in London, implying that the growth of the capi- Demography tal was due to in-migration from the countryside. He also estimated the proportion of births surviv- Demography is the study of human populations with ing to a range of ages, thereby developing the basic respect to their size, structure, and dynamics. For concept of the life table. Graunt’s research prefig- ures modern applications of demographic science: demographers, a population is a group of individuals information on fertility and mortality and population that coexist at a point in time and share a defining estimates for small areas (see Small Area Estima- characteristic such as residence in the same geograph- tion) remain the fundamental results of demographic ical area. The structure or composition of a population analysis required by those engaged in policy formu- refers to the distribution of its members by age, sex, lation and planning. and other characteristics, such as place of residence and marital or health status. The age and sex structure of a population results from past trends in fertil- ity, mortality, and migration. Thus, these processes Demographic Data comprise the components of demographic change. In most developed countries, civil registration of The age and sex structure of a population, in turn, births and deaths is the primary source of fertility and affects birth rates, death rates, and rates of migra- mortality data. Government agencies routinely collect tion. Changes in status such as getting married or demographic information when births and deaths are divorced interact with population structure in a sim- certified for administrative purposes (see Vital Statis- ilar way. tics, Overview, Overview). The primary source of Some authorities reserve the term demography data on the size, structure and distribution of national for the mathematical and statistical study of the populations is the population census. Censuses aim interrelationships between population size and struc- to enumerate the whole population of a defined geo- ture and the components of demographic change. graphical area. They collect individual-level data on According to this terminology, demography can be the population’s characteristics that refer to a sin- contrasted with population studies, which investigate gle point in time. As well as collecting data on the the determinants and consequences of demographic size and composition of the population, most cen- phenomena drawing on the concepts and theories suses also ask about moves in a fixed period of of disciplines such as the social sciences, health time before the enumeration. In countries where vital sciences, and history. Others encompass population statistics data are incomplete, questions may also be studies within demography and use the term for- asked about fertility and mortality. Countries that mal demography to distinguish the statistical core of issue identity numbers and require their population to the discipline. Demography (according to this wider report their place of residence can maintain continu- definition) is a multidisciplinary field: subdisciplines ous population registers. In a few European countries such as economic demography, historical demogra- these registers now fuse the functions of the registra- phy, anthropological demography, and mathematical tion system with those of the census. demography exist. They differ not only in their sub- The evolution of demographic analysis and of ject of study but also in their theoretical orientation routine collection of data on populations by the and methods. government were interlinked. Standard demographic The term demography has been ascribed to a Bel- measures and techniques of analysis were developed gian statistician, Achille Guillard, who coined it in largely for the study of vital statistics with census- 1855. However, the origins of modern demography based denominators. In recent decades, however, are usually traced back to John Graunt’s quantita- demographers have relied increasingly on survey data tive analyses of the “Bills of Mortality” published in to supplement those from traditional sources (see 1662 [5]. The “Bills of Mortality” provided weekly Surveys, Health and Morbidity). In particular, in lists of burials and baptisms in the parishes of Lon- countries where registration of vital events is incom- don. Graunt used these data to examine the sex ratio plete national sample surveys are the main source of at birth and to estimate the population of London. vital statistics. One of the first subjects to be inves- He showed that more deaths than births occurred tigated in demographic surveys was family planning 2 Demography

(see Reproduction). Other early surveys collected an avowedly applied intent. Demography bears on the women-based fertility histories to supplement the efforts of international agencies and national govern- event-based data generated by birth registration. Fer- ments to promote family planning and improve health tility history and family planning data remain the in the developing world. In the developed world, pop- focus of many demographic surveys, including the ulation growth has slowed but low fertility and the two major international programs of surveys con- reduction in death rates in old age are producing an ducted since the 1970s, namely the World Fertility increasingly aged population. Recent changes in pat- Survey and the Demographic and Health Surveys. terns of marriage and divorce and of childbearing inside and outside marriage also have major implica- tions for the family and public policy. Issues Demographic studies of mortality tend to focus on the analysis of routine data. Demographers’ research Between the mid-nineteenth and mid-twentieth