# Demography

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Demography (from prefix demo- from Ancient Greek δῆμος dēmos meaning "the people", and -graphy from γράφω graphō, ies "writing, description or measurement" [1] ) is the statistical study of populations, especially human beings.

The ancient Greek language includes the forms of Greek used in Ancient Greece and the ancient world from around the 9th century BC to the 6th century AD. It is often roughly divided into the Archaic period, Classical period, and Hellenistic period. It is antedated in the second millennium BC by Mycenaean Greek and succeeded by Medieval Greek.

Statistics is the discipline that concerns the collection, organization, displaying, analysis, interpretation and presentation of data. In applying statistics to a scientific, industrial, or social problem, it is conventional to begin with a statistical population or a statistical model to be studied. Populations can be diverse groups of people or objects such as "all people living in a country" or "every atom composing a crystal". Statistics deals with every aspect of data, including the planning of data collection in terms of the design of surveys and experiments. See glossary of probability and statistics.

In biology, a population is all the organisms of the same group or species, which live in a particular geographical area, and have the capability of interbreeding. The area of a sexual population is the area where inter-breeding is potentially possible between any pair within the area, and where the probability of interbreeding is greater than the probability of cross-breeding with individuals from other areas.

## Contents

Demography encompasses the study of the size, structure, and distribution of these populations, and spatial or temporal changes in them in response to birth, migration, aging, and death. As a very general science, it can analyze any kind of dynamic living population, i.e., one that changes over time or space (see population dynamics). Demographics are quantifiable characteristics of a given population.

Birth is the act or process of bearing or bringing forth offspring. In mammals, the process is initiated by hormones which cause the muscular walls of the uterus to contract, expelling the fetus at a developmental stage when it is ready to feed and breathe. In some species the offspring is precocial and can move around almost immediately after birth but in others it is altricial and completely dependent on parenting. In marsupials, the fetus is born at a very immature stage after a short gestational period and develops further in its mother's womb's pouch.

Death is the permanent cessation of all biological functions that sustain a living organism. Phenomena which commonly bring about death include aging, predation, malnutrition, disease, suicide, homicide, starvation, dehydration, and accidents or major trauma resulting in terminal injury. In most cases, bodies of living organisms begin to decompose shortly after death.

Population dynamics is the branch of life sciences that studies the size and age composition of populations as dynamical systems, and the biological and environmental processes driving them. Example scenarios are ageing populations, population growth, or population decline.

Demographic analysis can cover whole societies or groups defined by criteria such as education, nationality, religion, and ethnicity. Educational institutions [2] usually treat demography as a field of sociology, though there are a number of independent demography departments. [3] Based on the demographic research of the earth, earth's population up to the year 2050 and 2100 can be estimated by demographers.

Demographic analysis includes the things that allow us to measure the dimensions and dynamics of populations. These methods have primarily been developed to study human populations, but are extended to a variety of areas where researchers want to know how populations of social actors can change across time through processes of birth, death, and migration. In the context of human biological populations, demographic analysis uses administrative records to develop an independent estimate of the population. Demographic analysis estimates are often considered a reliable standard for judging the accuracy of the census information gathered at any time. In the labor force, demographic analysis is used to estimate sizes and flows of populations of workers; in population ecology the focus is on the birth, death, migration and immigration of individuals in a population of living organisms, alternatively, in social human sciences could involve movement of firms and institutional forms. Demographic analysis is used in a wide variety of contexts. For example, it is often used in business plans, to describe the population connected to the geographic location of the business. Demographic analysis is usually abbreviated as DA. For the 2010 U.S. Census, The U.S. Census Bureau has expanded its DA categories. Also as part of the 2010 U.S. Census, DA now also includes comparative analysis between independent housing estimates, and census address lists at different key time points.

Education is the process of facilitating learning, or the acquisition of knowledge, skills, values, beliefs, and habits. Educational methods include storytelling, discussion, teaching, training, and directed research. Education frequently takes place under the guidance of educators, however learners may also educate themselves. Education can take place in formal or informal settings and any experience that has a formative effect on the way one thinks, feels, or acts may be considered educational. The methodology of teaching is called pedagogy.

Nationality is a legal relationship between an individual person and a state. Nationality affords the state jurisdiction over the person and affords the person the protection of the state. What these rights and duties are varies from state to state.

Formal demography limits its object of study to the measurement of population processes, while the broader field of social demography or population studies also analyses the relationships between economic, social, cultural, and biological processes influencing a population. [4]

## History

Demographic thoughts traced back to antiquity, and were present in many civilisations and cultures, like Ancient Greece, Ancient Rome, China and India. [5] Demography is made up of two word Demos and Graphy . The term Demography refers to the overall study of population.

