Demography is the statistical study of human populations. Demography examines the size, structure, and movements of populations over space and time. It uses methods from history, economics, anthropology, sociology, and other fields. Demography is useful for governments and private businesses as a means of analyzing and predicting social, cultural, and economic trends related to population.

While basic demographic studies, such as censuses, were conducted in the ancient world as far back as 6,000 years ago, demographers as we know them, such as John Graunt from the United Kingdom, came about in the 16th century. The earliest statistical studies were concerned mostly with mortality (how many people died and at what age). Through studying baptism and burial records, Graunt could estimate the number of men of military age, and the number of women of childbearing age. His study represents one of the earliest statistical examinations of the population of a region. Demographic studies were often carried out by early insurance agents to determine life insurance rates.

These early demographic studies were mostly concerned with mortality. However, in the 19th century, studies showed that there was a decline in the number of births, and researchers began to study fertility as well as mortality. These studies led to the idea of “differential fertility.” Differential fertility suggests that different groups within a population have different numbers of children due to factors, such as religion, cultural attitudes, poverty, and employment. Migration of people is the last main factor in demographic studies. It is these three variables (mortality, fertility, and migration) that contribute to population change.

Demographers gather data mainly through government censuses and government registries of births and deaths. However, these sources can be inaccurate depending on the precision of government records. Demographers also gather data indirectly through surveying smaller groups within a population. These samples are then examined using statistical models to draw conclusions about the whole population.