A task that demographers are often called on to perform is population forecasting. Professional demographers describe their statements on future population as projections, the working out of the consequences of a set of assumptions.

Forecasting has gone through many styles, starting with extrapolation of population numbers by exponential, logistics, or other curves. More acceptable is extrapolating the components of population-birth, death and migration-and assembling the population from the extrapolated values of these.

Sampling to ascertain childbearing intentions of women has been extensively tried. Demographers have no illusions about the predictability of the long-term future, but, on the other hand, estimates made by those who have studied the past are more worthy of attention than the simple-minded extrapolations that form the alternative. Some numerical estimates of future population are indispensable for virtually any kind of economic planning, whether by a corporation or a government.

The richness of demography today is in part due to the commitment of scholars from many disciplines. Actuaries developed much of the early theory, and statisticians and biostatisticians today add to their worth with the techniques of numerical analysis and determination of error.

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Sociologists see population change as both the cause and the result of major changes in social structures and attitudes; they study the increases of labour force participation by women, of divorce, of single person households, the apparently lessening importance of marriage and the decline in fertility rates.

Economists see fertility rising and falling as people try to maximise utility. Biologists employ an ecological framework relating human populations to the plant and animal populations among which they live and on which they depend.

Psychologists have brought their survey and other tools to the study of preferences of parents for number and sex of children.

Historians, in a synthesis with demography, are putting to use the enormous among of valuable data in census and other records to gain new insights on what happened to birth and death rates during the past several centuries.