Ancient Greece was a civilization belonging to a period of Greek history from the Greek Dark Ages of the 12th–9th centuries BC to the end of antiquity. Immediately following this period was the beginning of the Early Middle Ages and the Byzantine era. Roughly three centuries after the Late Bronze Age collapse of Mycenaean Greece, Greek urban poleis began to form in the 8th century BC, ushering in the Archaic period and colonization of the Mediterranean Basin. This was followed by the period of Classical Greece, an era that began with the Greco-Persian Wars, lasting from the 5th to 4th centuries BC. Due to the conquests by Alexander the Great of Macedon, Hellenistic civilization flourished from Central Asia to the western end of the Mediterranean Sea. The Hellenistic period came to an end with the conquests and annexations of the eastern Mediterranean world by the Roman Republic, which established the Roman province of Macedonia in Roman Greece, and later the province of Achaea during the Roman Empire.

In Historiography, ancient Rome is Roman civilization from the founding of the Italian city of Rome in the 8th century BC to the collapse of the Western Roman Empire in the 5th century AD, encompassing the Roman Kingdom, Roman Republic and Roman Empire until the fall of the western empire. The civilization began as an Italic settlement in the Italian Peninsula, conventionally founded in 753 BC, that grew into the city of Rome and which subsequently gave its name to the empire over which it ruled and to the widespread civilisation the empire developed. The Roman Empire expanded to become one of the largest empires in the ancient world, though still ruled from the city, with an estimated 50 to 90 million inhabitants and covering 5.0 million square kilometres at its height in AD 117.

China, officially known as the People's Republic of China (PRC), is a country in East Asia and the world's most populous country, with a population of around 1.404 billion in 2017. Covering approximately 9,600,000 square kilometers (3,700,000 sq mi), it is the third or fourth largest country by total area. Governed by the Communist Party of China, the state exercises jurisdiction over 22 provinces, five autonomous regions, four direct-controlled municipalities, and the special administrative regions of Hong Kong and Macau.

In ancient Greece, this can be found in the writings of Herodotus, Thucidides, Hippocrates, Epicurus, Protagoras, Polus, Plato and Aristotle. [5] In Rome, writers and philosophers like Cicero, Seneca, Pliny the elder, Marcus Aurelius, Epictetus, Cato, and Columella also expressed important ideas on this ground. [5]

Herodotus was an ancient Greek historian who was born in Halicarnassus in the Persian Empire. He is known for having written the book The Histories, a detailed record of his "inquiry" on the origins of the Greco-Persian Wars. He is widely considered to have been the first writer to have treated historical subjects using a method of systematic investigation—specifically, by collecting his materials and then critically arranging them into an historiographic narrative. On account of this, he is often referred to as "The Father of History", a title first conferred on him by the first-century BC Roman orator Cicero.

Hippocrates of Kos, also known as Hippocrates II, was a Greek physician of the Age of Pericles, who is considered one of the most outstanding figures in the history of medicine. He is often referred to as the "Father of Medicine" in recognition of his lasting contributions to the field as the founder of the Hippocratic School of Medicine. This intellectual school revolutionized medicine in ancient Greece, establishing it as a discipline distinct from other fields with which it had traditionally been associated, thus establishing medicine as a profession.

Epicurus was an ancient Greek philosopher and sage who founded a highly influential school of philosophy now called Epicureanism. He was born on the Greek island of Samos to Athenian parents. Influenced by Democritus, Aristotle, Pyrrho, and possibly the Cynics, he turned against the Platonism of his day and established his own school, known as "the Garden", in Athens. Epicurus and his followers were known for eating simple meals and discussing a wide range of philosophical subjects, and he openly allowed women to join the school as a matter of policy. An extremely prolific writer, he is said to have originally written over 300 works on various subjects, but the vast majority of these writings have been lost. Only three letters written by him—the Letters to Menoeceus, Pythocles, and Herodotus—and two collections of quotes—the Principle Doctrines and the Vatican Sayings—have survived intact, along with a few fragments and quotations of his other writings. Most knowledge of his teachings comes from later authors, particularly the Roman poet Lucretius, the biographer Diogenes Laërtius, the statesman Cicero, and the philosophers Philodemus and Sextus Empiricus.

