenced by" population size, distribution, processes, structures, or characteristics (2005, p. 5).
It is no wonder that J. Mayone Stycos observed that "as a field with its own body of interrelated concepts, techniques, journals and professional associations, demography is clearly a discipline" (1987, p. 616). Caldwell also reached this conclusion, but more for methodological reasons: "Demography will remain a distinct discipline because of its approach: its demand that conclusions be in keeping with observable and testable data in the real world, that these data be used as shrewdly as possible to elicit their real meanings, and that the study should be representative of sizable or significant and definable populations" (1996, p. 333).
