Demography

Demography is the study of human population dynamics. It encompasses the study of the size, structure and distribution of populations, and how populations change over time due to births, deaths, migration and ageing. Demographic analysis can relate to whole societies or to groups defined by criteria such as education, nationality, religion and ethnicity.

The term demographics is often used erroneously for demography, but refers rather to selected population characteristics as used in marketing or opinion research, or the demographic profiles used in such research.

Data and methods
Demography may rely on the use of large amounts of data, including census returns and vital statistics registers, or incorporate survey data using indirect estimation techniques. The earliest modern census was carried out in United States in 1790, although several Scandinavian countires--notably Iceland and Denmark--have earlier censuses.

In many countries, particularly in the third world, reliable demographic data are still difficult to obtain; census is often equated in the minds of the people with taxation, so the people scatter when a census taker comes around. During the 1980s, for example, the population of Nigeria was widely estimated to be around 101 million people, before it was established to be as little as 89 million people (without adjustment for undercounting) in a census carried out in 1991.

Important concepts
Important concepts in demography include:
 * The crude birth rate, the annual number of live births per thousand people.
 * The general fertility rate, the annual number of live births per 1000 women of childbearing age (often taken to be from 15 to 49 years old, but sometimes from 15 to 44).
 * age-specific fertility rates, the annual number of live births per 1000 women in particular age groups (usually age 15-19, 20-24 etc.)
 * The crude death rate, the annual number of deaths per 1000 people.
 * The infant mortality rate, the annual number of deaths of children less than 1 year old per thousand live births.
 * The expectation of life (or life expectancy), the number of years which an individual at a given age can 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 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 rate is the number of daughters who would be born to a woman according to current age-specific fertility and mortality rates.

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 1000 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 relatively 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 summarises mortality separately at each age. A life table is necessary to give a good estimate of life expectancy.

History
The Natural and Political Observations ... upon the Bills of Mortality (1662) of John Graunt contains a primitive form of life table. Mathematicians, such as Edmond Halley, developed the life table as the basis for life insurance mathematics. At the end of the 18th. century Thomas Malthus concluded that, if unchecked, populations would be subject to exponential growth. He feared that population growth would tend to outstrip growth in food production, leading to ever increasing famine and poverty (see Malthusian catastrophe); he is seen as the intellectual father of ideas of overpopulation and the limits to growth. Later more sophisticated and realistic models were presented by e.g. Gompertz and Verhulst.

The demographic transition
Contrary to Malthus' predictions (though in line with his thoughts on moral restraint), natural population growth in most developed countries has diminished to close to zero, without being held in check by famine or lack of resources, as people in developed nations have shown a tendency to have fewer children. The fall in population growth has occurred despite large rises in life expectancy in these countries. This pattern of population growth, with slow (or no) growth in preindustrial societies, followed by fast growth as the society develops and industrialises, followed by slow growth again as it becomes more affluent, is known as the demographic transition.

Similar trends are now becoming visible in ever more developing countries, so that far from spiralling out of control, world population growth is expected to slow markedly in the next century, coming to an eventual standstill. The change is likely to be accompanied by major shifts in the proportion of world population in particular regions. The United Nations Population Division expects the absolute number of infants and toddlers in the world to begin to fall by 2015, and the number of children under 15 by 2025. Demographers at the International Institute for Applied Systems Analysis in Austria expect world population to peak at 9000 million by 2070. Throughout the 21st century, the average age of the population is likely to continue to rise.