cen- into health and mortality cannot be distinguished turies the more developed regions of the world clearly from that of epidemiologists. However, de- went through a demographic transition from a high- mographers tend to be concerned with the distribution fertility, high-mortality, and low-growth demographic of disease and premature death (see Descriptive Epi- regime to a low-fertility, low-mortality, and low- demiology) across social groups (see Social Classi- growth demographic regime. As mortality tended fications) and their implications for other aspects of to fall before fertility, this transition was marked social life, rather than with measuring risk factors for by rapid population growth. Since 1945, a similar specific conditions. transition has begun in most less developed countries. As a result, the world’s population has grown from Demographic Analysis about 2.5 billion to about 6 billion in the second half of the twentieth century. It is expected to grow to The aim of formal demographic analysis is to isolate between 9 and 16 billion by 2100. the components of demographic patterns by dividing Efforts to understand the determinants of the tran- a population into relatively homogeneous subgroups. sition of fertility and mortality to low levels are Analysis by age and sex has primacy over analy- a central concern of demography. Many demogra- sis by other compositional factors. Human biology phers now believe that explanations that focus on causes the propensity to die and to give birth to be economic factors and the provision of health and fam- differentiated by age and sex everywhere. It imposes ily planning programs are inadequate and need to be a degree of uniformity on age patterns of mortality supplemented by accounts that take into account the and fertility in all human populations. ideational and cultural determinants of demographic Classical demographic analysis is based on a fairly behavior. small set of measures and techniques. Most of these Thomas Malthus was the first author to develop are also used in cognate disciplines. Calculation of a systematic argument that high fertility leading to rates, ratios, and proportions represent the basic way population growth could have adverse effects on for controlling for population size. In demography, economic welfare [7]. He argued that a growing pop- rates calculated for the whole population that make ulation must eventually outstrip its subsistence base, no allowance for the influence of population structure bringing about rising mortality from famine, pesti- on the phenomenon of interest are referred to as crude lence, and war. Although the past two centuries of rates. Examples are the crude birth rate and crude human history have followed a very different path death rate (see Vital Statistics, Overview). from that envisaged by Malthus, concern still exists Calculation of age-specific rates and rates specific about the impact of population growth on economic to other subgroups of the population allow the ana- development and the environment. Today, however, lyst to isolate the propensity to experience the event economic demographers tend to be more sanguine being studied from the influence of population struc- about the consequences of population growth than ture. A range of methods of standardization are used those with a background in ecology [3]. to produce synthetic indices that summarize such spe- Many demographic outcomes are of concern to cific rates (see Standardization Methods). The dis- policy makers and much demographic research has tinction between cohort analysis and cross-sectional Demography 3 or period analysis is fundamental to demography. sexual intercourse, her probability of conceiving, and Demographers use the term cohort to refer to groups the probability that a pregnancy ends in a live birth. of individuals who experience a defining event at the The strength of the approach is that only a few of the same time. Examples include birth cohorts and mar- proximate determinants of individuals’ fertility differ riage cohorts. Cohort analysis studies the subsequent between populations in their impact at the aggregate experience of such groups. This contrasts with epi- level. Thus, the four main proximate determinants demiologic usage, which refers to all those eligible of fertility differences between groups and over time for recruitment into a longitudinal study as a cohort. are the proportion of women in sexual unions, post- Period measures are often treated as referring to partum infecundity associated with breast-feeding, a synthetic or hypothetical cohort, so that summary contraception, and abortion. Socioeconomic deter- indices can be calculated that indicate what would minants of fertility must operate through these few happen to a cohort that went through life experienc- proximate factors and a single characteristic may ing the specific rates of the period under study. For have countervailing effects on fertility via different example, the most widely used index of period fertil- proximate determinants. ity is the total fertility rate. This measures how many children women would bear on average if they went Demographic Models through life with the fertility of a specific period. It is calculated by summing the age-specific fertility rates Analysis of data on actual populations is paral- of a particular year, usually for five-year age groups, leled by mathematical models of the interrelationship over all ages at which women bear children. The total between population size and structure and the com- fertility rate is thus a form of directly standardized ponents of demographic change. Stable population rate, calculated using a uniform age distribution as theory as