In the Middle ages, Christian thinkers devoted much time in refuting the Classical ideas on demography. Important contributors to the field were William of Conches, [6] Bartholomew of Lucca, [6] William of Auvergne, [6] William of Pagula, [6] and Muslim sociologists like Ibn Khaldun. [7]

One of the earliest demographic studies in the modern period was Natural and Political Observations Made upon the Bills of Mortality (1662) by John Graunt, which contains a primitive form of life table. Among the study's findings were that one third of the children in London died before their sixteenth birthday. Mathematicians, such as Edmond Halley, developed the life table as the basis for life insurance mathematics. Richard Price was credited with the first textbook on life contingencies published in 1771, [8] followed later by Augustus de Morgan, ‘On the Application of Probabilities to Life Contingencies’ (1838). [9]

In 1755, Benjamin Franklin published his essay Observations Concerning the Increase of Mankind, Peopling of Countries, etc. , projecting exponential growth in British colonies. [10] His work influenced Thomas Robert Malthus, [11] who, writing at the end of the 18th century, feared that, if unchecked, population growth would tend to outstrip growth in food production, leading to ever-increasing famine and poverty (see Malthusian catastrophe). Malthus is seen as the intellectual father of ideas of overpopulation and the limits to growth. Later, more sophisticated and realistic models were presented by Benjamin Gompertz and Verhulst.

In 1855, a Belgian scholar Achille Guillard defined demography as the natural and social history of human species or the mathematical knowledge of populations, of their general changes, and of their physical, civil, intellectual and moral condition. [12]

The period 1860-1910 can be characterised as a period of transition wherein demography emerged from statistics as a separate field of interest. This period included a panoply of international ‘great demographers’ like Adolphe Quételet (1796–1874), William Farr (1807–1883), Louis-Adolphe Bertillon (1821–1883) and his son Jacques (1851–1922), Joseph Körösi (1844–1906), Anders Nicolas Kaier (1838–1919), Richard Böckh (1824–1907), Émile Durkheim (1858-1917), Wilhelm Lexis (1837–1914), and Luigi Bodio (1840–1920) contributed to the development of demography and to the toolkit of methods and techniques of demographic analysis. [13]

## Methods

There are two types of data collection—direct and indirect—with several different methods of each type.

### Direct methods

Direct data comes from vital statistics registries that track all births and deaths as well as certain changes in legal status such as marriage, divorce, and migration (registration of place of residence). In developed countries with good registration systems (such as the United States and much of Europe), registry statistics are the best method for estimating the number of births and deaths.

A census is the other common direct method of collecting demographic data. A census is usually conducted by a national government and attempts to enumerate every person in a country. In contrast to vital statistics data, which are typically collected continuously and summarized on an annual basis, censuses typically occur only every 10 years or so, and thus are not usually the best source of data on births and deaths. Analyses are conducted after a census to estimate how much over or undercounting took place. These compare the sex ratios from the census data to those estimated from natural values and mortality data.

Censuses do more than just count people. They typically collect information about families or households in addition to individual characteristics such as age, sex, marital status, literacy/education, employment status, and occupation, and geographical location. They may also collect data on migration (or place of birth or of previous residence), language, religion, nationality (or ethnicity or race), and citizenship. In countries in which the vital registration system may be incomplete, the censuses are also used as a direct source of information about fertility and mortality; for example the censuses of the People's Republic of China gather information on births and deaths that occurred in the 18 months immediately preceding the census.

### Indirect methods

Indirect methods of collecting data are required in countries and periods where full data are not available, such as is the case in much of the developing world, and most of historical demography. One of these techniques in contemporary demography is the sister method, where survey researchers ask women how many of their sisters have died or had children and at what age. With these surveys, researchers can then indirectly estimate birth or death rates for the entire population. Other indirect methods in contemporary demography include asking people about siblings, parents, and children. Other indirect methods are necessary in historical demography.

There are a variety of demographic methods for modelling population processes. They include models of mortality (including the life table, Gompertz models, hazards models, Cox proportional hazards models, multiple decrement life tables, Brass relational logits), fertility (Hernes model, Coale-Trussell models, parity progression ratios), marriage (Singulate Mean at Marriage, Page model), disability (Sullivan's method, multistate life tables), population projections (Lee-Carter model, the Leslie Matrix), and population momentum (Keyfitz).