developed by Lotka in the 1920s and 1930s the standard. demonstrates that any closed single-sex population Two basic aspects of any demographic process are subject to constant fertility and mortality rates con- its intensity, or quantum, and its timing, or tempo. verges on an unchanging age structure and a constant The intensity of a nonrenewable event such as death rate of growth. This stable outcome is independent or first marriage can be measured by the proportion of of the initial age structure of the population. The a cohort who eventually experience the event. Both special case of a stable population that is unchang- the expected timing of any nonrenewable process and ing in size is termed a stationary population. Its the distribution of times of its occurrence can be age structure is a function of the life table. Recent studied using life table methods. The intensity of a developments, known as generalized stable popu- renewable process such as birth or disease incidence lation theory, demonstrate that the mathematics of can be measured by the mean number of events per stable populations can be extended to populations in person, and their tempo by the characteristics of the which growth rates vary by age because of a history distribution of the events in time. Renewable events of fertility and mortality change and to populations can be categorized by the order of their occurrence, subject to decrements other than mortality [8]. and events of a particular order can be analyzed as One crucial application of demography is to the a nonrenewable process. For example, the proportion forecasting of future population change. This is usu- of women who have had a birth of order i that go on ally undertaken using cohort-component methods of to bear a child of order i 1 is known as a parity population projection [2]. These methods provide a + progression ratio. precise way of controlling for the influence of popu- Investigation of the determinants of fertility and lation structure and of working out the implications of mortality has been facilitated by making a distinc- any scenario postulated for future vital rates. Despite tion between proximate and distal determinants. The this, population forecasts have often proved wide of approach is most developed with respect to fertility. A the mark. Fertility, mortality, and migration remain proximate determinant is one that has a direct impact difficult to predict. Forecasts informed by a theo- on the outcome of interest while a distal determinant retical understanding of the determinants of these can only affect the outcome via a proximate deter- components of population change often perform little minant. The proximate determinants of fertility are better than the simple extrapolation of past trends in those factors that determine a woman’s exposure to vital rates. 4 Demography

The increasing availability of survey data and References information technology that makes it practicable to undertake individual-level analysis of data on [1] Bongaarts, J., Burch, T. & Wachter, K., eds. (1987). large samples, have facilitated convergence between Family Demography: Methods and their Applications. demographic methods and other forms of statistical Clarendon Press, Oxford. analysis. Thus, many of the developments in demo- [2] Brass, W. (1974). Perspectives in population prediction, graphic analysis during the past few decades have Journal of the Royal Statistical Society, Series A, 137, 532–583. been closely linked to those in statistical methods [3] Cassen, R. (1994). Overview, in Population and Devel- more generally. Demographers have both adopted opment: Old Debates, New Conclusions, R. Cassen and and contributed to the development of methods contributors. Transaction, Oxford. such as event history analysis [10], the modeling [4] Goldstein, H. (1995). Multilevel Statistical Models. of unobserved heterogeneity, and random-effects Edward Arnold, London. models [4]. Other fields of methodologic research [5] Graunt, J. (1964). Natural and Political Observations Mentioned in a Following Index, and Made upon the in recent years include the extension of life table Bills of Mortality; London, 1662. Reprinted, with an methods into multistate models that allow for incre- introduction by B. Benjamin, in Journal of the Institute ments as well as decrements from each state [6] and of Actuaries 90, 1–61. methods and models for the study of families and [6] Land, K.C. & Rogers, A., eds (1982). Multidimensional households [1]. (see Multilevel Models) Mathematical Demography. Academic Press, New York. One particularly successful field has been the [7] Malthus, T.R. (1970). An Essay on the Principle of development of indirect methods for estimating vital Population (London, 1798), A. Flew, ed. Penguin, Har- mondsworth. rates in populations with limited and defective vital [8] Preston, S.H. & Coale, A.J. (1982). Age structure, statistics [9]. Indirect methods use stable popula- growth, attrition, and accession: a new synthesis, Popu- tion theory and its extensions to describe the rela- lation Index 48, 217–259. tionship between conventional indices of fertility, [9] United Nations (1983). Indirect Techniques for Demo- mortality, and migration and items of information graphic Estimation. ST/ESA/Series A/81. United that can be collected more reliably in single-round Nations, New York. surveys and censuses in less developed countries. [10] Yamaguchi, K. (1991). Event History Analysis. Sage, London. For example, it is possible to estimate life table indices of child mortality from data on the pro- portion of women’s children ever-born who have (See also Actuarial Methods) died, tabulated by the age of the women con- cerned [9]. IAN M. TIMÆUS