The United Kingdom has a series of four national birth cohort studies, the first three spaced apart by 12 years: the 1946 National Survey of Health and Development, the 1958 National Child Development Study, [14] the 1970 British Cohort Study, [15] and the Millennium Cohort Study, begun much more recently in 2000. These have followed the lives of samples of people (typically beginning with around 17,000 in each study) for many years, and are still continuing. As the samples have been drawn in a nationally representative way, inferences can be drawn from these studies about the differences between four distinct generations of British people in terms of their health, education, attitudes, childbearing and employment patterns. [16]

## Common rates and ratios

• The crude birth rate , the annual number of live births per 1,000 people.
• The general fertility rate , the annual number of live births per 1,000 women of childbearing age (often taken to be from 15 to 49 years old, but sometimes from 15 to 44).
• The age-specific fertility rates, the annual number of live births per 1,000 women in particular age groups (usually age 15-19, 20-24 etc.)
• The crude death rate , the annual number of deaths per 1,000 people.
• The infant mortality rate , the annual number of deaths of children less than 1 year old per 1,000 live births.
• The expectation of life (or life expectancy), the number of years that an individual at a given age could expect to live at present mortality levels.
• The total fertility rate , the number of live births per woman completing her reproductive life, if her childbearing at each age reflected current age-specific fertility rates.
• The replacement level fertility, the average number of children women must have in order to replace the population for the next generation. For example, the replacement level fertility in the US is 2.11. [17]
• The gross reproduction rate , the number of daughters who would be born to a woman completing her reproductive life at current age-specific fertility rates.
• The net reproduction ratio is the expected number of daughters, per newborn prospective mother, who may or may not survive to and through the ages of childbearing.
• A stable population, one that has had constant crude birth and death rates for such a long period of time that the percentage of people in every age class remains constant, or equivalently, the population pyramid has an unchanging structure. [17]
• A stationary population, one that is both stable and unchanging in size (the difference between crude birth rate and crude death rate is zero). [17]

A stable population does not necessarily remain fixed in size. It can be expanding or shrinking. [17]

Note that the crude death rate as defined above and applied to a whole population can give a misleading impression. For example, the number of deaths per 1,000 people can be higher for developed nations than in less-developed countries, despite standards of health being better in developed countries. This is because developed countries have proportionally more older people, who are more likely to die in a given year, so that the overall mortality rate can be higher even if the mortality rate at any given age is lower. A more complete picture of mortality is given by a life table, which summarizes mortality separately at each age. A life table is necessary to give a good estimate of life expectancy.

## Basic equation

Suppose that a country (or other entity) contains Populationt persons at time t. What is the size of the population at time t + 1 ?

${\displaystyle {\text{Population}}_{t+1}={\text{Population}}_{t}+{\text{Natural Increase}}_{t}+{\text{Net Migration}}_{t}}$

Natural increase from time t to t + 1:

${\displaystyle {\text{Natural Increase}}_{t}={\text{Births}}_{t}-{\text{Deaths}}_{t}}$

Net migration from time t to t + 1:

${\displaystyle {\text{Net Migration}}_{t}={\text{Immigration}}_{t}-{\text{Emigration}}_{t}}$

This basic equation can also be applied to subpopulations. For example, the population size of ethnic groups or nationalities within a given society or country is subject to the same sources of change. When dealing with ethnic groups, however, "net migration" might have to be subdivided into physical migration and ethnic reidentification (assimilation). Individuals who change their ethnic self-labels or whose ethnic classification in government statistics changes over time may be thought of as migrating or moving from one population subcategory to another. [18]

More generally, while the basic demographic equation holds true by definition, in practice the recording and counting of events (births, deaths, immigration, emigration) and the enumeration of the total population size are subject to error. So allowance needs to be made for error in the underlying statistics when any accounting of population size or change is made.

The figure in this section shows the latest (2004) UN projections of world population out to the year 2150 (red = high, orange = medium, green = low). The UN "medium" projection shows world population reaching an approximate equilibrium at 9 billion by 2075. Working independently, demographers at the International Institute for Applied Systems Analysis in Austria expect world population to peak at 9 billion by 2070. [19] Throughout the 21st century, the average age of the population is likely to continue to rise.

### Science of population

Populations can change through three processes: fertility, mortality, and migration. Fertility involves the number of children that women have and is to be contrasted with fecundity (a woman's childbearing potential). [20] Mortality is the study of the causes, consequences, and measurement of processes affecting death to members of the population. Demographers most commonly study mortality using the Life Table, a statistical device that provides information about the mortality conditions (most notably the life expectancy) in the population. [21]

Migration refers to the movement of persons from a locality of origin to a destination place across some predefined, political boundary. Migration researchers do not designate movements 'migrations' unless they are somewhat permanent. Thus demographers do not consider tourists and travellers to be migrating. While demographers who study migration typically do so through census data on place of residence, indirect sources of data including tax forms and labour force surveys are also important. [22]

Demography is today widely taught in many universities across the world, attracting students with initial training in social sciences, statistics or health studies. Being at the crossroads of several disciplines such as sociology, economics, epidemiology, geography, anthropology and history, demography offers tools to approach a large range of population issues by combining a more technical quantitative approach that represents the core of the discipline with many other methods borrowed from social or other sciences. Demographic research is conducted in universities, in research institutes as well as in statistical departments and in several international agencies. Population institutions are part of the Cicred (International Committee for Coordination of Demographic Research) network while most individual scientists engaged in demographic research are members of the International Union for the Scientific Study of Population, [23] or a national association such as the Population Association of America in the United States, [24] or affiliates of the Federation of Canadian Demographers in Canada. [25]

## Notes

1. "The Science of Population". demographicpartitions.org. Archived from the original on 14 August 2015. Retrieved 4 August 2015.
2. Andrew Hinde Demographic Methods Ch. 1 ISBN   0-340-71892-7
3. Peter Biller,The measure of multitude: Population in medieval thought .
4. See, e.g., Andrey Korotayev, Artemy Malkov, & Daria Khaltourina (2006). Introduction to Social Macrodynamics: Compact Macromodels of the World System Growth. Moscow: URSS, ISBN   5-484-00414-4.
5. “Our Yesterdays: the History of the Actuarial Profession in North America, 1809-1979,” by E.J. (Jack) Moorhead, FSA, ( 1/23/10 – 2/21/04), published by the Society of Actuaries as part of the profession’s centennial celebration in 1989.
6. The History of Insurance, Vol 3, Edited by David Jenkins and Takau Yoneyama (1 85196 527 0): 8 Volume Set: ( 2000) Availability: Japan: Kinokuniya).
7. von Valtier, William F. (June 2011). ""An Extravagant Assumption": The Demographic Numbers behind Benjamin Franklin's Twenty-Five-Year Doubling Period" (PDF). Proceedings of the American Philosophical Society . 155 (2): 158–188. Archived from the original (PDF) on 5 March 2016. Retrieved 19 September 2018.
8. Zirkle, Conway (25 April 1941). "Natural Selection before the 'Origin of Species'". Proceedings of the American Philosophical Society . Philadelphia, PA: American Philosophical Society. 84 (1): 71–123. ISSN   0003-049X. JSTOR   984852.
9. Caves, R. W. (2004). Encyclopedia of the City. Routledge. p. 169.
10. de Gans, Henk and Frans van Poppel (2000) Contributions from the margins. Dutch statisticians, actuaries and medical doctors and the methods of demography in the time of Wilhelm Lexis. Workshop on ‘Lexis in Context: German and Eastern& Northern European Contributions to Demography 1860-1910’ at the Max Planck Institute for Demographic Research, Rostock, August 28 and 29, 2000.
11. Power C and Elliott J (2006). "Cohort profile: 1958 British Cohort Study". International Journal of Epidemiology. 35 (1): 34–41. doi:10.1093/ije/dyi183. PMID   16155052.
12. Elliott J and Shepherd P (2006). "Cohort profile: 1970 British Birth Cohort (BCS70)". International Journal of Epidemiology. 35 (4): 836–43. doi:10.1093/ije/dyl174. PMID   16931528.
13. The last three are run by the Centre for Longitudinal Studies
14. Introduction to environmental engineering and science by Masters and Ela, 2008, Pearson Education, chapter 3
15. See, for example, Barbara A. Anderson and Brian D. Silver, "Estimating Russification of Ethnic Identity Among Non-Russians in the USSR," Demography, Vol. 20, No. 4 (Nov., 1983): 461-489.
16. Lutz, Wolfgang; Sanderson, Warren; Scherbov, Sergei (19 June 1997). "Doubling of world population unlikely" (PDF). Nature. 387 (6635): 803–805. doi:10.1038/42935. PMID   9194559. Archived from the original (PDF) on 16 December 2008. Retrieved 2008-11-13.
17. John Bongaarts. The Fertility-Inhibiting Effects of the Intermediate Fertility Variables. Studies in Family Planning, Vol. 13, No. 6/7. (Jun. - Jul., 1982), pp. 179-189.
18. Donald T. Rowland Demographic Methods and Concepts Ch. 11 ISBN   0-19-875263-6
19. Canadian Population Society Archived 26 June 2011 at the Wayback Machine